CsgA is a pathogen-associated molecular pattern of Salmonella enterica serotype Typhimurium that is recognized by Toll-like receptor 2

The menace within: bacterial amyloids as a trigger for autoimmune and neurodegenerative diseases

Bacteria are known to produce amyloids, proteins characterized by a conserved cross-beta sheet structure, which exhibit structural and functional similarities to human amyloids. The deposition of human amyloids into fibrillar plaques within organs is closely linked to several debilitating human diseases, including Alzheimer's and Parkinson's disease. Recently, bacterial amyloids have garnered significant attention as potential initiators of human amyloid-associated diseases as well as autoimmune diseases. This review aims to explore how bacterial amyloid, particularly curli found in gut biofilms, can act as a trigger for neurodegenerative and autoimmune diseases. We will elucidate three primary mechanisms through which bacterial amyloids exert their influence: By delving into these three distinct modes of action, this review will provide valuable insights into the intricate relationship between bacterial amyloids and the onset or progression of neurodegenerative and autoimmune diseases. A comprehensive understanding of these mechanisms may open new avenues for therapeutic interventions and preventive strategies targeting amyloid-associated diseases.

Serum Amyloid A3 Fuels a Feed-Forward Inflammatory Response to the Bacterial Amyloid Curli in the Enteric Nervous System

BACKGROUND & AIMS

Mounting evidence suggests a role for the gastrointestinal microbiome as a determinant of peripheral immunity and central neurodegeneration, but the local disease mechanisms remain unknown. Given its potential relevance for early diagnosis and therapeutic intervention, we set out to map the pathogenic changes induced by bacterial amyloids in the gastrointestinal tract and its enteric nervous system.

METHODS

To examine the early response, we challenged primary murine myenteric networks with curli, the prototypic bacterial amyloid, and performed shotgun RNA sequencing and multiplex enzyme-linked immunosorbent assay. Using enteric neurosphere-derived glial and neuronal cell cultures, as well as in vivo curli injections into the colon wall, we further scrutinized curli-induced pathogenic pathways.

RESULTS

Curli induced a proinflammatory response, with marked up-regulation of serum amyloid A3 (Saa3) and the secretion of several cytokines. This proinflammatory state was induced primarily in enteric glia, was accompanied by increased levels of DNA damage and replication, and triggered the influx of immune cells in vivo. The addition of recombinant SAA3 was sufficient to recapitulate this specific proinflammatory phenotype while Saa3 knock-out attenuated curli-induced DNA damage and replication. Similar to curli, recombinant SAA3 caused a strong up-regulation of Saa3 transcripts, indicating a feedforward loop. Colonization of curli-producing Salmonella and dextran sulfate sodium-induced colitis caused a significant increase in Saa3 transcripts, indicating a central role for SAA3 in enteric dysfunction. Inhibition of dual leucine zipper kinase, an upstream regulator of the c-Jun N-terminal kinase pathway responsible for SAA3 production, attenuated curli- and SAA3-induced Saa3 up-regulation, DNA damage, and replication in enteric glia.

CONCLUSIONS

Our results position SAA3 as an important mediator of gastrointestinal vulnerability toward bacterial-derived amyloids and demonstrate the potential of dual leucine zipper kinase inhibition to dampen enteric pathology.

Differential regulatory control of curli (csg) gene expression in Salmonella enterica serovar Typhi requires more than a functional CsgD regulator

The human-specific Salmonella enterica serovar Typhi (S. Typhi) causes typhoid fever, a systemic disease with no known reservoir. Curli fimbriae are major components of biofilm produced by Salmonella and are encoded by the csg gene cluster (csgBAC and csgDEFG). The role of curli in S. Typhi is unknown, although detection of anti-curli antibodies suggests they are produced during host infection. In this study, we investigated curli gene expression in S. Typhi. We demonstrated that the CsgD regulatory protein binds weakly to the csgB promoter. Yet, replacing S. Typhi csgD with the csgD allele from S. Typhimurium did not modify the curli negative phenotype on Congo Red medium suggesting that differential regulation of curli gene expression in S. Typhi is not dependent on modification of the CsgD regulator. The entire csg gene cluster from S. Typhimurium was also cloned into S. Typhi, but again, despite introduction of a fully functional csg gene cluster from S. Typhimurium, curli were still not detected in S. Typhi. Thus, in addition to intrinsic genomic differences in the csg gene cluster that have resulted in production of a modified CsgD protein, S. Typhi has likely undergone other changes independent of the csg gene cluster that have led to distinctive regulation of csg genes compared to other Salmonella serovars.

Engineered Curli Nanofilaments as a Self-Adjuvanted Antigen Delivery Platform

Proteinaceous nanoparticles constitute efficient antigen delivery systems in vaccine formulations due to their size and repetitive nature that mimic most invading pathogens and promote immune activation. Nonetheless, the coadministration of an adjuvant with subunit nanovaccines is usually required to induce a robust, long-lasting, and protective immune response. Herein, the protein Curli-specific gene A (CsgA), which is known to self-assemble into nanofilaments contributing to bacterial biofilm, is exploited to engineer an intrinsically immunostimulatory antigen delivery platform. Three repeats of the M2e antigenic sequence from the influenza A virus matrix 2 protein are merged to the N-terminal domain of engineered CsgA proteins. These chimeric 3M2e-CsgA spontaneously self-assemble into antigen-displaying cross-β-sheet nanofilaments that activate the heterodimeric toll-like receptors 2 and 1. The resulting nanofilaments are avidly internalized by antigen-presenting cells and stimulate the maturation of dendritic cells. Without the need of any additional adjuvants, both assemblies show robust humoral and cellular immune responses, which translate into complete protection against a lethal experimental infection with the H1N1 influenza virus. Notably, these CsgA-based nanovaccines induce neither overt systemic inflammation, nor reactogenicity, upon mice inoculation. These results highlight the potential of engineered CsgA nanostructures as self-adjuvanted, safe, and versatile antigen delivery systems to fight infectious diseases.

Urinary Tract Infections Caused by Uropathogenic Escherichia coli: Mechanisms of Infection and Treatment Options

Urinary tract infections (UTIs) are common bacterial infections that represent a severe public health problem. They are often caused by Escherichia coli (E. coli), Klebsiella pneumoniae (K. pneumonia), Proteus mirabilis (P. mirabilis), Enterococcus faecalis (E. faecalis), and Staphylococcus saprophyticus (S. saprophyticus). Among these, uropathogenic E. coli (UPEC) are the most common causative agent in both uncomplicated and complicated UTIs. The adaptive evolution of UPEC has been observed in several ways, including changes in colonization, attachment, invasion, and intracellular replication to invade the urothelium and survive intracellularly. While antibiotic therapy has historically been very successful in controlling UTIs, high recurrence rates and increasing antimicrobial resistance among uropathogens threaten to greatly reduce the efficacy of these treatments. Furthermore, the gradual global emergence of multidrug-resistant UPEC has highlighted the need to further explore its pathogenesis and seek alternative therapeutic and preventative strategies. Therefore, a thorough understanding of the clinical status and pathogenesis of UTIs and the advantages and disadvantages of antibiotics as a conventional treatment option could spark a surge in the search for alternative treatment options, especially vaccines and medicinal plants. Such options targeting multiple pathogenic mechanisms of UPEC are expected to be a focus of UTI management in the future to help combat antibiotic resistance.

Role of TLRs in EGFR-mediated IL-8 secretion by enteroaggregative Escherichia coli-infected cultured human intestinal epithelial cells

Enteroaggregative Escherichia coli (EAEC) is an emerging enteric pathogen associated with persistent diarrhea in travelers, immunocompromised patients and children worldwide. However, the pathogenesis of this organism is yet to be established. In this study, the role of Toll-like receptors (TLRs) was evaluated in epidermal growth factor receptor (EGFR)-mediated IL-8 secretion by EAEC-infected human small intestinal and colonic epithelial cells (INT-407 and HCT-15, respectively). We observed that EAEC-induced upregulation of TLR2, TLR4 and TLR5 transcripts in both types of cells, and the maximum level of these transcripts was seen in cells infected with EAEC-T8 (an invasive clinical isolate). All these TLRs made a significant contribution to the EAEC-T8-mediated EGFR activation in these cells. Furthermore, these TLRs were found to be associated with activation of the downstream effectors (ERK-1/2, PI3 kinase and Akt) and transcription factors (NF-κB, c-Jun, c-Fos and STAT-3) of EGFR-mediated signal transduction pathways. Moreover, the involvement of these TLRs was also noted in IL-8 secretion by both EAEC-T8-infected cell types. Our findings suggest that EAEC-induced upregulation of TLR2, TLR4 and TLR5 is important for the IL-8 response via EGFR-mediated signal transduction pathways in these cells.

Emerging role of gut microbiota dysbiosis in neuroinflammation and neurodegeneration

Alzheimer's disease (AD), is a chronic age-related progressive neurodegenerative disorder, characterized by neuroinflammation and extracellular aggregation of Aβ peptide. Alzheimer's affects every 1 in 14 individuals aged 65 years and above. Recent studies suggest that the intestinal microbiota plays a crucial role in modulating neuro-inflammation which in turn influences Aβ deposition. The gut and the brain interact with each other through the nervous system and chemical means via the blood-brain barrier, which is termed the Microbiota Gut Brain Axis (MGBA). It is suggested that the gut microbiota can impact the host's health, and numerous factors, such as nutrition, pharmacological interventions, lifestyle, and geographic location, can alter the gut microbiota composition. Although, the exact relationship between gut dysbiosis and AD is still elusive, several mechanisms have been proposed as drivers of gut dysbiosis and their implications in AD pathology, which include, action of bacteria that produce bacterial amyloids and lipopolysaccharides causing macrophage dysfunction leading to increased gut permeability, hyperimmune activation of inflammatory cytokines (IL-1β, IL-6, IL-8, and NLRP3), impairment of gut- blood brain barrier causing deposition of Aβ in the brain, etc. The study of micro-organisms associated with dysbiosis in AD with the aid of appropriate model organisms has recognized the phyla Bacteroidetes and Firmicutes which contain organisms of the genus Escherichia, Lactobacillus, Clostridium, etc., to contribute significantly to AD pathology. Modulating the gut microbiota by various means, such as the use of prebiotics, probiotics, antibiotics or fecal matter transplantation, is thought to be a potential therapeutic intervention for the treatment of AD. This review aims to summarize our current knowledge on possible mechanisms of gut microbiota dysbiosis, the role of gut brain microbiota axis in neuroinflammation, and the application of novel targeted therapeutic approaches that modulate the gut microbiota in treatment of AD.

One species, different diseases: the unique molecular mechanisms that underlie the pathogenesis of typhoidal Salmonella infections

Salmonella enterica is one of the most widespread bacterial pathogens found worldwide, resulting in approximately 100 million infections and over 200 000 deaths per year. Salmonella isolates, termed 'serovars', can largely be classified as either nontyphoidal or typhoidal Salmonella, which differ in regard to disease manifestation and host tropism. Nontyphoidal Salmonella causes gastroenteritis in many hosts, while typhoidal Salmonella is human-restricted and causes typhoid fever, a systemic disease with a mortality rate of up to 30% without treatment. There has been considerable interest in understanding how different Salmonella serovars cause different diseases, but the molecular details that underlie these infections have not yet been fully characterized, especially in the case of typhoidal Salmonella. In this review, we highlight the current state of research into understanding the pathogenesis of both nontyphoidal and typhoidal Salmonella, with a specific interest in serovar-specific traits that allow human-adapted strains of Salmonella to cause enteric fever. Overall, a more detailed molecular understanding of how different Salmonella isolates infect humans will provide critical insights into how we can eradicate these dangerous enteric pathogens.

Systemic exposure to bacterial amyloid curli alters the gut mucosal immune response and the microbiome, exacerbating Salmonella-induced arthritis

The Salmonella biofilm-associated amyloid protein, curli, is a dominant instigator of systemic inflammation and autoimmune responses following Salmonella infection. Systemic curli injections or infection of mice with Salmonella Typhimurium induce the major features of reactive arthritis, an autoimmune disorder associated with Salmonella infection in humans. In this study, we investigated the link between inflammation and microbiota in exacerbating autoimmunity. We studied C57BL/6 mice from two sources, Taconic Farms and Jackson Labs. Mice from Taconic Farms have been reported to have higher basal levels of the inflammatory cytokine IL - 17 than do mice from Jackson Labs due to the differences in their microbiota. When we systemically injected mice with purified curli, we observed a significant increase in diversity in the microbiota of Jackson Labs mice but not in that of the Taconic mice. In Jackson Labs, mice, the most striking effect was the expansion of Prevotellaceae. Furthermore, there were increases in the relative abundance of the family Akkermansiaceae and decreases in families Clostridiaceae and Muribaculaceae in Jackson Labs mice. Curli treatment led to significantly aggravated immune responses in the Taconic mice compared to Jackson Labs counterparts. Expression and production of IL - 1β, a cytokine known to promote IL - 17 production, as well as expression of Tnfa increased in the gut mucosa of Taconic mice in the first 24 hours after curli injections, which correlated with significant increases in the number of neutrophils and macrophages in the mesenteric lymph nodes. A significant increase in the expression of Ccl3 in colon and cecum of Taconic mice injected with curli was detected. Taconic mice injected with curli also had elevated levels of inflammation in their knees. Overall, our data suggest that autoimmune responses to bacterial ligands, such as curli, are amplified in individuals with a microbiome that promote inflammation.

In silico design of epitope-based peptide vaccine against non-typhoidal Salmonella through immunoinformatic approaches

Non-typhoidal Salmonella (NTS) is one of the leading bacterial causes of many invasive human infections with a high antibiotic resistance profile. With this concern, the current study aimed to design an effective epitope-based peptide vaccine against NTS species as a successive and substitutive protective measure of invasive NTS disease. To design rationally, the current study considered a comprehensive in silico workflow combination of both immunoinformatics and molecular modeling approaches, including molecular docking and molecular dynamics (MD) simulation. We identified the two most promising T cell epitopes KVLYGIFAI and YGIFAITAL, and three B cell epitopes AAPVQVGEAAGS, TGGGDGSNT, and TGGGDGSNTGTTTT, in the outer membrane of NTS. Using these epitopes, a multiepitope vaccine was subsequently constructed along with appropriate adjuvant and linkers, which showed a good binding affinity and stability with toll-like receptor 2 (TLR2) in both molecular docking and MD simulation. Furthermore, in silico immune simulation described a strong immune response with a high number of antibodies, interferon-γ, and activated B and T cells. This study collectively suggests that predicted vaccine constructs could be considered potential vaccine candidates against common NTS species.Communicated by Ramaswamy H. Sarma.

The gut microbiome and Alzheimer's disease: Complex and bidirectional interactions

Structural and functional alterations to the gut microbiome, referred to as gut dysbiosis, have emerged as potential key mediators of neurodegeneration and Alzheimer disease (AD) pathogenesis through the "gut -brain" axis. Emerging data from animal and clinical studies support an important role for gut dysbiosis in mediating neuroinflammation, central and peripheral immune dysregulation, abnormal brain protein aggregation, and impaired intestinal and brain barrier permeability, leading to neuronal loss and cognitive impairment. Gut dysbiosis has also been shown to directly influence various mechanisms involved in neuronal growth and repair, synaptic plasticity, and memory and learning functions. Aging and lifestyle factors including diet, exercise, sleep, and stress influence AD risk through gut dysbiosis. Furthermore, AD is associated with characteristic gut microbial signatures which offer value as potential markers of disease severity and progression. Together, these findings suggest the presence of a complex bidirectional relationship between AD and the gut microbiome and highlight the utility of gut modulation strategies as potential preventative or therapeutic strategies in AD. We here review the current literature regarding the role of the gut-brain axis in AD pathogenesis and its potential role as a future therapeutic target in AD treatment and/or prevention.

Assembly of ordered DNA-curli fibril complexes during Salmonella biofilm formation correlates with strengths of the type I interferon and autoimmune responses

Deposition of human amyloids is associated with complex human diseases such as Alzheimer’s and Parkinson’s. Amyloid proteins are also produced by bacteria. The bacterial amyloid curli, found in the extracellular matrix of both commensal and pathogenic enteric bacterial biofilms, forms complexes with extracellular DNA, and recognition of these complexes by the host immune system may initiate an autoimmune response. Here, we isolated early intermediate, intermediate, and mature curli fibrils that form throughout the biofilm development and investigated the structural and pathogenic properties of each. Early intermediate aggregates were smaller than intermediate and mature curli fibrils, and circular dichroism, tryptophan, and thioflavin T analyses confirmed the establishment of a beta-sheet secondary structure as the curli conformations matured. Intermediate and mature curli fibrils were more immune stimulatory than early intermediate fibrils in vitro. The intermediate curli was cytotoxic to macrophages independent of Toll-like receptor 2. Mature curli fibrils had the highest DNA content and induced the highest levels of Isg15 expression and TNFα production in macrophages. In mice, mature curli fibrils induced the highest levels of anti-double-stranded DNA autoantibodies. The levels of autoantibodies were higher in autoimmune-prone NZBWxF/1 mice than wild-type C57BL/6 mice. Chronic exposure to all curli forms led to significant histopathological changes and synovial proliferation in the joints of autoimmune-prone mice; mature curli was the most detrimental. In conclusion, curli fibrils, generated during biofilm formation, cause pathogenic autoimmune responses that are stronger when curli complexes contain higher levels of DNA and in mice predisposed to autoimmunity.

Amyloid curli is an extracellular component of the biofilms of enteric bacteria like Salmonella and E. coli. During biofilm development, curli binds to extracellular DNA to form complexes that stimulate the immune system. Using complimentary techniques, we discovered that curli amyloids undergo changes in their physical and pathogenic properties throughout the development of the biofilm, polymerizing from early intermediate and intermediate to mature curli fibrils. While all three curli conformations activated TLR2, they bound different amounts of DNA that positively correlate with the complexity of the amyloids and their ability to induce type I IFNs and pro-inflammatory cytokines, and accelerate the onset of anti-dsDNA autoantibodies in autoimmune-prone mice. Synchrotron x-ray diffraction results suggest that DNA is organized by the beta sheet motif of curli monomers into a spatially periodic lattice that can amplify immune activation. Intermediate curli complexes were the only form that also showed cytotoxicity in macrophages independent of TLR2 and they induced higher levels of IL-1b, a cytokine involved in cell death. In mice prone to autoimmunity, the intermediate and mature curli/DNA induced also joint inflammation. Overall, the maturation of curli-DNA complexes through biofilm development stimulated distinct immune responses influenced by DNA content and promoted autoimmune features in mice.

Amyloid-containing biofilms and autoimmunity

Bacteria are microscopic, single-celled organisms known for their ability to adapt to their environment. In response to stressful environmental conditions or in the presence of a contact surface, they commonly form multicellular aggregates called biofilms. Biofilms form on various abiotic or biotic surfaces through a dynamic stepwise process involving adhesion, growth, and extracellular matrix production. Biofilms develop on tissues as well as on implanted devices during infections, providing the bacteria with a mechanism for survival under harsh conditions including targeting by the immune system and antimicrobial therapy. Like pathogenic bacteria, members of the human microbiota can form biofilms. Biofilms formed by enteric bacteria contribute to several human diseases including autoimmune diseases and cancer. However, until recently the interactions of immune cells with biofilms had been mostly uncharacterized. Here, we will discuss how components of the enteric biofilm produced in vivo, specifically amyloid curli and extracellular DNA, could be interacting with the host's immune system causing an unpredicted immune response.

New hope for Parkinson's disease treatment: Targeting gut microbiota

There might be more than 10 million confirmed cases of Parkinson's disease (PD) worldwide by 2040. However, the pathogenesis of PD is still unclear. Host health is closely related to gut microbiota, which are affected by factors such as age, diet, and exercise. Recent studies have found that gut microbiota may play key roles in the progression of a wide range of diseases, including PD. Changes in the abundance of gut bacteria, such as Helicobacter pylori, Enterococcus faecalis, and Desulfovibrio, might be involved in PD pathogenesis or interfere with PD therapy. Gut microbiota and the distal brain achieve action on each other through a gut‐brain axis composed of the nervous system, endocrine system, and immune system. Here, this review focused on the current understanding of the connection between Parkinson's disease and gut microbiota, to provide potential therapeutic targets for PD.

Purification of the Bacterial Amyloid "Curli" from Salmonella enterica Serovar Typhimurium and Detection of Curli from Infected Host Tissues

Microbiologists are learning to appreciate the importance of "functional amyloids" that are produced by numerous bacterial species and have impacts beyond the microbial world. These structures are used by bacteria to link together, presumably to increase survival, protect against harsh conditions, and perhaps to influence cell-cell communication. Bacterial functional amyloids are also beginning to be appreciated in the context of host-pathogen interactions, where there is evidence that they can trigger the innate immune system and are recognized as non-self-molecular patterns. The characteristic three-dimensional fold of amyloids renders them similar across the bacterial kingdom and into the eukaryotic world, where amyloid proteins can be undesirable and have pathological consequences. The bacterial protein curli, produced by pathogenic Salmonella enterica and Escherichia coli strains, was one of the first functional amyloids discovered. Curli have since been well characterized in terms of function, and we are just starting to scratch the surface about their potential impact on eukaryotic hosts. In this manuscript, we present step-by-step protocols with pictures showing how to purify these bacterial surface structures. We have described the purification process from S. enterica, acknowledging that the same method can be applied to E. coli. In addition, we describe methods for detection of curli within animal tissues (i.e., GI tract) and discuss purifying curli intermediates in a S. enterica msbB mutant strain as they are more cytotoxic than mature curli fibrils. Some of these methods were first described elsewhere, but we wanted to assemble them together in more detail to make it easier for researchers who want to purify curli for use in biological experiments. Our aim is to provide methods that are useful for specialists and non-specialists as bacterial amyloids become of increasing importance.

Comparative Genomics Applied to Systematically Assess Pathogenicity Potential in Shiga Toxin-Producing Escherichia coli O145:H28

Shiga toxin-producing Escherichia coli (STEC) O145:H28 can cause severe disease in humans and is a predominant serotype in STEC O145 environmental isolates. Here, comparative genomics was applied to a set of clinical and environmental strains to systematically evaluate the pathogenicity potential in environmental strains. While the core genes-based tree separated all O145:H28 strains from the non O145:H28 reference strains, it failed to segregate environmental strains from the clinical. In contrast, the accessory genes-based tree placed all clinical strains in the same clade regardless of their genotypes or serotypes, apart from the environmental strains. Loss-of-function mutations were common in the virulence genes examined, with a high frequency in genes related to adherence, autotransporters, and the type three secretion system. Distinct differences in pathogenicity islands LEE, OI-122, and OI-57, the acid fitness island, and the tellurite resistance island were detected between the O145:H28 and reference strains. A great amount of genetic variation was detected in O145:H28, which was mainly attributed to deletions, insertions, and gene acquisition at several chromosomal “hot spots”. Our study demonstrated a distinct virulence gene repertoire among the STEC O145:H28 strains originating from the same geographical region and revealed unforeseen contributions of loss-of-function mutations to virulence evolution and genetic diversification in STEC.

Relationship between Escherichia coli virulence factors, notably kpsMTII, and symptoms of clinical metritis and endometritis in dairy cows

Although Escherichia coli is a commensal bacterium of the bovine vaginal microbiota, it is an important pathogenic bacterium that causes diseases of the reproductive tract and sub-fertility. Recent studies have focused on virulence factors (VFs) of intrauterine E. coli; however, actual endometrial VFs have not been clearly identified. The purpose of this study was to identify the VFs of E. coli associated with clinical metritis and endometritis. Thirty-two strains of E. coli and four mixed Trueperella pyogenes (TP) strains were detected in the uterus of 19 Holstein dairy cows with obvious clinical signs (between 8 and 66 days postpartum). The presence of six E. coli VFs (fimH, fyuA, kpsMTII, hra1, csgA, and astA) was examined by PCR, and clinical signs and reproductive performance (mixed TP, the percentage of polymorphonuclear neutrophils [PMN%], days to uterine involution, etc.) were evaluated. Four VFs (fimH, hra1, csgA, and astA) were detected in all E. coli strains, whereas fyuA and kpsMTII were detected in 94% and 50% of strains, respectively. Cows with E. coli strains harboring kpsMTII exhibited significantly severe clinical scores (vaginal discharge score, PMN%, uterine involution), suggesting that kpsMTII is a key VF for progression of clinical metritis and endometritis. In the present study, we clearly identified six VFs associated with clinical metritis and endometritis. In addition, E. coli strains with kpsMTII probably play a crucial role in the progression of clinical metritis and endometritis.

Genetic Determinants in Salmonella enterica Serotype Typhimurium Required for Overcoming In Vitro Stressors in the Mimicking Host Environment

Salmonella enterica serotype Typhimurium, a nontyphoidal Salmonella (NTS), results in a range of enteric diseases, representing a major disease burden worldwide. There is still a significant portion of Salmonella genes whose mechanistic basis to overcome host innate defense mechanisms largely remains unknown. Here, we have applied transposon insertion sequencing (Tn-seq) method to unveil the genetic factors required for the growth or survival of S. Typhimurium under various host stressors simulated in vitro. A highly saturating Tn5 library of S. Typhimurium 14028s was subjected to selection during growth in the presence of short-chain fatty acid (100 mM propionate), osmotic stress (3% NaCl), or oxidative stress (1 mM H2O2) or survival in extreme acidic pH (30 min in pH 3) or starvation (12 days in 1× phosphate-buffered saline [PBS]). We have identified a total of 339 conditionally essential genes (CEGs) required to overcome at least one of these conditions mimicking host insults. Interestingly, all eight genes encoding FoF1-ATP synthase subunit proteins were required for fitness in all five stresses. Intriguingly, a total of 88 genes in Salmonella pathogenicity islands (SPI), including SPI-1, SPI-2, SPI-3, SPI-5, SPI-6, and SPI-11, are also required for fitness under the in vitro conditions. Additionally, by comparative analysis of the genes identified in this study and the genes previously shown to be required for in vivo fitness, we identified novel genes (marBCT, envF, barA, hscA, rfaQ, rfbI, and the genes encoding putative proteins STM14_1138, STM14_3334, STM14_4825, and STM_5184) that have compelling potential for the development of vaccines and antibacterial drugs to curb Salmonella infection. IMPORTANCE Salmonella enterica serotype Typhimurium is a major human bacterial pathogen that enters the food chain through meat animals asymptomatically carrying this pathogen. Despite the rich genome sequence data, a significant portion of Salmonella genes remain to be characterized for their potential contributions to virulence. In this study, we used transposon insertion sequencing (Tn-seq) to elucidate the genetic factors required for growth or survival under various host stressors, including short-chain fatty acids, osmotic stress, oxidative stress, extreme acid, and starvation. Among the total of 339 conditionally essential genes (CEGs) that are required under at least one of these five stress conditions were 221 previously known virulence genes required for in vivo fitness during infection in at least one of four animal species, including mice, chickens, pigs, and cattle. This comprehensive map of virulence phenotype-genotype in S. Typhimurium provides a roadmap for further interrogation of the biological functions encoded by the genome of this important human pathogen to survive in hostile host environments.

Salmonella fimbrial protein StcD induces cyclooxygenase-2 expression via Toll-like receptor 4

INTRODUCTION

The genome of Salmonella enterica serovar Typhimurium contains 13 operons with homology to fimbrial genes.

METHODS

To investigate the involvement of these fimbrial gene clusters in the expression of cyclooxygenase-2 (COX-2), which is an inducible enzyme involved in the synthesis of prostanoids, in J774 macrophages infected with S. enterica serovar Typhimurium, we constructed strains carrying a mutation in genes encoding the putative subunit proteins in 12 fimbrial operons.

RESULTS

The level of COX-2 expression was lower in macrophages infected with fimA or stcA mutant Salmonella than in those infected with wild-type Salmonella. Therefore, we focused on putative subunit protein StcA and adhesive like protein StcD encoded in the stc operon. Treatment of macrophages with purified recombinant StcD protein, but not StcA, resulted in the activation of the mitogen-activated protein kinase and nuclear factor kappa B signaling pathways, leading to the expression of not only COX-2 but also of pro-inflammatory cytokines such as interleukin (IL)-1β, IL-6, and tumor necrosis factor alpha. The expression of StcD-induced COX-2 was inhibited by treatment with the Toll-like receptor 4 (TLR4) inhibitor TAK-242, but not by treatment with the lipopolysaccharide (LPS) antagonist polymyxin B. Furthermore, StcD treatment stimulated HEK293 cells expressing TLR4 in the presence of CD14 and MD-2.

CONCLUSION

StcD is a pathogen-associated molecular pattern of S. enterica serovar Typhimurium that is recognized by TLR4 and plays a significant role in the induction of COX-2 expression in macrophages.

Virulence genes identification and characterization revealed the presence of the Yersinia High Pathogenicity Island (HPI) in Salmonella from Brazil

Salmonella spp. is one of the major agents of foodborne disease worldwide, and its virulence genes are responsible for the main pathogenic mechanisms of this micro-organism. The whole-genome sequencing (WGS) of pathogens has become a lower-cost and more accessible genotyping tool providing many gene analysis possibilities. This study provided an in silico investigation of 129 virulence genes, including plasmidial and bacteriophage genes from Brazilian strains' public Salmonella genomes. The frequency analysis of the four most sequenced serovars and a temporal analysis over the past four decades was also performed. The NCBI sequence reads archive (SRA) database comprised 1077 Salmonella public whole-genome sequences of strains isolated in Brazil between 1968 and 2018. Among the 1077 genomes, 775 passed in Salmonella in silico Typing (SISTR) quality control, which also identified 41 different serovars in which the four most prevalent were S. Enteritidis, S. Typhimurium, S. Dublin, and S. Heidelberg. Among these, S. Heidelberg presented the most distinct virulence profile, besides presenting Yersinia High Pathogenicity Island (HPI), rare and first reported in Salmonella from Brazil. The genes mgtC, csgC, ssaI and ssaS were the most prevalent within the 775 genomes with more than 99% prevalence. On the other hand, the less frequent genes were astA, iucBCD, tptC and shdA, with less than 1% frequency. All of the plasmids and bacteriophages virulence genes presented a decreasing trend between the 2000 s and 2010 s decades, except for the phage gene grvA, which increased in this period. This study provides insights into Salmonella virulence genes distribution in Brazil using freely available bioinformatics tools. This approach could guide in vivo and in vitro studies besides being an interesting method for the investigation and surveillance of Salmonella virulence. Moreover, here we propose the genes mgtC, csgC, ssaI and ssaS as additional targets for PCR identification of Salmonella in Brazil due to their very high frequency in the studied genomes.

A SNP in the Cache 1 Signaling Domain of Diguanylate Cyclase STM1987 Leads to Increased In Vivo Fitness of Invasive Salmonella Strains

Nontyphoidal Salmonella (NTS) strains are associated with gastroenteritis worldwide but are also the leading cause of bacterial bloodstream infections in sub-Saharan Africa. The invasive NTS (iNTS) strains that cause bloodstream infections differ from standard gastroenteritis-causing strains by >700 single-nucleotide polymorphisms (SNPs). These SNPs are known to alter metabolic pathways and biofilm formation and to contribute to serum resistance and are thought to signify iNTS strains becoming human adapted, similar to typhoid fever-causing Salmonella strains. Identifying SNPs that contribute to invasion or increased virulence has been more elusive. In this study, we identified a SNP in the cache 1 signaling domain of diguanylate cyclase STM1987 in the invasive Salmonella enterica serovar Typhimurium type strain D23580. This SNP was conserved in 118 other iNTS strains analyzed and was comparatively absent in global S Typhimurium isolates associated with gastroenteritis. STM1987 catalyzes the formation of bis-(3',5')-cyclic dimeric GMP (c-di-GMP) and is proposed to stimulate production of cellulose independent of the master biofilm regulator CsgD. We show that the amino acid change in STM1987 leads to a 10-fold drop in cellulose production and increased fitness in a mouse model of acute infection. Reduced cellulose production due to the SNP led to enhanced survival in both murine and human macrophage cell lines. In contrast, loss of CsgD-dependent cellulose production did not lead to any measurable change in in vivo fitness. We hypothesize that the SNP in stm1987 represents a pathoadaptive mutation for iNTS strains.

Microbiome or Infections: Amyloid-Containing Biofilms as a Trigger for Complex Human Diseases

The human microbiota is the community of microorganisms that live upon or within their human host. The microbiota consists of various microorganisms including bacteria, fungi, viruses, and archaea; the gut microbiota is comprised mostly of bacteria. Many bacterial species within the gut microbiome grow as biofilms, which are multicellular communities embedded in an extracellular matrix. Studies have shown that the relative abundances of bacterial species, and therefore biofilms and bacterial byproducts, change during progression of a variety of human diseases including gastrointestinal, autoimmune, neurodegenerative, and cancer. Studies have shown the location and proximity of the biofilms within the gastrointestinal tract might impact disease outcome. Gram-negative enteric bacteria secrete the amyloid curli, which makes up as much as 85% of the extracellular matrix of enteric biofilms. Curli mediates cell-cell attachment and attachment to various surfaces including extracellular matrix components such as fibronectin and laminin. Structurally, curli is strikingly similar to pathological and immunomodulatory human amyloids such as amyloid-β, which has been implicated in Alzheimer's disease, α-synuclein, which is involved in Parkinson's disease, and serum amyloid A, which is secreted during the acute phase of inflammation. The immune system recognizes both bacterial amyloid curli and human amyloids utilizing the same receptors, so curli also induces inflammation. Moreover, recent work indicates that curli can participate in the self-assembly process of pathological human amyloids. Curli is found within biofilms of commensal enteric bacteria as well as invasive pathogens; therefore, evidence suggests that curli contributes to complex human diseases. In this review, we summarize the recent findings on how bacterial biofilms containing curli participate in the pathological and immunological processes in gastrointestinal diseases, systemic autoimmune diseases, and neurodegenerative diseases.

Outer Membrane Structural Defects in Salmonella enterica Serovar Typhimurium Affect Neutrophil Chemokinesis but Not Chemotaxis

Neutrophils, the first line of defense against pathogens, are critical in the host response to acute and chronic infections. In Gram-negative pathogens, the bacterial outer membrane (OM) is a key mediator of pathogen detection; nonetheless, the effects of variations in its molecular structure on the neutrophil migratory response to bacteria remain largely unknown. Here, we developed a quantitative microfluidic assay that precludes physical contact between bacteria and neutrophils while maintaining chemical communication, thus allowing investigation of both transient and steady-state responses of neutrophils to a library of Salmonella enterica serovar Typhimurium OM-related mutants at single-cell resolution. Using single-cell quantitative metrics, we found that transient neutrophil chemokinesis is highly gradated based upon OM structure, while transient and steady-state chemotaxis responses differ little between mutants. Based on our finding of a lack of correlation between chemokinesis and chemotaxis, we define "stimulation score" as a metric that comprehensively describes the neutrophil response to pathogens. Complemented with a killing assay, our results provide insight into how OM modifications affect neutrophil recruitment and pathogen survival. Altogether, our platform enables the discovery of transient and steady-state migratory responses and provides a new path for quantitative interrogation of cell decision-making processes in a variety of host-pathogen interactions.IMPORTANCE Our findings provide insights into the previously unexplored effects of Salmonella envelope defects on fundamental innate immune cell behavior, which advance the knowledge in pathogen-host cell biology and potentially inspire the rational design of attenuated strains for vaccines or immunotherapeutic strains for cancer therapy. Furthermore, the microfluidic assay platform and analytical tools reported herein enable high-throughput, sensitive, and quantitative screening of microbial strains' immunogenicity in vitro This approach could be particularly beneficial for rapid in vitro screening of engineered microbial strains (e.g., vaccine candidates) as the quantitative ranking of the overall strength of the neutrophil response, reported by "stimulation score," agrees with in vivo cytokine response trends reported in the literature.

Nitrate Is an Environmental Cue in the Gut for Salmonella enterica Serovar Typhimurium Biofilm Dispersal through Curli Repression and Flagellum Activation via Cyclic-di-GMP Signaling

Curli, a major component of the bacterial biofilms in the intestinal tract, activates pattern recognition receptors and triggers joint inflammation after infection with Salmonella enterica serovar Typhimurium. The factors that allow S. Typhimurium to disperse from biofilms and invade the epithelium to establish a successful infection during acute inflammation remain unknown. Here, we studied S. Typhimurium biofilms in vitro and in vivo to understand how the inflammatory environment regulates the switch between multicellular and motile S. Typhimurium in the gut. We discovered that nitrate generated by the host is an environmental cue that induces S. Typhimurium to disperse from the biofilm. Nitrate represses production of an important biofilm component, curli, and activates flagella via the modulation of intracellular cyclic-di-GMP levels. We conclude that nitrate plays a central role in pathogen fitness by regulating the sessile-to-motile lifestyle switch during infection. IMPORTANCE Recent studies provided important insight into our understanding of the role of c-di-GMP signaling and the regulation of enteric biofilms. Despite an improved understanding of how c-di-GMP signaling regulates S. Typhimurium biofilms, the processes that affect the intracellular c-di-GMP levels and the formation of multicellular communities in vivo during infections remain unknown. Here, we show that nitrate generated in the intestinal lumen during infection with S. Typhimurium is an important regulator of biofilm formation in vivo.

Salmonella versus the Microbiome

A balanced gut microbiota contributes to health, but the mechanisms maintaining homeostasis remain elusive. Microbiota assembly during infancy is governed by competition between species and by environmental factors, termed habitat filters, that determine the range of successful traits within the microbial community. These habitat filters include the diet, host-derived resources, and microbiota-derived metabolites, such as short-chain fatty acids. Once the microbiota has matured, competition and habitat filtering prevent engraftment of new microbes, thereby providing protection against opportunistic infections. Competition with endogenous Enterobacterales, habitat filtering by short-chain fatty acids, and a host-derived habitat filter, epithelial hypoxia, also contribute to colonization resistance against Salmonella serovars. However, at a high challenge dose, these frank pathogens can overcome colonization resistance by using their virulence factors to trigger intestinal inflammation. In turn, inflammation increases the luminal availability of host-derived resources, such as oxygen, nitrate, tetrathionate, and lactate, thereby creating a state of abnormal habitat filtering that enables the pathogen to overcome growth inhibition by short-chain fatty acids. Thus, studying the process of ecosystem invasion by Salmonella serovars clarifies that colonization resistance can become weakened by disrupting host-mediated habitat filtering. This insight is relevant for understanding how inflammation triggers dysbiosis linked to noncommunicable diseases, conditions in which endogenous Enterobacterales expand in the fecal microbiota using some of the same growth-limiting resources required by Salmonella serovars for ecosystem invasion. In essence, ecosystem invasion by Salmonella serovars suggests that homeostasis and dysbiosis simply represent states where competition and habitat filtering are normal or abnormal, respectively.

Biological Functions of Prokaryotic Amyloids in Interspecies Interactions: Facts and Assumptions

Amyloids are fibrillar protein aggregates with an ordered spatial structure called "cross-β". While some amyloids are associated with development of approximately 50 incurable diseases of humans and animals, the others perform various crucial physiological functions. The greatest diversity of amyloids functions is identified within prokaryotic species where they, being the components of the biofilm matrix, function as adhesins, regulate the activity of toxins and virulence factors, and compose extracellular protein layers. Amyloid state is widely used by different pathogenic bacterial species in their interactions with eukaryotic organisms. These amyloids, being functional for bacteria that produce them, are associated with various bacterial infections in humans and animals. Thus, the repertoire of the disease-associated amyloids includes not only dozens of pathological amyloids of mammalian origin but also numerous microbial amyloids. Although the ability of symbiotic microorganisms to produce amyloids has recently been demonstrated, functional roles of prokaryotic amyloids in host-symbiont interactions as well as in the interspecies interactions within the prokaryotic communities remain poorly studied. Here, we summarize the current findings in the field of prokaryotic amyloids, classify different interspecies interactions where these amyloids are involved, and hypothesize about their real occurrence in nature as well as their roles in pathogenesis and symbiosis.

Variation in the ratio of curli and phosphoethanolamine cellulose associated with biofilm architecture and properties

Bacterial biofilms are communities of bacteria entangled in a self-produced extracellular matrix (ECM). Escherichia coli direct the assembly of two insoluble biopolymers, curli amyloid fibers, and phosphoethanolamine (pEtN) cellulose, to build remarkable biofilm architectures. Intense curiosity surrounds how bacteria harness these amyloid-polysaccharide composites to build biofilms, and how these biopolymers function to benefit bacterial communities. Defining ECM composition involving insoluble polymeric assemblies poses unique challenges to analysis and, thus, to comparing strains with quantitative ECM molecular correlates. In this work, we present results from a sum-of-the-parts ¹³ C solid-state nuclear magnetic resonance (NMR) analysis to define the curli-to-pEtN cellulose ratio in the isolated ECM of the E. coli laboratory K12 strain, AR3110. We compare and contrast the compositional analysis and comprehensive biofilm phenotypes for AR3110 and a well-studied clinical isolate, UTI89. The ECM isolated from AR3110 contains approximately twice the amount of pEtN cellulose relative to curli content as UTI89, revealing plasticity in matrix assembly principles among strains. The two parent strains and a panel of relevant gene mutants were investigated in three biofilm models, examining: (a) macrocolonies on agar, (b) pellicles at the liquid-air interface, and (c) biomass accumulation on plastic. We describe the influence of curli, cellulose, and the pEtN modification on biofilm phenotypes with power in the direct comparison of these strains. The results suggest that curli more strongly influence adhesion, while pEtN cellulose drives cohesion. Their individual and combined influence depends on both the biofilm modality (agar, pellicle, or plastic-associated) and the strain itself.

Functional Reciprocity of Amyloids and Antimicrobial Peptides: Rethinking the Role of Supramolecular Assembly in Host Defense, Immune Activation, and Inflammation

Pathological self-assembly is a concept that is classically associated with amyloids, such as amyloid-β (Aβ) in Alzheimer's disease and α-synuclein in Parkinson's disease. In prokaryotic organisms, amyloids are assembled extracellularly in a similar fashion to human amyloids. Pathogenicity of amyloids is attributed to their ability to transform into several distinct structural states that reflect their downstream biological consequences. While the oligomeric forms of amyloids are thought to be responsible for their cytotoxicity via membrane permeation, their fibrillar conformations are known to interact with the innate immune system to induce inflammation. Furthermore, both eukaryotic and prokaryotic amyloids can self-assemble into molecular chaperones to bind nucleic acids, enabling amplification of Toll-like receptor (TLR) signaling. Recent work has shown that antimicrobial peptides (AMPs) follow a strikingly similar paradigm. Previously, AMPs were thought of as peptides with the primary function of permeating microbial membranes. Consistent with this, many AMPs are facially amphiphilic and can facilitate membrane remodeling processes such as pore formation and fusion. We show that various AMPs and chemokines can also chaperone and organize immune ligands into amyloid-like ordered supramolecular structures that are geometrically optimized for binding to TLRs, thereby amplifying immune signaling. The ability of amphiphilic AMPs to self-assemble cooperatively into superhelical protofibrils that form structural scaffolds for the ordered presentation of immune ligands like DNA and dsRNA is central to inflammation. It is interesting to explore the notion that the assembly of AMP protofibrils may be analogous to that of amyloid aggregates. Coming full circle, recent work has suggested that Aβ and other amyloids also have AMP-like antimicrobial functions. The emerging perspective is one in which assembly affords a more finely calibrated system of recognition and response: the detection of single immune ligands, immune ligands bound to AMPs, and immune ligands spatially organized to varying degrees by AMPs, result in different immunologic outcomes. In this framework, not all ordered structures generated during multi-stepped AMP (or amyloid) assembly are pathological in origin. Supramolecular structures formed during this process serve as signatures to the innate immune system to orchestrate immune amplification in a proportional, situation-dependent manner.

In vivo synthesis of bacterial amyloid curli contributes to joint inflammation during S. Typhimurium infection

Reactive arthritis, an autoimmune disorder, occurs following gastrointestinal infection with invasive enteric pathogens, such as Salmonella enterica. Curli, an extracellular, bacterial amyloid with cross beta-sheet structure can trigger inflammatory responses by stimulating pattern recognition receptors. Here we show that S. Typhimurium produces curli amyloids in the cecum and colon of mice after natural oral infection, in both acute and chronic infection models. Production of curli was associated with an increase in anti-dsDNA autoantibodies and joint inflammation in infected mice. The negative impacts on the host appeared to be dependent on invasive systemic exposure of curli to immune cells. We hypothesize that in vivo synthesis of curli contributes to known complications of enteric infections and suggest that cross-seeding interactions can occur between pathogen-produced amyloids and amyloidogenic proteins of the host.

Our manuscript focuses on curli, a ‘functional amyloid’ produced by Salmonella as well as other enteric bacteria. We present the first biochemical evidence that these fibers are produced in the gastrointestinal tract of mice after oral infection, the natural route for Salmonella infections. This finding is significant because of the immune impacts on the host; we show that curli cause an increase in autoimmunity and inflammation in the knee joints of infected mice. Reactive arthritis is a known autoimmune complication after enteric infections and our results indicate that presence of curli in the gut provides a novel linchpin of pathogenesis. As curli or curli-like amyloids are also produced by the commensal bacteria, it is possible that the unintended release of amyloids produced by the microbiota could trigger similar autoimmune reactions. Finally, our work provides conceptual evidence for the possibility of cross-seeding between bacterial amyloids like curli and human amyloids involved in amyloid-associated diseases such as Alzheimer’s Disease via the gut microbiome or infections.

Oncolytic bacteria: past, present and future

More than a century ago, independent groups raised the possibility of using bacteria to selectively infect tumours. Such treatment induces an immune reaction that can cause tumour rejection and protect the patient against further recurrences. One of the first holistic approximations to use bacteria in cancer treatment was performed by William Coley, considered the father of immune-therapy, at the end of XIX century. Since then, many groups have used different bacteria to test their antitumour activity in animal models and patients. The basis for this reactivity implies that innate immune responses activated upon bacteria recognition, also react against the tumour. Different publications have addressed several aspects of oncolytic bacteria. In the present review, we will focus on revisiting the historical aspects using bacteria as oncolytic agents and how they led to the current clinical trials. In addition, we address the molecules present in oncolytic bacteria that induce specific toxic effects against the tumors as well as the activation of host immune responses in order to trigger antitumour immunity. Finally, we discuss future perspectives that could be considered in the different fields implicated in the implementation of this kind of therapy in order to improve the current use of bacteria as oncolytic agents.

Evaluation of Phosphoethanolamine Cellulose Production among Bacterial Communities Using Congo Red Fluorescence

Bacterial biofilms are surface-associated communities of bacterial cells enmeshed in an extracellular matrix (ECM). The biofilm lifestyle results in physiological heterogeneity across the community, promotes persistence, and protects cells from external insults such as antibiotic treatment. Escherichia coli was recently discovered to produce a chemically modified form of cellulose, phosphoethanolamine (pEtN) cellulose, which contributes to the formation of its extracellular matrix and elaboration of its hallmark wrinkled macrocolony architectures. Both pEtN cellulose and unmodified cellulose bind dyes such as calcofluor white and Congo red (CR). Here, we present the use of CR fluorescence to distinguish between pEtN cellulose and unmodified cellulose producers. We demonstrate the utility of this tool in the evaluation of a uropathogenic E. coli clinical isolate that appeared to produce curli and a cellulosic component but did not exhibit macrocolony wrinkling. We determined that lack of macrocolony wrinkling was attributed to a single-nucleotide mutation and introduction of a stop codon in bcsG, abrogating production of BcsG, the pEtN transferase. Thus, this work underscores the important contribution of the pEtN cellulose modification to the E. coli agar-based macrocolony wrinkling phenotype and introduces a facile approach to distinguish between modified and unmodified cellulose.IMPORTANCE E. coli bacteria produce amyloid fibers, termed curli, and a cellulosic component to assemble biofilm communities. Cellulose is the most abundant biopolymer on Earth, and we recently discovered that the cellulosic component in E. coli biofilms was not standard cellulose, but a newly identified cellulosic polymer, phosphoethanolamine cellulose. Studies involving the biological and functional impact of this cellulose modification among E. coli and other organisms are just beginning. Convenient methods for distinguishing pEtN cellulose from unmodified cellulose in E. coli and for estimating production are needed to facilitate further research. Dissecting the balance of pEtN cellulose and curli production by E. coli commensal strains and clinical isolates will improve our understanding of the host microbiome and of factors contributing to bacterial pathogenesis.

The Rich Tapestry of Bacterial Protein Translocation Systems

The translocation of proteins across membranes is a fundamental cellular function. Bacteria have evolved a striking array of pathways for delivering proteins into or across cytoplasmic membranes and, when present, outer membranes. Translocated proteins can form part of the membrane landscape, reside in the periplasmic space situated between the inner and outer membranes of Gram-negative bacteria, deposit on the cell surface, or be released to the extracellular milieu or injected directly into target cells. One protein translocation system, the general secretory pathway, is conserved in all domains of life. A second, the twin-arginine translocation pathway, is also phylogenetically distributed among most bacteria and plant chloroplasts. While all cell types have evolved additional systems dedicated to the translocation of protein cargoes, the number of such systems in bacteria is now known to exceed nine. These dedicated protein translocation systems, which include the types 1 through 9 secretion systems (T1SSs-T9SSs), the chaperone-usher pathway, and type IV pilus system, are the subject of this review. Most of these systems were originally identified and have been extensively characterized in Gram-negative or diderm (two-membrane) species. It is now known that several of these systems also have been adapted to function in Gram-positive or monoderm (single-membrane) species, and at least one pathway is found only in monoderms. This review briefly summarizes the distinctive mechanistic and structural features of each dedicated pathway, as well as the shared properties, that together account for the broad biological diversity of protein translocation in bacteria.

Salmonella Typhimurium biofilm disruption by a human antibody that binds a pan-amyloid epitope on curli

Bacterial biofilms, especially those associated with implanted medical devices, are difficult to eradicate. Curli amyloid fibers are important components of the biofilms formed by the Enterobacteriaceae family. Here, we show that a human monoclonal antibody with pan-amyloid-binding activity (mAb 3H3) can disrupt biofilms formed by Salmonella enterica serovar Typhimurium in vitro and in vivo. The antibody disrupts the biofilm structure, enhancing biofilm eradication by antibiotics and immune cells. In mice, 3H3 injections allow antibiotic-mediated clearance of catheter-associated S. Typhimurium biofilms. Thus, monoclonal antibodies that bind a pan-amyloid epitope have potential to prevent or eradicate bacterial biofilms.

Curli amyloid fibers are important components of bacterial biofilms formed by E. coli and Salmonella. Here, Tursi et al. show that a human monoclonal antibody with pan-amyloid binding activity can disrupt biofilms formed by Salmonella Typhimurium in vitro and in vivo.

Salmonella Extracellular Polymeric Substances Modulate Innate Phagocyte Activity and Enhance Tolerance of Biofilm-Associated Bacteria to Oxidative Stress

Salmonella enterica serovar Typhi causes 14.3 million acute cases of typhoid fever that are responsible for 136,000 deaths each year. Chronic infections occur in 3%-5% of those infected and S. Typhi persists primarily in the gallbladder by forming biofilms on cholesterol gallstones, but how these bacterial communities evade host immunity is not known. Salmonella biofilms produce several extracellular polymeric substances (EPSs) during chronic infection, which are hypothesized to prevent pathogen clearance either by protecting biofilm-associated bacteria from direct humoral attack or by modulating innate phagocyte interaction with biofilms. Using wild-type and EPS-deficient planktonic and biofilm Salmonella, the direct attack hypothesis was tested by challenging biofilms with human serum and antimicrobial peptides. Biofilms were found to be tolerant to these molecules, but these phenotypes were independent of the tested EPSs. By examining macrophage and neutrophil responses, new roles for biofilm-associated capsular polysaccharides and slime polysaccharides were identified. The S. Typhi Vi antigen was found to modulate innate immunity by reducing macrophage nitric oxide production and neutrophil reactive oxygen species (ROS) production. The slime polysaccharides colanic acid and cellulose were found to be immune-stimulating and represent a key difference between non-typhoidal serovars and typhoidal serovars, which do not express colanic acid. Furthermore, biofilm tolerance to the exogenously-supplied ROS intermediates hydrogen peroxide (H₂O₂) and hypochlorite (ClO) indicated an additional role of the capsular polysaccharides for both serovars in recalcitrance to H₂O₂ but not ClO, providing new understanding of the stalemate that arises during chronic infections and offering new directions for mechanistic and clinical studies.

Cytotoxic Curli Intermediates Form during Salmonella Biofilm Development

Enterobacteriaceae produce amyloid proteins called curli that are the major proteinaceous component of biofilms. Amyloids are also produced by humans and are associated with diseases such as Alzheimer's. During the multistep process of amyloid formation, monomeric subunits form oligomers, protofibrils, and finally mature fibrils. Amyloid β oligomers are more cytotoxic to cells than the mature amyloid fibrils. Oligomeric intermediates of curli had not been previously detected. We determined that turbulence inhibited biofilm formation and that, intriguingly, curli aggregates purified from cultures grown under high-turbulence conditions were structurally smaller and contained less DNA than curli preparations from cultures grown with less turbulence. Using flow cytometry analysis, we demonstrated that CsgA was expressed in cultures exposed to higher turbulence but that these cultures had lower levels of cell death than less-turbulent cultures. Our data suggest that the DNA released during cell death drives the formation of larger fibrillar structures. Consistent with this idea, addition of exogenous genomic DNA increased the size of the curli intermediates and led to binding to thioflavin T at levels observed with mature aggregates. Similar to the intermediate oligomers of amyloid β, intermediate curli aggregates were more cytotoxic than the mature curli fibrils when incubated with bone marrow-derived macrophages. The discovery of cytotoxic curli intermediates will enable research into the roles of amyloid intermediates in the pathogenesis of Salmonella and other bacteria that cause enteric infections.IMPORTANCE Amyloid proteins are the major proteinaceous components of biofilms, which are associated with up to 65% of human bacterial infections. Amyloids produced by human cells are also associated with diseases such as Alzheimer's. The amyloid monomeric subunits self-associate to form oligomers, protofibrils, and finally mature fibrils. Amyloid β oligomers are more cytotoxic to cells than the mature amyloid fibrils. Here we detected oligomeric intermediates of curli for the first time. Like the oligomers of amyloid β, intermediate curli fibrils were more cytotoxic than the mature curli fibrillar aggregates when incubated with bone marrow-derived macrophages. The discovery of cytotoxic curli intermediates will enable research into the roles of amyloid intermediates in the pathogenesis of Salmonella and other bacteria that cause enteric infections.

Bacterial Amyloids: The Link between Bacterial Infections and Autoimmunity

Molecular mimicry is a common mechanism used by many bacteria to evade immune responses. In recent years, it has become evident that bacteria also decorate the extracellular matrix (ECM) of their biofilms with molecules that resemble those of the host. These molecules include amyloids and other proteins, polysaccharides, and extracellular DNA. Bacterial amyloids, like curli, and extracellular DNA are found in the biofilms of many species. Recent work demonstrated that curli and DNA form unique molecular structures that are recognized by the immune system, causing activation of autoimmune pathways. Although a variety of mechanisms have been suggested as the means by which infections initiate and/or exacerbate autoimmune diseases, the mechanism remains unknown. In this article, we discuss recent work on biofilms that highlight the role of amyloids as a carrier for DNA and potentiator of autoimmune responses, and we propose a novel link between bacterial infections and autoimmune diseases.

Curli Biogenesis: Bacterial Amyloid Assembly by the Type VIII Secretion Pathway

In 1989, Normark and coworkers reported on fibrous surface structures called curli on strains of Escherichia coli that were suspected of causing bovine mastitis. Subsequent work by many groups has revealed an elegant and highly regulated curli biogenesis pathway also referred to as the type VIII secretion system. Curli biogenesis is governed by two divergently transcribed operons, csgBAC and csgDEFG. The csgBAC operon encodes the structural subunits of curli, CsgA and CsgB, along with a chaperone-like protein, CsgC. The csgDEFG operon encodes the accessory proteins required for efficient transcription, secretion, and assembly of the curli fiber. CsgA and CsgB are secreted as largely unstructured proteins and transition to β-rich structures that aggregate into regular fibers at the cell surface. Since both of these proteins have been shown to be amyloidogenic in nature, the correct spatiotemporal synthesis of the curli fiber is of paramount importance for proper functioning and viability. Gram-negative bacteria have evolved an elegant machinery for the safe handling, secretion, and extracellular assembly of these amyloidogenic proteins.

Targeting Bacterial Biofilms by the Green Tea Polyphenol EGCG

Bacterial biofilms are multicellular aggregates in which cells are embedded in an extracellular matrix of self-produced biopolymers. Being refractory to antibiotic treatment and host immune systems, biofilms are involved in most chronic infections, and anti-biofilm agents are being searched for urgently. Epigallocatechin-3-gallate (EGCG) was recently shown to act against biofilms by strongly interfering with the assembly of amyloid fibres and the production of phosphoethanolamin-modified cellulose fibrils. Mechanistically, this includes a direct inhibition of the fibre assembly, but also triggers a cell envelope stress response that down-regulates the synthesis of these widely occurring biofilm matrix polymers. Based on its anti-amyloidogenic properties, EGCG seems useful against biofilms involved in cariogenesis or chronic wound infection. However, EGCG seems inefficient against or may even sometimes promote biofilms which rely on other types of matrix polymers, suggesting that searching for 'magic bullet' anti-biofilm agents is an unrealistic goal. Combining molecular and ecophysiological aspects in this review also illustrates why plants control the formation of biofilms on their surfaces by producing anti-amyloidogenic compounds such as EGCG. These agents are not only helpful in combating certain biofilms in chronic infections but even seem effective against the toxic amyloids associated with neuropathological diseases.

Parallel evolution leading to impaired biofilm formation in invasive Salmonella strains

Pathogenic Salmonella strains that cause gastroenteritis are able to colonize and replicate within the intestines of multiple host species. In general, these strains have retained an ability to form the rdar morphotype, a resistant biofilm physiology hypothesized to be important for Salmonella transmission. In contrast, Salmonella strains that are host-adapted or even host-restricted like Salmonella enterica serovar Typhi, tend to cause systemic infections and have lost the ability to form the rdar morphotype. Here, we investigated the rdar morphotype and CsgD-regulated biofilm formation in two non-typhoidal Salmonella (NTS) strains that caused invasive disease in Malawian children, S. Typhimurium D23580 and S. Enteritidis D7795, and compared them to a panel of NTS strains associated with gastroenteritis, as well as S. Typhi strains. Sequence comparisons combined with luciferase reporter technology identified key SNPs in the promoter region of csgD that either shut off biofilm formation completely (D7795) or reduced transcription of this key biofilm regulator (D23580). Phylogenetic analysis showed that these SNPs are conserved throughout the African clades of invasive isolates, dating as far back as 80 years ago. S. Typhi isolates were negative for the rdar morphotype due to truncation of eight amino acids from the C-terminus of CsgD. We present new evidence in support of parallel evolution between lineages of nontyphoidal Salmonella associated with invasive disease in Africa and the archetypal host-restricted invasive serovar; S. Typhi. We hypothesize that the African invasive isolates are becoming human-adapted and ‘niche specialized’ with less reliance on environmental survival, as compared to gastroenteritis-causing isolates.

African clades of nontyphoidal Salmonella cause invasive disease on a daily basis and thousands of deaths each year. Although it is generally accepted that the transmission route for these organisms is fecal-oral, we know very little about their behaviour in the environment between hosts. In this paper, we have identified both a genotype and a phenotype that suggest environmental niche specialization that is distinct from lineages of Salmonella Typhimurium and Salmonella Enteritidis associated with industrialized food supply chains in resource-rich settings. We also compared with strains of Salmonella Typhi, which cause systemic typhoid fever infections exclusively in humans. In each invasive lineage, regulatory or structural gene mutations leading to loss or impairment of biofilm were identified, all associated with curli and cellulose production, the two main structures that comprise the biofilm matrix. This suggests that similar evolutionary pressures are acting on invasive Salmonella isolates. Public health strategies aimed at reducing the burden of invasive Salmonella disease must prevent transmission to vulnerable adults and children via water sanitation and hygiene practices–a process that starts with identification of environmental reservoirs. The results of our study will raise the profile of this neglected aspect of invasive salmonellosis and will challenge researchers and clinicians to search in new places for potential environmental reservoirs of these pathogens.

Microbial functional amyloids serve diverse purposes for structure, adhesion and defence

The functional amyloid state of proteins has in recent years garnered much attention for its role in serving crucial and diverse biological roles. Amyloid is a protein fold characterised by fibrillar morphology, binding of the amyloid-specific dyes Thioflavin T and Congo Red, insolubility and underlying cross-β structure. Amyloids were initially characterised as an aberrant protein fold associated with mammalian disease. However, in the last two decades, functional amyloids have been described in almost all biological systems, from viruses, to bacteria and archaea, to humans. Understanding the structure and role of these amyloids elucidates novel and potentially ancient mechanisms of protein function throughout nature. Many of these microbial functional amyloids are utilised by pathogens for invasion and maintenance of infection. As such, they offer novel avenues for therapies. This review examines the structure and mechanism of known microbial functional amyloids, with a particular focus on the pathogenicity conferred by the production of these structures and the strategies utilised by microbes to interfere with host amyloid structures. The biological importance of microbial amyloid assemblies is highlighted by their ubiquity and diverse functionality.

TLR4 Participates in the Inflammatory Response Induced by the AAF/II Fimbriae From Enteroaggregative Escherichia coli on Intestinal Epithelial Cells

Enteroaggregative Escherichia coli (EAEC) infections are one of the most frequent causes of persistent diarrhea in children, immunocompromised patients and travelers worldwide. The most prominent colonization factors of EAEC are aggregative adherence fimbriae (AAF). EAEC prototypical strain 042 harbors the AAF/II fimbriae variant, which mediates adhesion to intestinal epithelial cells and participates in the induction of an inflammatory response against this pathogen. However, the mechanism and the cell receptors implicated in eliciting this response have not been fully characterized. Since previous reports have shown that TLR4 recognize fimbriae from different pathogens, we evaluated the role of this receptor in the response elicited against EAEC by intestinal cells. Using a mutual antagonist against TLR2 and TLR4 (OxPAPC), we observed that blocking of these receptors significantly reduces the secretion of the inflammatory marker IL-8 in response to EAEC and AAF/II fimbrial extract in HT-29 cells. Using a TLR4-specific antagonist (TAK-242), we observed that the secretion of this cytokine was significantly reduced in HT-29 cells infected with EAEC or incubated with AAF/II fimbrial extract. We evaluated the participation of AAF/II fimbriae in the TLR4-mediated secretion of 38 cytokines, chemokines, and growth factors involved in inflammation. A reduction in the secretion of IL-8, GRO, and IL-4 was observed. Our results suggest that TLR4 participates in the secretion of several inflammation biomarkers in response to AAF/II fimbriae.

Salmonella Fimbrial Protein FimH Is Involved in Expression of Proinflammatory Cytokines in a Toll-Like Receptor 4-Dependent Manner

Type 1 fimbriae are proteinaceous filamentous structures present on bacterial surfaces and are mainly composed of the major fimbrial protein subunit FimA and the adhesive protein FimH, which is located at the tip of the fimbrial shaft. Here, we investigated the involvement of type 1 fimbriae in the expression of proinflammatory cytokines in macrophages infected with Salmonella enterica serovar Typhimurium. The level of interleukin-1β (IL-1β) mRNA was lower in macrophages infected with fimA or fimH mutant strains than in those infected with wild-type Salmonella Treatment of macrophages with purified recombinant FimH protein, but not FimA, resulted in the activation of the mitogen-activated protein kinase and nuclear factor κB signaling pathways, leading to the expression of not only IL-1β but also other proinflammatory cytokines, such as IL-6 and tumor necrosis factor alpha. However, FimH carrying an N-terminal region deletion or heat-treated FimH did not show such effects. The expression of FimH-induced IL-1β was inhibited by treatment with the Toll-like receptor 4 (TLR4) inhibitor TAK-242 but not by treatment with polymyxin B, a lipopolysaccharide antagonist. Furthermore, FimH treatment stimulated HEK293 cells expressing TLR4 and MD-2/CD14 but did not stimulate HEK293 cells expressing only TLR4. Collectively, FimH is a pathogen-associated molecular pattern of S. enterica serovar Typhimurium that is recognized by TLR4 in the presence of MD-2 and CD14 and plays a significant role in the expression of proinflammatory cytokines in Salmonella-infected macrophages.

Hydrogen peroxide stimulates uropathogenic Escherichia coli strains to cellulose production

Reactive oxygen intermediates, such as hydrogen peroxide, are toxic molecules produced by immune cells in response to bacterial invasion into the host. Bacteria try to protect themselves against the immune system through specific properties such as biofilm formation. This phenomenon occurs also during urinary tract infections. Cellulose is an important factor of Escherichia coli biofilm and contributes to building a protective shield around bacterial cells upon the host immune response. In this study, we aimed to analyze the effect of hydrogen peroxide on the production of this biofilm component. To achieve this goal, 25 clinical E. coli strains isolated from patients with urinary tract infections were used. These bacterial strains were characterized based on their growth characteristics, their ability to form biofilm and their capacity to produce cellulose upon exposure to sub-lethal concentrations of hydrogen peroxide growth, and the biofilm formation of these strains was analyzed. Our results revealed that the analyzed uropathogenic E. coli strains slightly, but significantly, reduced growth and biofilm production upon hydrogen peroxide treatment. However, when separating these strains regarding their ability to produce cellulose, we found that general biofilm production was reduced but cellulose expression was induced upon peroxide treatment. This finding contributes to a better understanding of how bacterial biofilm formation is triggered and provides interesting insights into how uropathogenic E. coli protect themselves in an inhospitable environment.

Curli-Containing Enteric Biofilms Inside and Out: Matrix Composition, Immune Recognition, and Disease Implications

Biofilms of enteric bacteria are highly complex, with multiple components that interact to fortify the biofilm matrix. Within biofilms of enteric bacteria such as Escherichia coli and Salmonella species, the main component of the biofilm is amyloid curli. Other constituents include cellulose, extracellular DNA, O antigen, and various surface proteins, including BapA. Only recently, the roles of these components in the formation of the enteric biofilm individually and in consortium have been evaluated. In addition to enhancing the stability and strength of the matrix, the components of the enteric biofilm influence bacterial virulence and transmission. Most notably, certain components of the matrix are recognized as pathogen-associated molecular patterns. Systemic recognition of enteric biofilms leads to the activation of several proinflammatory innate immune receptors, including the Toll-like receptor 2 (TLR2)/TLR1/CD14 heterocomplex, TLR9, and NLRP3. In the model of Salmonella enterica serovar Typhimurium, the immune response to curli is site specific. Although a proinflammatory response is generated upon systemic presentation of curli, oral administration of curli ameliorates the damaged intestinal epithelial barrier and reduces the severity of colitis. Furthermore, curli (and extracellular DNA) of enteric biofilms potentiate the autoimmune disease systemic lupus erythematosus (SLE) and promote the fibrillization of the pathogenic amyloid α-synuclein, which is implicated in Parkinson's disease. Homologues of curli-encoding genes are found in four additional bacterial phyla, suggesting that the biomedical implications involved with enteric biofilms are applicable to numerous bacterial species.

Phosphoethanolamine cellulose enhances curli-mediated adhesion of uropathogenic Escherichia coli to bladder epithelial cells

Uropathogenic Escherichia coli (UPEC) are the major causative agents of urinary tract infections, employing numerous molecular strategies to contribute to adhesion, colonization, and persistence in the bladder niche. Identifying strategies to prevent adhesion and colonization is a promising approach to inhibit bacterial pathogenesis and to help preserve the efficacy of available antibiotics. This approach requires an improved understanding of the molecular determinants of adhesion to the bladder urothelium. We designed experiments using a custom-built live cell monolayer rheometer (LCMR) to quantitatively measure individual and combined contributions of bacterial cell surface structures [type 1 pili, curli, and phosphoethanolamine (pEtN) cellulose] to bladder cell adhesion. Using the UPEC strain UTI89, isogenic mutants, and controlled conditions for the differential production of cell surface structures, we discovered that curli can promote stronger adhesive interactions with bladder cells than type 1 pili. Moreover, the coproduction of curli and pEtN cellulose enhanced adhesion. The LCMR enables the evaluation of adhesion under high-shear conditions to reveal this role for pEtN cellulose which escaped detection using conventional tissue culture adhesion assays. Together with complementary biochemical experiments, the results support a model wherein cellulose serves a mortar-like function to promote curli association with and around the bacterial cell surface, resulting in increased bacterial adhesion strength at the bladder cell surface.

Whole Cell Cross-Linking to Discover Host-Microbe Protein Cognate Receptor/Ligand Pairs

Bacterial surface ligands mediate interactions with the host cell during association that determines the specific outcome for the host–microbe association. The association begins with receptors on the host cell binding ligands on the microbial cell to form a partnership that initiates responses in both cells. Methods to determine the specific cognate partnerships are lacking. Determining these molecular interactions between the host and microbial surfaces are difficult, yet crucial in defining biologically important events that are triggered during association of the microbiome, and critical in defining the initiating signal from the host membrane that results in pathology or commensal association. In this study, we designed an approach to discover cognate host–microbe receptor/ligand pairs using a covalent cross-linking strategy with whole cells. Protein/protein cross-linking occurred when the interacting molecules were within 9–12 Å, allowing for identification of specific pairs of proteins from the host and microbe that define the molecular interaction during association. To validate the method three different bacteria with three previously known protein/protein partnerships were examined. The exact interactions were confirmed and led to discovery of additional partnerships that were not recognized as cognate partners, but were previously reported to be involved in bacterial interactions. Additionally, three unknown receptor/ligand partners were discovered and validated with in vitro infection assays by blocking the putative host receptor and deleting the bacterial ligand. Subsequently, Salmonella enterica sv. Typhimurium was cross-linked to differentiated colonic epithelial cells (caco-2) to discover four previously unknown host receptors bound to three previously undefined host ligands for Salmonella. This approach resulted in a priori discovery of previously unknown and biologically important molecules for host/microbe association that were casually reported to mediate bacterial invasion. The whole cell cross-linking approach promises to enable discovery of possible targets to modulate interaction of the microbiome with the host that are important in infection and commensalism, both of with initiate a host response.

Structure-Function Analysis of the Curli Accessory Protein CsgE Defines Surfaces Essential for Coordinating Amyloid Fiber Formation

Curli amyloid fibers are produced as part of the extracellular biofilm matrix and are composed primarily of the major structural subunit CsgA. The CsgE chaperone facilitates the secretion of CsgA through CsgG by forming a cap at the base of the nonameric CsgG outer membrane pore. We elucidated a series of finely tuned nonpolar and charge-charge interactions that facilitate the oligomerization of CsgE and its ability to transport unfolded CsgA to CsgG for translocation. CsgE oligomerization in vitro is temperature dependent and is disrupted by mutations in the W48 and F79 residues. Using nuclear magnetic resonance (NMR), we identified two regions of CsgE involved in the CsgE-CsgA interaction: a head comprising a positively charged patch centered around R47 and a stem comprising a negatively charged patch containing E31 and E85. Negatively charged residues in the intrinsically disordered N- and C-terminal "tails" were not implicated in this interaction. Head and stem residues were mutated and interrogated using in vivo measurements of curli production and in vitro amyloid polymerization assays. The R47 head residue of CsgE is required for stabilization of CsgA- and CsgE-mediated curli fiber formation. Mutation of the E31 and E85 stem residues to positively charged side chains decreased CsgE-mediated curli fiber formation but increased CsgE-mediated stabilization of CsgA. No single-amino-acid substitutions in the head, stem, or tail regions affected the ability of CsgE to cap the CsgG pore as determined by a bile salt sensitivity assay. These mechanistic insights into the directed assembly of functional amyloids in extracellular biofilms elucidate possible targets for biofilm-associated bacterial infections.IMPORTANCE Curli represent a class of functional amyloid fibers produced by Escherichia coli and other Gram-negative bacteria that serve as protein scaffolds in the extracellular biofilm matrix. Despite the lack of sequence conservation among different amyloidogenic proteins, the structural and biophysical properties of functional amyloids such as curli closely resemble those of amyloids associated with several common neurodegenerative diseases. These parallels are underscored by the observation that certain proteins and chemicals can prevent amyloid formation by the major curli subunit CsgA and by alpha-synuclein, the amyloid-forming protein found in Lewy bodies during Parkinson's disease. CsgA subunits are targeted to the CsgG outer membrane pore by CsgE prior to secretion and assembly into fibers. Here, we use biophysical, biochemical, and genetic approaches to elucidate a mechanistic understanding of CsgE function in curli biogenesis.

The Role of Functional Amyloids in Bacterial Virulence

Amyloid fibrils are best known as a product of human and animal protein misfolding disorders, where amyloid formation is associated with cytotoxicity and disease. It is now evident that for some proteins, the amyloid state constitutes the native structure and serves a functional role. These functional amyloids are proving widespread in bacteria and fungi, fulfilling diverse functions as structural components in biofilms or spore coats, as toxins and surface-active fibers, as epigenetic material, peptide reservoirs or adhesins mediating binding to and internalization into host cells. In this review, we will focus on the role of functional amyloids in bacterial pathogenesis. The role of functional amyloids as virulence factor is diverse but mostly indirect. Nevertheless, functional amyloid pathways deserve consideration for the acute and long-term effects of the infectious disease process and may form valid antimicrobial targets.

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Functional amyloids are widespread in bacteria, pathogenic and non-pathogenic.

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Bacterial biofilms most commonly function as structural support in the extracellular matrix of biofilms or spore coats, and in cell–cell and cell-surface adherence.

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The amyloid state can be the sole structured and functional state, or can be facultative, as a secondary state to folded monomeric subunits.

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Bacterial amyloids can enhance virulence by increasing persistence, cell adherence and invasion, intracellular survival, and pathogen spread by increased environmental survival.

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Bacterial amyloids may indirectly inflict disease by triggering inflammation, contact phase activation and possibly induce or aggravate human pathological aggregation disorders.

Host-bacteria interaction and adhesin study for development of therapeutics

Host-pathogen interaction is one of the most important areas of study to understand the adhesion of the pathogen to the host organisms. To adhere on the host cell surface, bacteria assemble the diverse adhesive structures on its surface, which play a foremost role in targeting to the host cell. We have highlighted different bacterial adhesins which are either protein mediated or glycan mediated. The present article listed examples of different bacterial adhesin proteins involved in the interactions with their host, types and subtypes of the fimbriae and non-fimbriae bacterial adhesins. Different bacterial surface adhesin subunits interact with host via different host surface biomolecules. We have also discussed the interactome of some of the pathogens with their host. Therefore, the present study will help researchers to have a detailed understanding of different interacting bacterial adhesins and henceforth, develop new therapies, adhesin specific antibodies and vaccines, which can effectively control pathogenicity of the pathogens.

The curli regulator CsgD mediates stationary phase counter-silencing of csgBA in Salmonella Typhimurium

Integration of horizontally acquired genes into transcriptional networks is essential for the regulated expression of virulence in bacterial pathogens. In Salmonella enterica, expression of such genes is repressed by the nucleoid-associated protein H-NS, which recognizes and binds to AT-rich DNA. H-NS-mediated silencing must be countered by other DNA-binding proteins to allow expression under appropriate conditions. Some genes that can be transcribed by RNA polymerase (RNAP) associated with the alternative sigma factor σ^S or the housekeeping sigma factor σ⁷⁰ in vitro appear to be preferentially transcribed by σ^S in the presence of H-NS, suggesting that σ^S may act as a counter-silencer. To determine whether σ^S directly counters H-NS-mediated silencing and whether co-regulation by H-NS accounts for the σ^S selectivity of certain promoters, we examined the csgBA operon, which is required for curli fimbriae expression and is known to be regulated by both H-NS and σ^S . Using genetics and in vitro biochemical analyses, we found that σ^S is not directly required for csgBA transcription, but rather up-regulates csgBA via an indirect upstream mechanism. Instead, the biofilm master regulator CsgD directly counter-silences the csgBA promoter by altering the DNA-protein complex structure to disrupt H-NS-mediated silencing in addition to directing the binding of RNAP.