Salmonella Typhimurium type III secretion effectors stimulate innate immune responses in cultured epithelial cells

Invasive Salmonella Typhimurium colonizes gallbladder and contributes to gallbladder carcinogenesis through activation of host epigenetic modulator KDM6B

Gallbladder stones alone do not explain the risk of gallbladder cancer (GBC) as the sole etiological factor. Chronic microbial infection, particularly Salmonella, has been implicated in GB carcinogenesis, but its causative role and the underlying mechanisms are largely unknown. We studied gut and gallbladder tissue microbiome through targeted metagenomics to identify pathogenic bacteria in GBC. Virulence and pathogenicity of identified Salmonella Typhimurium from GBC tissue were studied after culture by whole genome sequencing, phylogenetic analysis, mutational profiling, and pangenome analysis. Mechanistic studies for GBC carcinogenesis were carried out in a mouse model of gallstones and chronic Salmonella infection, a cellular model using GBC (NOZ) cell lines, and a xenograft tumor model. We found an increased abundance of Salmonella in the gut microbiome of patients with GBC and culturable S. Typhimurium from the gallbladder cancer tissue. Comparative genomics of S. Typhimurium isolated from the GBC tissue showed a high invasive index. S. Typhimurium isolates harbored horizontally acquired virulence functions in their accessory genome. Chronic S. Typhimurium infection caused chronic inflammation, pre-malignant changes, and tumor-promoting mechanisms in the mouse model with gallbladder stones with activation of the epigenetic modulator KDM6B both in the mouse model and human GBC. Inhibition of KDM6B reduced engrafted tumor size in SCID mice. Of the differentially regulated genes in human GBC tissue, ADAMTSL5, CX3CR1, and SPSB4 were also significantly dysregulated in NOZ cells infected with Salmonella. Chronic Salmonella infection contributes to gallbladder carcinogenesis through a host epigenetic mechanism involving KDM6B.

The interplay between Salmonella and host: Mechanisms and strategies for bacterial survival

Salmonella, an intracellular pathogen, infects both humans and animals, causing diverse diseases such as gastroenteritis and enteric fever. The Salmonella type III secretion system (T3SS), encoded within its pathogenicity islands (SPIs), is critical for bacterial virulence by directly delivering multiple effectors into eukaryotic host cells. Salmonella utilizes these effectors to facilitate its survival and replication within the host through modulating cytoskeletal dynamics, inflammatory responses, the biogenesis of Salmonella-containing vacuole (SCV), and host cell survival. Moreover, these effectors also interfere with immune responses via inhibiting innate immunity or antigen presentation. In this review, we summarize the current progress in the survival strategies employed by Salmonella and the molecular mechanisms underlying its interactions with the host. Understanding the interplay between Salmonella and host can enhance our knowledge of the bacterium's pathogenic processes and provide new insights into how it manipulates host cellular physiological activities to ensure its survival.

Dietary supplement of Acanthopanax senticosus decoction formula improves immune response via intestine flora of rabbits

Young rabbits are sensitive to surrounding changes and conditioned pathogens in intestine which might result in slow inflammation and diarrhea after microbial invasion. Traditional medicine herbs could provide efficacious treatment on slow infection and inflammation. The present research designed an Acanthopanax senticosus (ACS) formula consisted of five types of Chinese herbs including Acanthopanax senticosus (Rupr & Maxim) Harms (Ciwujia in Chinese), Astragalus membranceus (Fisch) Bge (Huangqi in Chinese), Indigo naturalis (Qingdai in Chinese), Houttuynia cordata Thunb (Yuxingcao in Chinese), and Glycyrrhizae radix et Rhizoma (Gancao in Chinese). The effects of ACS decoction supplement were investigated via determination of cytokines and growth performances of young rabbits, and the flora in intestinal digesta from six fragments were further explored using 16S rRNA gene sequencing technology. Compared to the control group, rabbits supplied with different doses of ACS decoction possessed lower diarrhea and death rates, together with the IL-10 concentration, while the declined IL-1β and IL-12 levels and inflammatory factor gene expressions in intestinal tissues. Additionally, ACS addition changed the diversity of flora in each segments of intestine. Functional prediction on abundances of genera enriched to seven KEGG immunity pathways. Moreover, strong correlations were determined between the abundance of bacteria with interleukins contents, and the predictive immune signaling abundances, respectively. Especially, ACS exhibited anti-inflammation effects via decreasing the abundances of Bacteroides, Clostridia_vadinBB60_group, NK4A214_group, and dgA_11_gut_group in intestine of young rabbits. In conclusion, dietary supplement with ACS exerted diarrhea-reducing effects, and improved immunity homeostasis by modulating intestinal flora diversity in young rabbits.

Type III Secretion System in Intestinal Pathogens and Metabolic Diseases

Modern lifestyle changes, especially the consumption of a diet high in salt, sugar, and fat, have contributed to the increasing incidence and prevalence of chronic metabolic diseases such as diabetes, obesity, and gout. Changing lifestyles continuously shape the gut microbiota which is closely related to the occurrence and development of metabolic diseases due to its specificity of composition and structural diversity. A large number of pathogenic bacteria such as Yersinia, Salmonella, Shigella, and pathogenic E. coli in the gut utilize the type III secretion system (T3SS) to help them resist host defenses and cause disease. Although the T3SS is critical for the virulence of many important human pathogens, its relationship with metabolic diseases remains unknown. This article reviews the structure and function of the T3SS, the disruption of intestinal barrier integrity by the T3SS, the changes in intestinal flora containing the T3SS in metabolic diseases, the possible mechanisms of the T3SS affecting metabolic diseases, and the application of the T3SS in the treatment of metabolic diseases. The aim is to provide insights into metabolic diseases targeting the T3SS, thereby serving as a valuable reference for future research on disease diagnosis, prevention, and treatment.

Critical role of msgA in invasive capacity and intracellular survivability of Salmonella

Salmonella enterica serovar Typhimurium, which is a common foodborne pathogen, causes both intestinal and systemic infections in hosts. Salmonella has a complex pathogenic mechanism that involves invasive capacity and intracellular survivability, which hampers research on virulence of Salmonella. The virulence of Salmonella is primarily studied through Salmonella pathogenicity islands (SPIs). However, there are also genes outside these SPIs that significantly impact virulence. Macrophage survival gene msgA is positioned at a region independent of the SPIs and conserved in Salmonella. However, there has been limited research on msgA to date. This study aims to investigate the virulent function of msgA to deepen our understanding of Salmonella virulence. Proteomic and RT-qPCR analyses reveal that MsgA influences multiple metabolic pathways and the expression of SPIs. The depletion of msgA led to the significantly reduced invasive capacity and intracellular survivability, and thus the decreased virulence of Salmonella. In conclusion, our study suggests that MsgA is an important regulator that mainly regulates virulence. Further research into the function of MsgA will enhance the understanding of Salmonella pathogenesis and promote the application of Salmonella for medical treatment.

IMPORTANCE

Salmonella enterica serovar Typhimurium is a common foodborne pathogen, it has a complex pathogenic mechanism that involves invasive capacity and intracellular survivability. The virulence of Salmonella is primarily studied through its pathogenicity islands. In contrast, virulence genes located outside the Salmonella pathogenicity islands (SPIs) have received less attention. Macrophage survival gene (MsgA) is positioned at a region independent of the SPIs and conserved in Salmonella. Our research indicates that MsgA is a novel global regulator influencing the metabolic pathways and SPIs. Further research into the function of MsgA will enhance the understanding of Salmonella pathogenesis and promote the application of Salmonella for medical treatment.

Single cell genomics based insights into the impact of cell-type specific microbial internalization on disease severity

Host-microbe interactions are complex and ever-changing, especially during infections, which can significantly impact human physiology in both health and disease by influencing metabolic and immune functions. Infections caused by pathogens such as bacteria, viruses, fungi, and parasites are the leading cause of global mortality. Microbes have evolved various immune evasion strategies to survive within their hosts, which presents a multifaceted challenge for detection. Intracellular microbes, in particular, target specific cell types for survival and replication and are influenced by factors such as functional roles, nutrient availability, immune evasion, and replication opportunities. Identifying intracellular microbes can be difficult because of the limitations of traditional culture-based methods. However, advancements in integrated host microbiome single-cell genomics and transcriptomics provide a promising basis for personalized treatment strategies. Understanding host-microbiota interactions at the cellular level may elucidate disease mechanisms and microbial pathogenesis, leading to targeted therapies. This article focuses on how intracellular microbes reside in specific cell types, modulating functions through persistence strategies to evade host immunity and prolong colonization. An improved understanding of the persistent intracellular microbe-induced differential disease outcomes can enhance diagnostics, therapeutics, and preventive measures.

Salmonella T3SS-elicited inflammatory innate immune response inhibits type I IFN response in macrophages

As a gram-negative intracellular bacterial pathogen, Salmonella enterica serovar Typhimurium (S. Typhimurium) invades different cell types including macrophages. Its infection in macrophages induces robust innate immune responses that are featured by proinflammatory and type I interferon (IFN) responses. The type III secretion systems (T3SSs) of S. Typhimurium play a crucial role in activating host inflammasome pathways. It has been recognized that the inflammasome pathways inhibit the type I IFN cascade. However, the potential role of T3SS in regulating the type I IFN response and the underlying mechanisms are largely unknown. In this study, we showed that S. Typhimurium infection activated strong proinflammatory, type I IFN and IFN-stimulated genes (ISGs) expression in macrophages. Furthermore, we showed that T3SS-defective S. Typhimurium mutant ΔinvC elicited attenuated inflammatory response but enhanced type I IFN and ISGs expression. Additionally, the inhibition of caspase-1 by a specific inhibitor VX-765 resulted in increased type I IFN response. Moreover, cell-permeable pan-caspase inhibitor Z-VAD-FMK also enhanced the type I IFN response upon S. Typhimurium infection. Intriguingly, compared with exponential phase S. Typhimurium infection, stationary phase bacteria triggered higher levels of type I IFN responses. Finally, the inhibition of caspase-1 by VX-765 substantially increased the intracellular S. Typhimurium burden. In conclusion, we demonstrated that the proinflammatory response induced by S. Typhimurium T3SS can inhibit the type I IFN response, which provides insight into the role of T3SS in orchestrating innate immunity during S. Typhimurium infection.

A two-step activation mechanism enables mast cells to differentiate their response between extracellular and invasive enterobacterial infection

Mast cells localize to mucosal tissues and contribute to innate immune defense against infection. How mast cells sense, differentiate between, and respond to bacterial pathogens remains a topic of ongoing debate. Using the prototype enteropathogen Salmonella Typhimurium (S.Tm) and other related enterobacteria, here we show that mast cells can regulate their cytokine secretion response to distinguish between extracellular and invasive bacterial infection. Tissue-invasive S.Tm and mast cells colocalize in the mouse gut during acute Salmonella infection. Toll-like Receptor 4 (TLR4) sensing of extracellular S.Tm, or pure lipopolysaccharide, causes a modest induction of cytokine transcripts and proteins, including IL-6, IL-13, and TNF. By contrast, type-III-secretion-system-1 (TTSS-1)-dependent S.Tm invasion of both mouse and human mast cells triggers rapid and potent inflammatory gene expression and >100-fold elevated cytokine secretion. The S.Tm TTSS-1 effectors SopB, SopE, and SopE2 here elicit a second activation signal, including Akt phosphorylation downstream of effector translocation, which combines with TLR activation to drive the full-blown mast cell response. Supernatants from S.Tm-infected mast cells boost macrophage survival and maturation from bone-marrow progenitors. Taken together, this study shows that mast cells can differentiate between extracellular and host-cell invasive enterobacteria via a two-step activation mechanism and tune their inflammatory output accordingly.

Salmonella actively modulates TFEB in murine macrophages in a growth-phase and time-dependent manner

IMPORTANCE

Activation of the host transcription factor TFEB helps mammalian cells adapt to stresses such as starvation and infection by upregulating lysosome, autophagy, and immuno-protective gene expression. Thus, TFEB is generally thought to protect host cells. However, it may also be that pathogenic bacteria like Salmonella orchestrate TFEB in a spatio-temporal manner to harness its functions to grow intracellularly. Indeed, the relationship between Salmonella and TFEB is controversial since some studies showed that Salmonella actively promotes TFEB, while others have observed that Salmonella degrades TFEB and that compounds that promote TFEB restrict bacterial growth. Our work provides a path to resolve these apparent discordant observations since we showed that stationary-grown Salmonella actively delays TFEB after infection, while late-log Salmonella is permissive of TFEB activation. Nevertheless, the exact function of this manipulation remains unclear, but conditions that erase the conditional control of TFEB by Salmonella may be detrimental to the microbe.

Quantitative Proteomics of the Intracellular Bacterial Pathogen Salmonella enterica Serovar Typhimurium

Mass spectrometry-based proteomics provides a wealth of information about changes in protein production and abundance under diverse conditions, as well as mechanisms of regulation, signaling cascades, interaction partners, and communication patterns across biological systems. For profiling of intracellular pathogens, proteomic profiling can be performed in the absence of a host to singularly define the pathogenic proteome or during an infection-like setting to identify dual perspectives of infection. In this chapter, we present techniques to extract proteins from the human bacterial intracellular pathogen, Salmonella enterica serovar Typhimurium, in the presence of macrophages, an important innate immune cell in host defense. We outline sample preparation, including protein extraction, digestion, and purification, as well as mass spectrometry measurements and bioinformatics analysis. The data generated from our dual perspective profiling approach provides new insight into pathogen and host protein modulation under infection-like conditions.

Epithelial cell invasion by salmonella typhimurium induces modulation of genes controlled by aryl hydrocarbon receptor signaling and involved in extracellular matrix biogenesis

Salmonella is the only bacterium able to enter a host cell by the two known mechanisms: trigger and zipper. The trigger mechanism relies on the injection of bacterial effectors into the host cell through the Salmonella type III secretion system 1. In the zipper mechanism, mediated by the invasins Rck and PagN, the bacterium takes advantage of a cellular receptor for invasion. This study describes the transcriptomic reprogramming of the IEC-6 intestinal epithelial cell line to Salmonella Typhimurium strains that invaded cells by a trigger, a zipper, or both mechanisms. Using S. Typhimurium strains invalidated for one or other entry mechanism, we have shown that IEC-6 cells could support both entries. Comparison of the gene expression profiles of exposed cells showed that irrespective of the mechanism used for entry, the transcriptomic reprogramming of the cell was nearly the same. On the other hand, when gene expression was compared between cells unexposed or exposed to the bacterium, the transcriptomic reprogramming of exposed cells was significantly different. It is particularly interesting to note the modulation of expression of numerous target genes of the aryl hydrocarbon receptor showing that this transcription factor was activated by S. Typhimurium infection. Numerous genes associated with the extracellular matrix were also modified. This was confirmed at the protein level by western-blotting showing a dramatic modification in some extracellular matrix proteins. Analysis of a selected set of modulated genes showed that the expression of the majority of these genes was modulated during the intracellular life of S. Typhimurium.

Salmonella Bloodstream Infections

Salmonella is a major foodborne pathogen of both animals and humans. This bacterium is responsible for considerable morbidity and mortality world-wide. Different serovars of this genus cause diseases ranging from self-limiting gastroenteritis to a potentially fatal systemic disease known as enteric fever. Gastrointestinal infections with Salmonella are usually self-limiting and rarely require medical intervention. Bloodstream infections, on the other hand, are often fatal even with hospitalization. This review describes the routes and underlying mechanisms of the extraintestinal dissemination of Salmonella and the chronic infections that sometimes result. It includes information on the pathogenicity islands and individual virulence factors involved in systemic dissemination as well as a discussion of the host factors that mediate susceptibility. Also, the major outbreaks of invasive Salmonella disease in the tropics are described.

Differential Salmonella Typhimurium intracellular replication and host cell responses in caecal and ileal organoids derived from chicken

Chicken infection with Salmonella Typhimurium is an important source of foodborne human diseases. Salmonella colonizes the avian intestinal tract and more particularly the caecum, without causing symptoms. This thus poses a challenge for the prevention of foodborne transmission. Until now, studies on the interaction of Salmonella with the avian gut intestine have been limited by the absence of in vitro intestinal culture models. Here, we established intestinal crypt-derived chicken organoids to better decipher the impact of Salmonella intracellular replication on avian intestinal epithelium. Using a 3D organoid model, we observed a significantly higher replication rate of the intracellular bacteria in caecal organoids than in ileal organoids. Our model thus recreates intracellular environment, allowing Salmonella replication of avian epithelium according to the intestinal segment. Moreover, an inhibition of the cellular proliferation was observed in infected ileal and caecal organoids compared to uninfected organoids. This appears with a higher effect in ileal organoids, as well as a higher cytokine and signaling molecule response in infected ileal organoids at 3 h post-infection (hpi) than in caecal organoids that could explain the lower replication rate of Salmonella observed later at 24 hpi. To conclude, this study demonstrates that the 3D organoid is a model allowing to decipher the intracellular impact of Salmonella on the intestinal epithelium cell response and illustrates the importance of the gut segment used to purify stem cells and derive organoids to specifically study epithelial cell -Salmonella interaction.

Mitogen-Activated Protein Kinases (MAPKs) and Enteric Bacterial Pathogens: A Complex Interplay

Diverse extracellular and intracellular cues activate mammalian mitogen-activated protein kinases (MAPKs). Canonically, the activation starts at cell surface receptors and continues via intracellular MAPK components, acting in the host cell nucleus as activators of transcriptional programs to regulate various cellular activities, including proinflammatory responses against bacterial pathogens. For instance, binding host pattern recognition receptors (PRRs) on the surface of intestinal epithelial cells to bacterial pathogen external components trigger the MAPK/NF-κB signaling cascade, eliciting cytokine production. This results in an innate immune response that can eliminate the bacterial pathogen. However, enteric bacterial pathogens evolved sophisticated mechanisms that interfere with such a response by delivering virulent proteins, termed effectors, and toxins into the host cells. These proteins act in numerous ways to inactivate or activate critical components of the MAPK signaling cascades and innate immunity. The consequence of such activities could lead to successful bacterial colonization, dissemination, and pathogenicity. This article will review enteric bacterial pathogens' strategies to modulate MAPKs and host responses. It will also discuss findings attempting to develop anti-microbial treatments by targeting MAPKs.

Novel secreted STPKLRR from Vibrio splendidus AJ01 promotes pathogen internalization via mediating tropomodulin phosphorylation dependent cytoskeleton rearrangement

We previously demonstrated that the flagellin of intracellular Vibrio splendidus AJ01 could be specifically identified by tropomodulin (Tmod) and further mediate p53-dependent coelomocyte apoptosis in the sea cucumber Apostichopus japonicus. In higher animals, Tmod serves as a regulator in stabilizing the actin cytoskeleton. However, the mechanism on how AJ01 breaks the AjTmod-stabilized cytoskeleton for internalization remains unclear. Here, we identified a novel AJ01 Type III secretion system (T3SS) effector of leucine-rich repeat-containing serine/threonine-protein kinase (STPKLRR) with five LRR domains and a serine/threonine kinase (STYKc) domain, which could specifically interact with tropomodulin domain of AjTmod. Furthermore, we found that STPKLRR directly phosphorylated AjTmod at serine 52 (S52) to reduce the binding stability between AjTmod and actin. After AjTmod dissociated from actin, the F-actin/G-actin ratio decreased to induce cytoskeletal rearrangement, which in turn promoted the internalization of AJ01. The STPKLRR knocked out strain could not phosphorylated AjTmod and displayed lower internalization capacity and pathogenic effect compared to AJ01. Overall, we demonstrated for the first time that the T3SS effector STPKLRR with kinase activity was a novel virulence factor in Vibrio and mediated self-internalization by targeting host AjTmod phosphorylation dependent cytoskeleton rearrangement, which provided a candidate target to control AJ01 infection in practice.

A Bittersweet Kiss of Gram-Negative Bacteria: The Role of ADP-Heptose in the Pathogenesis of Infection

Due to the global crisis caused by the dramatic rise of drug resistance among Gram-negative bacteria, there is an urgent need for a thorough understanding of the pathogenesis of infections of such an etiology. In light of the limited availability of new antibiotics, therapies aimed at host-pathogen interactions emerge as potential treatment modalities. Thus, understanding the mechanism of pathogen recognition by the host and immune evasion appear to be the key scientific issues. Until recently, lipopolysaccharide (LPS) was recognized as a major pathogen-associated molecular pattern (PAMP) of Gram-negative bacteria. However, recently, ADP-L-glycero-β-D-manno-heptose (ADP-heptose), an intermediate carbohydrate metabolite of the LPS biosynthesis pathway, was discovered to activate the hosts' innate immunity. Therefore, ADP-heptose is regarded as a novel PAMP of Gram-negative bacteria that is recognized by the cytosolic alpha kinase-1 (ALPK1) protein. The conservative nature of this molecule makes it an intriguing player in host-pathogen interactions, especially in the context of changes in LPS structure or even in its loss by certain resistant pathogens. Here, we present the ADP-heptose metabolism, outline the mechanisms of its recognition and the activation of its immunity, and summarize the role of ADP-heptose in the pathogenesis of infection. Finally, we hypothesize about the routes of the entry of this sugar into cytosol and point to emerging questions that require further research.

Research Note: Therapeutic effect of a Salmonella phage combination on chicks infected with Salmonella Typhimurium

Antibiotic treatment failure is increasingly encountered for the emergence of pandrug-resistant isolates, including the prototypical broad-host-range Salmonella enterica serovar (S.) Typhimurium, which mainly transmitted to humans through poultry products. In this study we explored the therapeutic potential of a Salmonella phage composition containing a virulent phage and a nonproductive phage that does not produce progeny phage against chicks infected with a pandrug-resistant S. Typhimurium strain of avian origin. After approximately 10⁷ CFU of S. Typhimurium strain ST149 were administrated to chicks by intraperitoneal injection, the phage combination (∼10⁸ PFU) was gavaged at 8-h, 32-h, and 54-h postinfection. At d 10 postinfection, phage treatment completely protected chicks from Salmonella-induced death compared to 91.7% survival in the Salmonella challenge group. In addition, phage treatment also greatly reduced the bacterial load in various organs, with Salmonella colonization levels decreasing more significantly in spleen and bursa than in liver and cecal contents, possibly due to higher phage titers in these immune organs. However, phages could not alleviate the decreased body weight gain and the enlargement of spleen and bursa of infected chicks. Further examination of the bacterial flora in the cecal contents of chicks found that S. Typhimurium infection caused a remarkable decrease in abundance of Clostridia vadin BB60 group and Mollicutes RF39 (the dominant genus in chicks), making Lactobacillus the dominate genus. Although phage treatment partially restored the decline of Clostridia vadin BB60 group and Mollicutes RF39 and increased abundance of Lactobacillus caused by S. Typhimurium infection, Fournierella that may aggravate intestinal inflammation became the major genus, followed by increased Escherichia-Shigella as the second dominate bacterial genus. These results suggested that successive phage treatment modulated the structural composition and abundance of bacterial communities, but failed to normalize the intestinal microbiome disrupted by S. Typhimurium infection. Phages need to be combined with other means to control the spread of S. Typhimurium in poultry.

Manipulation of host immune defenses by effector proteins delivered from multiple secretion systems of Salmonella and its application in vaccine research

Salmonella is an important zoonotic bacterial species and hazardous for the health of human beings and livestock globally. Depending on the host, Salmonella can cause diseases ranging from gastroenteritis to life-threatening systemic infection. In this review, we discuss the effector proteins used by Salmonella to evade or manipulate four different levels of host immune defenses: commensal flora, intestinal epithelial-mucosal barrier, innate and adaptive immunity. At present, Salmonella has evolved a variety of strategies against host defense mechanisms, among which various effector proteins delivered by the secretory systems play a key role. During its passage through the digestive system, Salmonella has to face the intact intestinal epithelial barrier as well as competition with commensal flora. After invasion of host cells, Salmonella manipulates inflammatory pathways, ubiquitination and autophagy processes with the help of effector proteins. Finally, Salmonella evades the adaptive immune system by interfering the migration of dendritic cells and interacting with T and B lymphocytes. In conclusion, Salmonella can manipulate multiple aspects of host defense to promote its replication in the host.

Determination of the Salmonella intracellular lifestyle by the diversified interaction of Type III secretion system effectors and host GTPases

Intracellular bacteria have developed sophisticated strategies to subvert the host endomembrane system to establish a stable replication niche. Small GTPases are critical players in regulating each step of membrane trafficking events, such as vesicle biogenesis, cargo transport, tethering, and fusion events. Salmonella is a widely studied facultative intracellular bacteria. Salmonella delivers several virulence proteins, termed effectors, to regulate GTPase dynamics and subvert host trafficking for their benefit. In this review, we summarize an updated and systematic understanding of the interactions between bacterial effectors and host GTPases in determining the intracellular lifestyle of Salmonella. This article is categorized under: Infectious Diseases > Molecular and Cellular Physiology.

Immune evasion and persistence in enteric bacterial pathogens

The major function of the mammalian immune system is to prevent and control infections caused by enteropathogens that collectively have altered human destiny. In fact, as the gastrointestinal tissues are the major interface of mammals with the environment, up to 70% of the human immune system is dedicated to patrolling them The defenses are multi-tiered and include the endogenous microflora that mediate colonization resistance as well as physical barriers intended to compartmentalize infections. The gastrointestinal tract and associated lymphoid tissue are also protected by sophisticated interleaved arrays of active innate and adaptive immune defenses. Remarkably, some bacterial enteropathogens have acquired an arsenal of virulence factors with which they neutralize all these formidable barriers to infection, causing disease ranging from mild self-limiting gastroenteritis to in some cases devastating human disease.

Profound gene expression changes in the epithelial monolayer of active ulcerative colitis and Crohn's disease

In recent years it has become apparent that the epithelium is highly involved in inflammatory bowel disease (IBD) pathophysiology. The majority of gene expression studies of IBD are generated from heterogeneous biopsies, providing no distinction between immune cells, the epithelium and other mucosal cells. By using laser capture microdissection (LCM) coupled with RNA sequencing, we aimed to characterize the expressional changes of the isolated colonic epithelial monolayer from ulcerative colitis (UC) and Crohn’s disease (CD) patients compared to healthy controls (HC). The analysis identified 3706 genes as differentially expressed between active IBD epithelium and HC. Weighted gene co-expression network analysis was used to stratify genes into modules, which were subsequently characterized using enrichment analysis. Our data show a distinct upregulation of the antigen presentation machinery during inflammation, including major histocompatibility complex class II molecules (e.g. HLA-DPA1, HLA-DPB1, HLA-DRA) and key transcription factors/activators (STAT1, IRF1, CIITA). We also see an epithelial downregulation of retinoic acid-responsive nuclear receptors (RARA, RARB, RXRA), but upregulation of retinoid-metabolizing enzymes (RDH11, ALDH1A2, ALDH1A3), which together suggest a perturbation of epithelial vitamin A signaling during active IBD. Lastly, we identified a cluster of stress-related genes, including activator protein 1 components JUNB and ATF3, as significantly upregulated in active UC but not in CD, revealing an interesting aspect of IBD heterogeneity. The results represent a unique resource for enhanced understanding of epithelial involvement in IBD inflammation and is a valuable tool for further studies on these processes.

Kinase-independent synthesis of 3-phosphorylated phosphoinositides by a phosphotransferase

Despite their low abundance, phosphoinositides play a central role in membrane traffic and signalling. PtdIns(3,4,5)P3 and PtdIns(3,4)P2 are uniquely important, as they promote cell growth, survival and migration. Pathogenic organisms have developed means to subvert phosphoinositide metabolism to promote successful infection and their survival in host organisms. We demonstrate that PtdIns(3,4)P2 is a major product generated in host cells by the effectors of the enteropathogenic bacteria Salmonella and Shigella. Pharmacological, gene silencing and heterologous expression experiments revealed that, remarkably, the biosynthesis of PtdIns(3,4)P2 occurs independently of phosphoinositide 3-kinases. Instead, we found that the Salmonella effector SopB, heretofore believed to be a phosphatase, generates PtdIns(3,4)P2 de novo via a phosphotransferase/phosphoisomerase mechanism. Recombinant SopB is capable of generating PtdIns(3,4,5)P3 and PtdIns(3,4)P2 from PtdIns(4,5)P2 in a cell-free system. Through a remarkable instance of convergent evolution, bacterial effectors acquired the ability to synthesize 3-phosphorylated phosphoinositides by an ATP- and kinase-independent mechanism, thereby subverting host signalling to gain entry and even provoke oncogenic transformation.

Probiotic Bacillus subtilis LF11 Protects Intestinal Epithelium Against Salmonella Infection

Enteric diseases caused by Salmonella are prevalent in poultry farming. With the forbiddance of antibiotics in feedstuff industry, Bacillus subtilis (B. subtilis) preparation as antibiotic alternatives against Salmonella infection has gained increasing attention recently. However, the protection modes of B. subtilis against Salmonella infection in broilers are strain-specific. In this study, probiotic B. subtilis LF11 significantly reduced diarrhea and mortality of broilers caused by Salmonella braenderup (S. braenderup) in spite of no inhibition effect on it in vitro. Here, the intestinal epithelial cells NCM460 were incubated to explore the protection of B. subtilis LF11 on intestinal epithelium against Salmonella. The results revealed that B. subtilis LF11 showed obvious exclusion activity with the decrease of adhesion and invasion of S. braenderup to NCM460 cells, accordingly with the increase of NCM460 cell survival compared with S. braenderup challenge alone. Meanwhile, RT-PCR and Western blot proved that the gene transcription and expression levels of four tight junction proteins in NCM 460 cells were upregulated, which was further confirmed by immunofluorescence observation. Besides, B. subtilis LF11 downregulated the gene transcription levels of the proinflammatory cytokines IL-6, IL-8, and TNF-α induced by S. braenderup H9812. ELISA analysis also verified that B. subtilis LF11 reduced the IL-8 production significantly. In general, B. subtilis LF11 has the ability to protect the intestinal epithelium against Salmonella infection by reducing the Salmonella adhesion and invasion, enhancing the intestinal barrier and attenuating the enterocyte inflammatory responses, and has the potential as probiotics to prevent enteric diseases in broilers.

Pathogenicity of Salmonella enterica subspecies enterica serovar Enteritidis phage type 1 in one-day-old specific pathogen-free chicks

Background

The studies about Salmonella infection in newly hatched chicks were not extensive.

Aim

The objective of this study was to determine the pathogenicity of Salmonella enterica subspecies enterica serovar Enteritidis (SE) phage type (PT) 1 in one-day-old specific pathogen-free (SPF) chicks.

Methods

Seventy, one-day-old SPF chicks, were divided into SE group (30 chicks), mortality group (10 chicks), both orally inoculated (1.0 ml) with SE PT1 (1 × 10⁸ colony-forming unit per 1.0 ml), and one control group (30 chicks). The chicks were sacrificed at 6 and 12 hours, and days 1, 2, 3, 5, 7, 10, 14, and 21 post-inoculation (pi). Samples were collected for bacterial isolation, histological examination, and ultrastructural examination.

Results

Starting from day 2 pi, the body weight in the SE group significantly (p < 0.05) decreased. The SE isolation percentages from the liver, spleen, mid-intestinal content, cecal content, cecal tonsil, blood, and cloacal swab were 0.73, 0.77, 0.33, 0.33, 0.36, 0.40, and 0.30, respectively. The isolation percentage in the liver was significantly (p < 0.05) higher than the blood and cloacal swab. The villi heights and crypt depths in the SE group were significantly (p < 0.05) greater and smaller, respectively. Ultrastructurally, erosion and necrosis were observed in the microvilli of the cecal tonsil. The bacteria were engulfed by macrophages at the interepithelial clefts of the M-like M cells.

Conclusion

It was concluded that the inoculation of SE PT 1 in one-day-old chicks caused a systemic infection with diarrhea, a decrease in the body weight and villi height in the duodenum, jejunum, and ileum, and high bacterial loading in the liver with mild gross and histological lesions of organs, erosion, and necrosis of microvilli and low mortality. The bacteria entered the body system from the intestinal tract through the interepithelial clefts of the M-like M cells of the cecal tonsil.

Salmonella Typhimurium and inflammation: a pathogen-centric affair

Microbial infections are controlled by host inflammatory responses that are initiated by innate immune receptors after recognition of conserved microbial products. As inflammation can also lead to disease, tissues that are exposed to microbial products such as the intestinal epithelium are subject to stringent regulatory mechanisms to prevent indiscriminate signalling through innate immune receptors. The enteric pathogen Salmonella enterica subsp. enterica serovar Typhimurium, which requires intestinal inflammation to sustain its replication in the intestinal tract, uses effector proteins of its type III secretion systems to trigger an inflammatory response without the engagement of innate immune receptors. Furthermore, S. Typhimurium uses a different set of effectors to restrict the inflammatory response to preserve host homeostasis. The S. Typhimurium-host interface is a remarkable example of the unique balance that emerges from the co-evolution of a pathogen and its host.

Comparative transcriptional profiling of the early host response to infection by typhoidal and non-typhoidal Salmonella serovars in human intestinal organoids

Salmonella enterica represents over 2500 serovars associated with a wide-ranging spectrum of disease; from self-limiting gastroenteritis to invasive infections caused by non-typhoidal serovars (NTS) and typhoidal serovars, respectively. Host factors strongly influence infection outcome as malnourished or immunocompromised individuals can develop invasive infections from NTS, however, comparative analyses of serovar-specific host responses have been constrained by reliance on limited model systems. Here we used human intestinal organoids (HIOs), a three-dimensional “gut-like” in vitro system derived from human embryonic stem cells, to elucidate similarities and differences in host responses to NTS and typhoidal serovars. HIOs discriminated between the two most prevalent NTS, Salmonella enterica serovar Typhimurium (STM) and Salmonella enterica serovar Enteritidis (SE), and typhoidal serovar Salmonella enterica serovar Typhi (ST) in epithelial cell invasion, replication and transcriptional responses. Pro-inflammatory signaling and cytokine output was reduced in ST-infected HIOs compared to NTS infections, consistent with early stages of NTS and typhoidal diseases. While we predicted that ST would induce a distinct transcriptional profile from the NTS strains, more nuanced expression profiles emerged. Notably, pathways involved in cell cycle, metabolism and mitochondrial functions were downregulated in STM-infected HIOs and upregulated in SE-infected HIOs. These results correlated with suppression of cellular proliferation and induction of host cell death in STM-infected HIOs and in contrast, elevated levels of reactive oxygen species production in SE-infected HIOs. Collectively, these results suggest that the HIO model is well suited to reveal host transcriptional programming specific to infection by individual Salmonella serovars, and that individual NTS may provoke unique host epithelial responses during intestinal stages of infection.

Salmonella enterica is the major causative agent of bacterial infections associated with contaminated food and water. Salmonella enterica consists of over 2500 serovars of which Typhimurium (STM), Enteritidis (SE) and Typhi (ST) are the three major serovars with medical relevance to humans. These serovars elicit distinctive immune responses and cause different diseases in humans, including self-limiting diarrhea, gastroenteritis and typhoid fever. Differences in the human host response to these serovars are likely to be a major contributing factor to distinct disease outcomes but are not well characterized, possibly due to the limitations of human-derived physiological infection models. Distinct from immortalized epithelial cell culture models, human intestinal organoids (HIOs) are three-dimensional structures derived from embryonic stem cells that differentiate into intestinal mesenchymal and epithelial cells, mirroring key organizational aspects of the intestine. In this study, we used HIOs to monitor transcriptional changes during early stages of STM, SE and ST infection. Our comparative analysis showed that HIO inflammatory responses are the dominant response in all infections, but ST infection induces the weakest upregulation of inflammatory mediators relative to the other serovars. In addition, we identified several cellular processes, including cell cycle and mitochondrial functions, that were inversely regulated between STM and SE infection despite these serovars causing similar localized intestinal infection in humans. Our findings reinforce HIOs as an emerging model system to study Salmonella serovar infection and define global host transcriptional response profiles as a foundation for understanding human infection outcomes.

Systematic reconstruction of an effector-gene network reveals determinants of Salmonella cellular and tissue tropism

The minimal genetic requirements for microbes to survive within multiorganism communities, including host-pathogen interactions, remain poorly understood. Here, we combined targeted gene mutagenesis with phenotype-guided genetic reassembly to identify a cooperative network of SPI-2 T3SS effector genes that are sufficient for Salmonella Typhimurium (STm) to cause disease in a natural host organism. Five SPI-2 effector genes support pathogen survival within the host cell cytoplasm by coordinating bacterial replication with Salmonella-containing vacuole (SCV) division. Unexpectedly, this minimal genetic repertoire does not support STm systemic infection of mice. In vivo screening revealed a second effector-gene network, encoded by the spv operon, that expands the life cycle of STm from growth in cells to deep-tissue colonization in a murine model of typhoid fever. Comparison between Salmonella infection models suggests how cooperation between effector genes drives tissue tropism in a pathogen group.

Association of quorum sensing and biofilm formation with Salmonella virulence: story beyond gathering and cross-talk

Enteric fever (typhoid and paratyphoid fever) is a public health concern which contributes to mortality and morbidity all around the globe. It is caused mainly due to ingestion of contaminated food and water with a gram negative, rod-shaped, flagellated bacterium known as Salmonella enterica serotype typhi (typhoid fever) or paratyphi (paratyphoid fever). Clinical problems associated with Salmonellosis are mainly bacteraemia, gastroenteritis and enteric fever. The bacteria undergo various mechanisms to escape itself from immune reaction of the host, modulating immune response at the site of infection leading to virulence factor production and anti-microbial resistance. Biofilm is one of the adaptation mechanisms through which Salmonella survives in unfavourable conditions and thus is considered as a major threat to public health. Another property of the bacteria is "Quorum Sensing", which is a cell-cell communication and most of the pathogenic bacteria use it to coordinate the production of several virulence factors and other behaviours such as swarming and biofilm formation. Earlier, quorum sensing was believed to be just a medium for communication but, later on, its role in virulence has been studied. However, there are negligible information relating to interaction between quorum sensing and biofilm formation and how these events play crucial role in Salmonella pathogenesis. The review is a summary of updated information regarding how Salmonella uses these properties to spread more and survive better, making a challenge for clinicians and public health experts. Therefore, this review would help bring an insight regarding how biofilm formation and quorum sensing are inter-related and their role in pathogenesis and virulence of Salmonella.

Lactobacillus johnsonii L531 Alleviates the Damage Caused by Salmonella Typhimurium via Inhibiting TLR4, NF-κB, and NLRP3 Inflammasome Signaling Pathways

Salmonella Typhimurium (S. Typhimurium) is an aggressive zoonotic pathogen that causes enteritis and diarrhea. Antibiotic therapy is still the primary method at present. However, the increasing emergence of multi-drug resistant bacteria weakens the therapeutic efficacy of antibiotics. Probiotics have been widely studied as an alternative antibiotic therapy. In this study, we established an IPEC-J2 cell model of S. Typhimurium infection, aiming to determine the protective effect of Lactobacillus johnsonii L531 (L. johnsonii L531) on S. Typhimurium infection. As our data showed, S. Typhimurium infection resulted in a robust inflammatory response demonstrated by promoted protein levels of the inflammatory-related pathway (TLR4, MyD88, p-IκBα, and p-p65), increased cytokine levels of IL-6, IL-1β, IL-18, and TNF-α, and activated the NLRP3 inflammasome via promoting its assembly. However, L. johnsonii L531 pre-incubation inhibited the activation of the above inflammatory signaling pathways and reduced the expression levels of pro-inflammatory cytokines. In addition, L. johnsonii L531 alleviated the damage of S. Typhimurium to tight junctions ZO-1, Occludin, and Claudin-1. In summary, our findings suggested that L. johnsonii L531 alleviated S. Typhimurium-induced tight junction injury by inhibiting the TLR4/NF-κB/NLRP3 inflammasome signaling pathway.

ARHGEF26 enhances Salmonella invasion and inflammation in cells and mice

Salmonella hijack host machinery in order to invade cells and establish infection. While considerable work has described the role of host proteins in invasion, much less is known regarding how natural variation in these invasion-associated host proteins affects Salmonella pathogenesis. Here we leveraged a candidate cellular GWAS screen to identify natural genetic variation in the ARHGEF26 (Rho Guanine Nucleotide Exchange Factor 26) gene that renders lymphoblastoid cells susceptible to Salmonella Typhi and Typhimurium invasion. Experimental follow-up redefined ARHGEF26’s role in Salmonella epithelial cell infection. Specifically, we identified complex serovar-by-host interactions whereby ARHGEF26 stimulation of S. Typhi and S. Typhimurium invasion into host cells varied in magnitude and effector-dependence based on host cell type. While ARHGEF26 regulated SopB- and SopE-mediated S. Typhi (but not S. Typhimurium) infection of HeLa cells, the largest effect of ARHGEF26 was observed with S. Typhimurium in polarized MDCK cells through a SopB- and SopE2-independent mechanism. In both cell types, knockdown of the ARHGEF26-associated protein DLG1 resulted in a similar phenotype and serovar specificity. Importantly, we show that ARHGEF26 plays a critical role in S. Typhimurium pathogenesis by contributing to bacterial burden in the enteric fever murine model, as well as inflammation in the colitis infection model. In the enteric fever model, SopB and SopE2 are required for the effects of Arhgef26 deletion on bacterial burden, and the impact of sopB and sopE2 deletion in turn required ARHGEF26. In contrast, SopB and SopE2 were not required for the impacts of Arhgef26 deletion on colitis. A role for ARHGEF26 on inflammation was also seen in cells, as knockdown reduced IL-8 production in HeLa cells. Together, these data reveal pleiotropic roles for ARHGEF26 during infection and highlight that many of the interactions that occur during infection that are thought to be well understood likely have underappreciated complexity.

During infection, Salmonella manipulates host cells into engulfing the bacteria and establishing an intracellular niche. While many studies have identified genes involved in different stages of this Salmonella invasion process, few studies have examined how differences between human hosts contribute to infection susceptibility. Here we leveraged a candidate genetic screen to identify natural genetic variation in the human ARHGEF26 gene that correlates with Salmonella invasion. Springboarding from this result, we experimentally tested and redefined ARHGEF26’s role in Salmonella invasion, discovered a new role for ARHGEF26 in regulating inflammation during Salmonella disease, and demonstrated the relevance of these findings in mouse models. Building on how ARHGEF26 functions in other contexts, we implicated two ARHGEF26-interacting host proteins as contributors to Salmonella pathobiology. Collectively, these results identify a potential source of inter-person diversity in susceptibility to Salmonella disease and expand our molecular understanding of Salmonella infection to include a multifaceted role for ARHGEF26. They further identify important future directions in understanding how Salmonella recruit and manipulate ARHGEF26 as well as how ARHGEF26 is able to drive Salmonella-beneficial processes.

New Insights on the Early Interaction Between Typhoid and Non-typhoid Salmonella Serovars and the Host Cells

Salmonella enterica is a common source of food and water-borne infections, causing a wide range of clinical ailments in both human and animal hosts. Immunity to Salmonella involves an interplay between different immune responses, which are rapidly initiated to control bacterial burden. However, Salmonella has developed several strategies to evade and modulate the host immune responses. In this sense, the main knowledge about the pathogenicity of this bacterium has been obtained by the study of mouse models with non-typhoidal serovars. However, this knowledge is not representative of all the pathologies caused by non-typhoidal serovars in the human. Here we review the most important features of typhoidal and non-typhoidal serovars and the diseases they cause in the human host, describing the virulence mechanisms used by these pathogens that have been identified in different models of infection.

Reprogramming of microRNA expression via E2F1 downregulation promotes Salmonella infection both in infected and bystander cells

Cells infected with pathogens can contribute to clearing infections by releasing signals that instruct neighbouring cells to mount a pro-inflammatory cytokine response, or by other mechanisms that reduce bystander cells’ susceptibility to infection. Here, we show the opposite effect: epithelial cells infected with Salmonella Typhimurium secrete host factors that facilitate the infection of bystander cells. We find that the endoplasmic reticulum stress response is activated in both infected and bystander cells, and this leads to activation of JNK pathway, downregulation of transcription factor E2F1, and consequent reprogramming of microRNA expression in a time-dependent manner. These changes are not elicited by infection with other bacterial pathogens, such as Shigella flexneri or Listeria monocytogenes. Remarkably, the protein HMGB1 present in the secretome of Salmonella-infected cells is responsible for the activation of the IRE1 branch of the endoplasmic reticulum stress response in non-infected, neighbouring cells. Furthermore, E2F1 downregulation and the associated microRNA alterations promote Salmonella replication within infected cells and prime bystander cells for more efficient infection.

Cells infected with pathogens can release signals that instruct neighbouring cells to mount an immune response or that reduce these cells’ susceptibility to infection. Here, Aguilar et al. show the opposite effect: cells infected with Salmonella Typhimurium secrete host factors that facilitate the infection of bystander cells by activating their ER-stress response.

Bacterial microcompartments and their role in pathogenicity

Catabolic bacterial microcompartments (BMC), or metabolosomes, are self-assembling structures formed by enzymes enclosed by porous protein shells. They provide a specialised environment inside bacterial cells separating a short catabolic pathway with reactive or toxic intermediates from the cytoplasm. Substrates for microcompartment metabolism like ethanolamine and 1,2-propanediol are constantly produced in the human intestine by bacterial metabolism of food or host cell components. Enteric pathogens gain a competitive advantage in the intestine by metabolising these substrates, an advantage enhanced by the host inflammatory response. They exploit the intestinal specificity of signature metabolosome substrates by adopting substrate sensors and regulators encoded by BMC operons for governance of non-metabolic processes in pathogenesis. In turn, products of microcompartment metabolism regulate the host immune system.

Salmonella enterica Serovar Typhimurium SPI-1 and SPI-2 Shape the Global Transcriptional Landscape in a Human Intestinal Organoid Model System

The intestinal epithelium is a primary interface for engagement of the host response by foodborne pathogens, like Salmonella enterica Typhimurium. While the interaction of S Typhimurium with the mammalian host has been well studied in transformed epithelial cell lines or in the complex intestinal environment in vivo, few tractable models recapitulate key features of the intestine. Human intestinal organoids (HIOs) contain a polarized epithelium with functionally differentiated cell subtypes, including enterocytes and goblet cells and a supporting mesenchymal cell layer. HIOs contain luminal space that supports bacterial replication, are more amenable to experimental manipulation than animals and are more reflective of physiological host responses. Here, we use the HIO model to define host transcriptional responses to S Typhimurium infection, also determining host pathways dependent on Salmonella pathogenicity island-1 (SPI-1)- and -2 (SPI-2)-encoded type 3 secretion systems (T3SS). Consistent with prior findings, we find that S Typhimurium strongly stimulates proinflammatory gene expression. Infection-induced cytokine gene expression was rapid, transient, and largely independent of SPI-1 T3SS-mediated invasion, likely due to continued luminal stimulation. Notably, S Typhimurium infection led to significant downregulation of host genes associated with cell cycle and DNA repair, leading to a reduction in cellular proliferation, dependent on SPI-1 and SPI-2 T3SS. The transcriptional profile of cell cycle-associated target genes implicates multiple miRNAs as mediators of S Typhimurium-dependent cell cycle suppression. These findings from Salmonella-infected HIOs delineate common and distinct contributions of SPI-1 and SPI-2 T3SSs in inducing early host responses during enteric infection and reinforce host cell proliferation as a process targeted by Salmonella IMPORTANCE Salmonella enterica serovar Typhimurium (S Typhimurium) causes a significant health burden worldwide, yet host responses to initial stages of intestinal infection remain poorly understood. Due to differences in infection outcome between mice and humans, physiological human host responses driven by major virulence determinants of Salmonella have been more challenging to evaluate. Here, we use the three-dimensional human intestinal organoid model to define early responses to infection with wild-type S Typhimurium and mutants defective in the SPI-1 or SPI-2 type-3 secretion systems. While both secretion system mutants show defects in mouse models of oral Salmonella infection, the specific contributions of each secretion system are less well understood. We show that S Typhimurium upregulates proinflammatory pathways independently of either secretion system, while the downregulation of the host cell cycle pathways relies on both SPI-1 and SPI-2. These findings lay the groundwork for future studies investigating how SPI-1- and SPI-2-driven host responses affect infection outcome and show the potential of this model to study host-pathogen interactions with other serovars to understand how initial interactions with the intestinal epithelium may affect pathogenesis.

SIK2 orchestrates actin-dependent host response upon Salmonella infection

Salmonella is an intracellular pathogen of a substantial global health concern. In order to identify key players involved in Salmonella infection, we performed a global host phosphoproteome analysis subsequent to bacterial infection. Thereby, we identified the kinase SIK2 as a central component of the host defense machinery upon Salmonella infection. SIK2 depletion favors the escape of bacteria from the Salmonella-containing vacuole (SCV) and impairs Xenophagy, resulting in a hyperproliferative phenotype. Mechanistically, SIK2 associates with actin filaments under basal conditions; however, during bacterial infection, SIK2 is recruited to the SCV together with the elements of the actin polymerization machinery (Arp2/3 complex and Formins). Notably, SIK2 depletion results in a severe pathological cellular actin nucleation and polymerization defect upon Salmonella infection. We propose that SIK2 controls the formation of a protective SCV actin shield shortly after invasion and orchestrates the actin cytoskeleton architecture in its entirety to control an acute Salmonella infection after bacterial invasion.

The Salmonella effector protein SopD targets Rab8 to positively and negatively modulate the inflammatory response

The food-borne bacterial pathogen Salmonella Typhimurium uses a type III protein secretion system to deliver multiple proteins into host cells. These secreted effectors modulate the functions of host cells and activate specific signalling cascades that result in the production of pro-inflammatory cytokines and intestinal inflammation. Some of the Salmonella-encoded effectors counteract this inflammatory response and help to preserve host homeostasis. Here, we demonstrate that the Salmonella effector protein SopD, which is required for pathogenesis, functions to both activate and inhibit the inflammatory response by targeting the Rab8 GTPase, which is a negative regulator of inflammation. We show that SopD has GTPase-activating protein activity for Rab8 and, therefore, inhibits this GTPase and stimulates inflammation. We also show that SopD activates Rab8 by displacing it from its cognate guanosine dissociation inhibitor, resulting in the stimulation of a signalling cascade that suppresses inflammation. We solved the crystal structure of SopD in association with Rab8 to a resolution of 2.3 Å, which reveals a unique contact interface that underlies these complex interactions. These findings show the remarkable evolution of a bacterial effector protein to exert both agonistic and antagonistic activities towards the same host cellular target to modulate the inflammatory response.

RhoGTPases and inflammasomes: Guardians of effector-triggered immunity

Pathogens have evolved smart strategies to invade hosts and hijack their immune responses. One such strategy is the targeting of the host RhoGTPases by toxins or virulence factors to hijack the cytoskeleton dynamic and immune processes. In response to this microbial attack, the host has evolved an elegant strategy to monitor the function of virulence factors and toxins by sensing the abnormal activity of RhoGTPases. This innate immune strategy of sensing bacterial effector targeting RhoGTPase appears to be a bona fide example of effector-triggered immunity (ETI). Here, we review recently discovered mechanisms by which the host can sense the activity of these toxins through NOD and NOD-like receptors (NLRs).

Interactions between Salmonella and host macrophages - Dissecting NF-κB signaling pathway responses

Salmonella not only invades host cells, but also replicates intracellularly to cause a range of diseases, including gastroenteritis and systemic infections such as typhoid fever. The body's first line of defense against pathogens is the innate immune response system that can protect against Salmonella invasion and replication. Nuclear factor κB (NF-κB) is an important transcriptional regulator that plays an important role in host inflammatory responses to pathogens. Both the canonical and non-canonical NF-κB signaling pathways are activated by Salmonella in many different ways through its virulence factors, leading to the release of inflammatory factors and the activation of inflammatory responses in mammalian hosts. Equally, Salmonella, as an enteropathogen, has accordingly evolved strategies to disturb NF-κB activation, such as secreting some effector proteins by type III secretion systems as well as inducing host cells to express NF-κB pathway inhibitors, allowing it to colonize and persistently infect the hosts. This review focuses on how Salmonella activates NF-κB signaling pathway and the strategies used by Salmonella to interfere with the NF-κB pathway activation.

Escherichia coli Rho GTPase-activating toxin CNF1 mediates NLRP3 inflammasome activation via p21-activated kinases-1/2 during bacteraemia in mice

Inflammasomes are signalling platforms that are assembled in response to infection or sterile inflammation by cytosolic pattern recognition receptors. The consequent inflammasome-triggered caspase-1 activation is critical for the host defence against pathogens. During infection, NLRP3, which is a pattern recognition receptor that is also known as cryopyrin, triggers the assembly of the inflammasome-activating caspase-1 through the recruitment of ASC and Nek7. The activation of the NLRP3 inflammasome is tightly controlled both transcriptionally and post-translationally. Despite the importance of the NLRP3 inflammasome regulation in autoinflammatory and infectious diseases, little is known about the mechanism controlling the activation of NLRP3 and the upstream signalling that regulates the NLRP3 inflammasome assembly. We have previously shown that the Rho-GTPase-activating toxin from Escherichia coli cytotoxic necrotizing factor-1 (CNF1) activates caspase-1, but the upstream mechanism is unclear. Here, we provide evidence of the role of the NLRP3 inflammasome in sensing the activity of bacterial toxins and virulence factors that activate host Rho GTPases. We demonstrate that this activation relies on the monitoring of the toxin's activity on the Rho GTPase Rac2. We also show that the NLRP3 inflammasome is activated by a signalling cascade that involves the p21-activated kinases 1 and 2 (Pak1/2) and the Pak1-mediated phosphorylation of Thr 659 of NLRP3, which is necessary for the NLRP3-Nek7 interaction, inflammasome activation and IL-1β cytokine maturation. Furthermore, inhibition of the Pak-NLRP3 axis decreases the bacterial clearance of CNF1-expressing UTI89 E. coli during bacteraemia in mice. Taken together, our results establish that Pak1 and Pak2 are critical regulators of the NLRP3 inflammasome and reveal the role of the Pak-NLRP3 signalling axis in vivo during bacteraemia in mice.

Phylogeny of Salmonella enterica subspecies arizonae by whole-genome sequencing reveals high incidence of polyphyly and low phase 1 H antigen variability

Salmonella enterica subspecies arizonae is frequently associated with animal reservoirs, particularly reptiles, and can cause illness in some mammals, including humans. Using whole-genome sequencing data, core genome phylogenetic analyses were performed using 112 S. enterica subsp. arizonae isolates, representing 46 of 102 described serovars. Nearly one-third of these are polyphyletic, including two serovars that appear in four and five distinct evolutionary lineages. Subspecies arizonae has a monophasic H antigen. Among the 46 serovars investigated, only 8 phase 1 H antigens were identified, demonstrating high conservation for this antigen. Prophages and plasmids were found throughout this subspecies, including five novel prophages. Polyphyly was also reflected in prophage content, although some clade-specific enrichment for some phages was observed. IncFII(S) was the most frequent plasmid replicon identified and was found in a quarter of S. enterica subsp. arizonae genomes. Salmonella pathogenicity islands (SPIs) 1 and 2 are present across all Salmonella, including this subspecies, although effectors sipA, sptP and arvA in SPI-1 and sseG and ssaI in SPI-2 appear to be lost in this lineage. SPI-20, encoding a type VI secretion system, is exclusive to this subspecies and is well maintained in all genomes sampled. A number of fimbral operons were identified, including the sas operon that appears to be a synapomorphy for this subspecies, while others exhibited more clade-specific patterns. This work reveals evolutionary patterns in S. enterica subsp. arizonae that make this subspecies a unique lineage within this very diverse species.

Cathelicidin-mediated lipopolysaccharide signaling via intracellular TLR4 in colonic epithelial cells evokes CXCL8 production

We hypothesized that the antimicrobial peptide cathelicidin has a physiological role in regulating gut inflammatory homeostasis. We determined that cathelicidin synergizes with LPS to facilitate its internalization and signaling via endosomic TLR4 in colonic epithelium, evoking synthesis of the human neutrophil chemoattractant, CXCL8 (or murine homolog, CXCL1). Interaction of cathelicidin with LPS in the control of CXCL8/CXCL1 synthesis was assessed in human colon epithelial cells, murine colonoids and cathelicidin-null mice (Camp^-/- ). Mechanistically, human cathelicidin (LL-37), as an extracellular complex with LPS, interacted with lipid raft-associated GM1 gangliosides to internalize and activate intracellular TLR4. Two signaling pathways converged on CXCL8/CXCL1 production: (1) a p38MAPK-dependent pathway regulated by Src-EGFR kinases; and, (2) a p38MAPK-independent, NF-κB-dependent pathway, regulated by MEK1/2-MAPK. Increased cathelicidin-dependent CXCL8 secretion in the colonic mucosa activated human blood-derived neutrophils. These cathelicidin effects occurred in vitro at concentrations well below those needed for microbicidal function. The important immunomodulatory role of cathelicidins was evident in cathelicidin-null/Camp^-/- mice, which had diminished colonic CXCL1 secretion, decreased neutrophil recruitment-activation and reduced bacterial clearance when challenged with the colitis-inducing murine pathogen, Citrobacter rodentium. We conclude that in addition to its known microbicidal action, cathelicidin has a unique pathogen-sensing role, facilitating LPS-mediated intestinal responses, including the production of CXCL8/CXCL1 that would contribute to an integrated tissue response to recruit neutrophils during colitis.

Regulation and Functions of ROP GTPases in Plant-Microbe Interactions

Rho proteins of plants (ROPs) form a specific clade of Rho GTPases, which are involved in either plant immunity or susceptibility to diseases. They are intensively studied in grass host plants, in which ROPs are signaling hubs downstream of both cell surface immune receptor kinases and intracellular nucleotide-binding leucine-rich repeat receptors, which activate major branches of plant immune signaling. Additionally, invasive fungal pathogens may co-opt the function of ROPs for manipulation of the cytoskeleton, cell invasion and host cell developmental reprogramming, which promote pathogenic colonization. Strikingly, mammalian bacterial pathogens also initiate both effector-triggered susceptibility for cell invasion and effector-triggered immunity via Rho GTPases. In this review, we summarize central concepts of Rho signaling in disease and immunity of plants and briefly compare them to important findings in the mammalian research field. We focus on Rho activation, downstream signaling and cellular reorganization under control of Rho proteins involved in disease progression and pathogen resistance.

Immune modulations and survival strategies of evolved hypervirulent Salmonella Typhimurium strains

BACKGROUND

Evolving multidrug-resistance and hypervirulence in Salmonella is due to multiple host-pathogen, and non-host environmental interactions. Previously we had studied Salmonella adaptation upon repeated exposure in different in-vitro and in-vivo environmental conditions. This study deals with the mechanistic basis of hypervirulence of the passaged hypervirulent Salmonella strains reported previously.

METHODS

Real-time PCR, flow cytometry, western blotting, and confocal microscopy were employed to check the alteration of signaling pathways by the hypervirulent strains. The hypervirulence was also looked in-vivo in the Balb/c murine model system.

RESULTS

The hypervirulent strains altered cytokine production towards anti-inflammatory response via NF-κB and Akt-NLRC4 signaling in RAW-264.7 and U-937 cells. They also impaired lysosome number, as well as co-localization with the lysosome as compared to unpassaged WT-STM. In Balb/c mice also they caused decreased antimicrobial peptides, reduced nitric oxide level, altered cytokine production, and reduced CD4+ T cell population leading to increased organ burden.

CONCLUSIONS

Hypervirulent Salmonella strains infection resulted in an anti-inflammatory environment by upregulating IL-10 and down-regulating IL-1β expression. They also evaded lysosomal degradation for their survival. With inhibition of NF-κB and Akt signaling, cytokine expression, lysosome number, as well as the bacterial burden was reverted, indicating the infection mediated immune modulation by the hypervirulent Salmonella strains through these pathways.

GENERAL SIGNIFICANCE

Understanding the mechanism of adaptation can provide better disease prognosis by either targeting the bacterial gene or by strengthening the host immune system that might ultimately help in controlling salmonellosis.

NOD1 and NOD2 Activation by Diverse Stimuli: a Possible Role for Sensing Pathogen-Induced Endoplasmic Reticulum Stress

Prompt recognition of microbes by cells is critical to eliminate invading pathogens. Some cell-associated pattern recognition receptors (PRRs) recognize and respond to microbial ligands. However, others can respond to cellular perturbations, such as damage-associated molecular patterns (DAMPs). Nucleotide oligomerization domains 1 and 2 (NOD1/2) are PRRs that recognize and respond to multiple stimuli of microbial and cellular origin, such as bacterial peptidoglycan, viral infections, parasitic infections, activated Rho GTPases, and endoplasmic reticulum (ER) stress. How NOD1/2 are stimulated by such diverse stimuli is not fully understood but may partly rely on cellular changes during infection that result in ER stress. NOD1/2 are ER stress sensors that facilitate proinflammatory responses for pathogen clearance; thus, NOD1/2 may help mount broad antimicrobial responses through detection of ER stress, which is often induced during a variety of infections. Some pathogens may subvert this response to promote infection through manipulation of NOD1/2 responses to ER stress that lead to apoptosis. Here, we review NOD1/2 stimuli and cellular responses. Furthermore, we discuss pathogen-induced ER stress and how it might potentiate NOD1/2 signaling.

Dynamics and Outcome of Macrophage Interaction Between Salmonella Gallinarum, Salmonella Typhimurium, and Salmonella Dublin and Macrophages From Chicken and Cattle

Salmonella Gallinarum only infects avian species, where it causes a severe systemic infection in birds of all ages. It is generally accepted that interaction with phagocytic cells plays an important role in the development of systemic, host-specific Salmonella infections. The current study detailed the interaction of S. Gallinarum with macrophages derived from chicken (HD11) and cattle (Bomac) compared to interaction of the broad host range serovar, Salmonella Typhimurium and the cattle adapted serovar Salmonella Dublin. Results showed a weaker invading ability of S. Gallinarum in both kinds of macrophages, regardless whether the bacteria were opsonized or not before infections. However, opsonization of S. Gallinarum by chicken serum increased its intracellular survival rate in chicken macrophages. No significant induction of nitrogen oxide was observed in the infected HD11 cells within the first 6 h, and levels of reactive oxygen species (ROS) were similar among the three serovars. S. Gallinarum infection was associated with low cell deaths in both chicken and cattle macrophages, whereas S. Dublin only induced a comparable high level of cell death in chicken macrophages, but not in macrophages of its preferred host species (Bomac) compared to host generalist S. Typhimurium. S. Gallinarum-infected HD11 macrophages exhibited low induction of pro-inflammation genes [interleukin (IL)1β, CXCLi1, and CXCLi2] compared to the two other serovars, and contrary to the other serovars, it did not induce significant downregulation of Toll-like receptor (TLR)2, TLR4, and TLR5. In in vivo infection of 1-week-old chicken, a significant upregulation of the TLR4 and TLR5 genes in the spleen was observed in S. Gallinarum-infected chickens, but not in S. Typhimurium-infected chicken at 5 days post-infections. Taken together, results show that S. Gallinarum infection of macrophages was characterized by low uptake and low cytotoxicity, possibly allowing long-term persistence in the intracellular environment, and it caused a low induction of pro-inflammatory responses.

Salmonella Effector SteA Suppresses Proinflammatory Responses of the Host by Interfering With IκB Degradation

Salmonella enterica serovar Typhimurium is known to cause its virulence by secreting various effector proteins directly into the host cytoplasm via two distinct type III secretion systems (T3SS-1 and T3SS-2). Generally, T3SS-1-delivered effectors help Salmonella Typhimurium in the early phases of infection including invasion and immune modulation of the host cells, whereas T3SS-2 effectors mainly help in the survival of Salmonella Typhimurium within the host cells including maintenance of Salmonella-containing vacuole, replication of the bacteria, and dissemination. Some of the effectors are secreted via both T3SS-1 and T3SS-2, suggesting their role in distinct phases of infection of host cells. SteA is such an effector that is secreted by both T3SS-1 and T3SS-2. It has been shown to control the membrane dynamics of the Salmonella-containing vacuole within the host cells in the late phases of infection. In this manuscript, toward characterizing the T3SS-1 function of SteA, we found that SteA suppresses inflammatory responses of the host by interfering with the nuclear factor kappa B pathway. Our initial observation showed that the mice infected with steA-deleted Salmonella Typhimurium (ΔsteA) died earlier compared to the wild-type bacteria due to heightened immune responses, which indicated that SteA might suppress immune responses. Furthermore, our study revealed that SteA suppresses immune responses in macrophages by interfering with the degradation of IκB, the inhibitor of nuclear factor kappa B. SteA suppresses the ubiquitination and hence degradation of IκB by acting on Cullin-1 of the Skp-1, Cullin-1, F-box (SCF)-E3 ligase complex. Our study revealed that SteA suppresses a key step necessary for E3 ligase activation, i.e., neddylation of Cullin-1 by interfering with dissociation of its inhibitor Cand-1.

Engineered E. coli Nissle 1917 for the delivery of matrix-tethered therapeutic domains to the gut

Mucosal healing plays a critical role in combatting the effects of inflammatory bowel disease, fistulae and ulcers. While most treatments for such diseases focus on systemically delivered anti-inflammatory drugs, often leading to detrimental side effects, mucosal healing agents that target the gut epithelium are underexplored. We genetically engineer Escherichia coli Nissle 1917 (EcN) to create fibrous matrices that promote gut epithelial integrity in situ. These matrices consist of curli nanofibers displaying trefoil factors (TFFs), known to promote intestinal barrier function and epithelial restitution. We confirm that engineered EcN can secrete the curli-fused TFFs in vitro and in vivo, and is non-pathogenic. We observe enhanced protective effects of engineered EcN against dextran sodium sulfate-induced colitis in mice, associated with mucosal healing and immunomodulation. This work lays a foundation for the development of a platform in which the in situ production of therapeutic protein matrices from beneficial bacteria can be exploited.