Bacterial signals and cell responses during Shigella entry into epithelial cells

Cortactin: A universal host cytoskeletal target of Gram-negative and Gram-positive bacterial pathogens

Pathogenic bacteria possess a great potential of causing infectious diseases and represent a serious threat to human and animal health. Understanding the molecular basis of infection development can provide new valuable strategies for disease prevention and better control. In host-pathogen interactions, actin-cytoskeletal dynamics play a crucial role in the successful adherence, invasion, and intracellular motility of many intruding microbial pathogens. Cortactin, a major cellular factor that promotes actin polymerization and other functions, appears as a central regulator of host-pathogen interactions and different human diseases including cancer development. Various important microbes have been reported to hijack cortactin signaling during infection. The primary regulation of cortactin appears to proceed via serine and/or tyrosine phosphorylation events by upstream kinases, acetylation, and interaction with various other host proteins, including the Arp2/3 complex, filamentous actin, the actin nucleation promoting factor N-WASP, focal adhesion kinase FAK, the large GTPase dynamin-2, the guanine nucleotide exchange factor Vav2, and the actin-stabilizing protein CD2AP. Given that many signaling factors can affect cortactin activities, several microbes target certain unique pathways, while also sharing some common features. Here we review our current knowledge of the hallmarks of cortactin as a major target for eminent Gram-negative and Gram-positive bacterial pathogens in humans.

Combatting intracellular pathogens using bacteriophage delivery

Intracellular pathogens reside in specialised compartments within the host cells restricting the access of antibiotics. Insufficient intracellular delivery of antibiotics along with several other resistance mechanisms weaken the efficacy of current therapies. An alternative to antibiotic therapy could be bacteriophage (phage) therapy. Although phage therapy has been in practice for a century against various bacterial infections, the efficacy of phages against intracellular bacteria is still being explored. In this review, we will discuss the advancement and challenges in phage therapy, particularly against intracellular bacterial pathogens. Finally, we will highlight the uptake mechanisms and approaches to overcome the challenges to phage therapy against intracellular bacteria.

Distinguishing Pathovars from Nonpathovars: Escherichia coli

Escherichia coli is one of the most well-adapted and pathogenically versatile bacterial organisms. It causes a variety of human infections, including gastrointestinal illnesses and extraintestinal infections. It is also part of the intestinal commensal flora of humans and other mammals. Groups of E. coli that cause diarrhea are often described as intestinal pathogenic E. coli (IPEC), while those that cause infections outside of the gut are called extraintestinal pathogenic E. coli (ExPEC). IPEC can cause a variety of diarrheal illnesses as well as extraintestinal syndromes such as hemolytic-uremic syndrome. ExPEC cause urinary tract infections, bloodstream infection, sepsis, and neonatal meningitis. IPEC and ExPEC have thus come to be referred to as pathogenic variants of E. coli or pathovars. While IPEC can be distinguished from commensal E. coli based on their characteristic virulence factors responsible for their associated clinical manifestations, ExPEC cannot be so easily distinguished. IPEC most likely have reservoirs outside of the human intestine but it is unclear if ExPEC represent nothing more than commensal E. coli that breach a sterile barrier to cause extraintestinal infections. This question has become more complicated by the advent of whole genome sequencing (WGS) that has raised a new question about the taxonomic characterization of E. coli based on traditional clinical microbiologic and phylogenetic methods. This review discusses how molecular epidemiologic approaches have been used to address these questions, and how answers to these questions may contribute to our better understanding of the epidemiology of infections caused by E. coli. *This article is part of a curated collection.

The type III secretion system needle, tip, and translocon

Many Gram-negative bacteria pathogenic to plants and animals deploy the type III secretion system (T3SS) to inject virulence factors into their hosts. All bacteria that rely on the T3SS to cause infectious diseases in humans have developed antibiotic resistance. The T3SS is an attractive target for developing new antibiotics because it is essential in virulence, and part of its structural component is exposed on the bacterial surface. The structural component of the T3SS is the needle apparatus, which is assembled from over 20 different proteins and consists of a base, an extracellular needle, a tip, and a translocon. This review summarizes the current knowledge on the structure and assembly of the needle, tip, and translocon.

The Role of Connexins in Gastrointestinal Diseases

Gap junctions are hexagonal arrays of protein molecules in the plasma membrane and were first described in Mauthner cell synapses of goldfish. They form pathways for coupling between cells, allowing passive, electrotonic spread of ions and also passage of larger molecules such as amino acids and nucleotides. They are expressed in both excitable and non-excitable tissues. Each gap junction is made of two connexons, which are hexameric proteins of the connexin subunit. In this review, the roles that connexins play in gastrointestinal motility, the mechanisms of altered connexin expression leading to inflammatory bowel disease, gastrointestinal infections, and gastrointestinal symptoms in autistic spectrum disorder are discussed in detail.

Inhibition of Shigella sonnei-induced epithelial barrier disruption by surface-layer associated proteins of lactobacilli from Chinese fermented food

Surface-layer associated proteins (SLAP) of Lactobacillus paracasei ssp. paracasei M5-L and Lactobacillus casei Q8-L were examined to identify the functional basis for their protection within intestinal epithelial cells. The results showed that SLAP of M5-L and Q8-L remained active in a trypsin solution and retained a 45-kDa protein band, similar to that observed in controls. In contrast, under conditions of simulated gastric juice, the SLAP were partially degraded. Inhibitory effects of SLAP on adherence of Shigella sonnei to HT-29 cells were assessed with use of exclusion, competition, and replacement assays. In response to M5-L at 50 μg/mL SLAP, an inhibition ratio of 33% was obtained, while for Q8-L at 400 μg/mL SLAP, the inhibition ratio was 48%. Hoechst 33258 test results showed that cells infected with S. sonnei and co-incubated with SLAP of M5-L and Q8-L were only partially apoptotic, with apoptosis rates of 37.67 and 43.67%, respectively. These levels of apoptosis were substantially lower than that observed with cells infected with S. sonnei alone. In addition, the SLAP of Q8-L and M5-L reduced downstream caspase-1 activity and further modified apoptotic cell damage. Finally, SLAP of M5-L and Q8-L were also able to prevent S. sonnei-induced membrane damage by inhibiting delocalization of zonula occludens (ZO)-1 and reducing the amount of occludin produced by S. sonnei.

Sulphate-reducing bacteria from ulcerative colitis patients induce apoptosis of gastrointestinal epithelial cells

The human microbiome consists of a multitude of bacterial genera and species which continuously interact with one another and their host establishing a metabolic equilibrium. The dysbiosis can lead to the development of pathology, such as inflammatory bowel diseases. Sulfide-producing prokaryotes, including sulphate-reducing bacteria (SRB) constituting different genera, including the Desulfovibrio, are commonly detected within the human microbiome recovered from fecal material or colonic biopsy samples. It has been proposed that SRB likely contribute to colonic pathology, for example ulcerative colitis (UC). The interaction of SRB with the human colon and intestinal epithelial cell lines has been investigated using Desulfovibrio indonesiensis as a model mono-culture and in a co-culture with E. coli isolate, and with SRB consortia from human biopsy samples. We find that D. indonesiensis, whether as a mono- or co-culture, was internalized and induced apoptosis in colon epithelial cells. This effect was enhanced in the presence of E. coli. The SRB combination obtained through enrichment of biopsies from UC patients induced apoptosis of a human intestinal epithelial cell line. This response was not observed in SRB enrichments from healthy (non-UC) controls. Importantly, apoptosis was detected in epithelial cells from UC patients and was not seen in epithelial cells of healthy donors. Furthermore, the antibody raised against exopolysaccharides (EPS) of D. indonesiensis cross reacted with the SRB population from UC patients but not with the SRB combination from non-UC controls. This study reinforces a correlation between the presence of sulphate-reducing bacteria and an inflammatory response in UC sufferers.

Caveolin 1 restricts Group A Streptococcus invasion of nonphagocytic host cells

Caveolae are composed of 2 major proteins, caveolin 1 (CAV1) and cavin 1 or polymerase transcript release factor I (CAVIN1). Here, we demonstrate that CAV1 levels modulate invasion of Group A Streptococcus (GAS) into nonphagocytic mammalian cells. GAS showed enhanced internalisation into CAV1-knockout mouse embryonic fibroblasts and CAV1 knockdown human epithelial HEp-2 cells, whereas overexpression of CAV1 in HEp-2 cells reduced GAS invasion. This effect was not dependent on the expression of the GAS fibronectin binding protein SfbI, which had previously been implicated in caveolae-mediated uptake. Nor was this effect dependent on CAVIN1, as knockout of CAVIN1 in mouse embryonic fibroblasts resulted in reduced GAS internalisation. Although CAV1 restricted GAS invasion into host cells, we observed only minimal association of invading GAS (strain M1T1⁵⁴⁴⁸ ) with CAV1 by immunofluorescence and very low association of invading M1T1⁵⁴⁴⁸ with caveolae by transmission electron microscopy. These observations suggest that physical interaction with caveolae is not needed for CAV1 restriction of invading GAS. An indirect mechanism of action is also consistent with the finding that changing membrane fluidity reverses the increased invasion observed in CAV1-null cells. Together, these results suggest that CAV1 protects host cells against GAS invasion by a caveola-independent mechanism.

Ezrin enhances line tension along transcellular tunnel edges via NMIIa driven actomyosin cable formation

Transendothelial cell macroaperture (TEM) tunnels control endothelium barrier function and are triggered by several toxins from pathogenic bacteria that provoke vascular leakage. Cellular dewetting theory predicted that a line tension of uncharacterized origin works at TEM boundaries to limit their widening. Here, by conducting high-resolution microscopy approaches we unveil the presence of an actomyosin cable encircling TEMs. We develop a theoretical cellular dewetting framework to interpret TEM physical parameters that are quantitatively determined by laser ablation experiments. This establishes the critical role of ezrin and non-muscle myosin II (NMII) in the progressive implementation of line tension. Mechanistically, fluorescence-recovery-after-photobleaching experiments point for the upstream role of ezrin in stabilizing actin filaments at the edges of TEMs, thereby favouring their crosslinking by NMIIa. Collectively, our findings ascribe to ezrin and NMIIa a critical function of enhancing line tension at the cell boundary surrounding the TEMs by promoting the formation of an actomyosin ring.

Novel insight into the role of microbiota in colorectal surgery

Recent literature undeniably supports the idea that the microbiota has a strong influence on the healing process of an intestinal anastomosis. Understanding the mechanisms by which the bacterial community of the gut influences intestinal healing could open the door for new preventive and therapeutic approaches. Among the different mechanisms, data have shown that the production of specific reactive oxygen species (ROS) and the activation of specific formyl peptide receptors (FPRs) regulate intestinal wound healing. Evidence suggests that specific gut microbes such as Lactobacillus spp and Akkermansia muciniphila help to regulate healing processes through both ROS-dependent and FPR-dependent mechanisms. In this review, we will discuss the current knowledge and future perspectives concerning the impact of microbiota on wound healing. We will further review available evidence on whether mechanical bowel preparation and the use of specific antibiotics are beneficial or harmful procedures, an ongoing matter of debate. These practices have a profound effect on the gut microbiota composition at the level of both the mucosal and the luminal compartments. Therefore, a key question remains unanswered: should we continue to prepare the gut before surgical intervention? Current knowledge and data do not clearly support the use of one technique or another to avoid complications such as anastomotic leak. There is an urgent need for appropriate interventions with a deep microbiota analysis to investigate both the surgical technical benefits of a proper anastomosis compared with the potential effect of the gut microbes (beneficial vs harmful) on the processes of wound healing and anastomotic leakage reduction.

The impact of hypoxia on intestinal epithelial cell functions: consequences for invasion by bacterial pathogens

The maintenance of oxygen homeostasis in human tissues is mediated by several cellular adaptations in response to low-oxygen stress, called hypoxia. A decrease in tissue oxygen levels is initially counteracted by increasing local blood flow to overcome diminished oxygenation and avoid hypoxic stress. However, studies have shown that the physiological oxygen concentrations in several tissues are much lower than atmospheric (normoxic) conditions, and the oxygen supply is finely regulated in individual cell types. The gastrointestinal tract has been described to subsist in a state of physiologically low oxygen level and is thus depicted as a tissue in the state of constant low-grade inflammation. The intestinal epithelial cell layer plays a vital role in the immune response to inflammation and infections that occur within the intestinal tissue and is involved in many of the adaptation responses to hypoxic stress. This is especially relevant in the context of inflammatory disorders, such as inflammatory bowel disease (IBD). Therefore, this review aims to describe the intestinal epithelial cellular response to hypoxia and the consequences for host interactions with invading gastrointestinal bacterial pathogens.

Shigella Diversity and Changing Landscape: Insights for the Twenty-First Century

Shigella is a pathovar of Escherichia coli comprising four groups, Shigella flexneri, Shigella sonnei, Shigella dysenteriae, and Shigella boydii, each of them, with the exception of S.sonnei, comprising several serotypes. Shigella accounts for the majority of dysentery causing infections occurring world-wide each year. Recent advancements in the Shigella field have led to a better understanding of the molecular mechanisms underlying host epithelial cell invasion and immune cell function manipulation, mainly using S. flexneri as a model. Host-cell invasion is the final step of the infection process, as Shigella's virulence strategy relies also on its ability to survive hostile conditions during its journey through the gastro-intestinal tract, to compete with the host microbiota and to cross the intestinal mucus layer. Hence, the diversity of the virulence strategies among the different Shigella species has not yet been deeply investigated, which might be an important step to understand the epidemiological spreading of Shigella species worldwide and a key aspect for the validation of novel vaccine candidates. The recent development of high-throughput screening and sequencing methods will facilitate these complex comparison studies. In this review we discuss several of the major avenues that the Shigella research field has taken over the past few years and hopefully gain some insights into the questions that remain surrounding this important human pathogen.

Endocytosis of viruses and bacteria

Of the many pathogens that infect humans and animals, a large number use cells of the host organism as protected sites for replication. To reach the relevant intracellular compartments, they take advantage of the endocytosis machinery and exploit the network of endocytic organelles for penetration into the cytosol or as sites of replication. In this review, we discuss the endocytic entry processes used by viruses and bacteria and compare the strategies used by these dissimilar classes of pathogens.

Pathogenicity of Shigella in chickens

Shigellosis in chickens was first reported in 2004. This study aimed to determine the pathogenicity of Shigella in chickens and the possibility of cross-infection between humans and chickens. The pathogenicity of Shigella in chickens was examined via infection of three-day-old SPF chickens with Shigella strain ZD02 isolated from a human patient. The virulence and invasiveness were examined by infection of the chicken intestines and primary chicken intestinal epithelial cells. The results showed Shigella can cause death via intraperitoneal injection in SPF chickens, but only induce depression via crop injection. Immunohistochemistry and transmission electron microscopy revealed the Shigella can invade the intestinal epithelia. Immunohistochemistry of the primary chicken intestinal epithelial cells infected with Shigella showed the bacteria were internalized into the epithelial cells. Electron microscopy also confirmed that Shigella invaded primary chicken intestinal epithelia and was encapsulated by phagosome-like membranes. Our data demonstrate that Shigella can invade primary chicken intestinal epithelial cells in vitro and chicken intestinal mucosa in vivo, resulting in pathogenicity and even death. The findings suggest Shigella isolated from human or chicken share similar pathogenicity as well as the possibility of human-poultry cross-infection, which is of public health significance.

Shigella flexneri Inhibits Intestinal Inflammation by Modulation of Host Sphingosine-1-Phosphate in Mice

Infection with invasive Shigella species results in intestinal inflammation in humans but no symptoms in adult mice. To investigate why adult mice are resistant to invasive shigellae, 6~8-week-old mice were infected orally with S. flexneri 5a. Shigellae successfully colonized the small and large intestines. Mild cell death was seen but no inflammation. The infected bacteria were cleared 24 hours later. Microarray analysis of infected intestinal tissue showed that several genes that are involved with the sphingosine-1-phosphate (S1P) signaling pathway, a lipid mediator which mediates immune responses, were altered significantly. Shigella infection of a human intestinal cell line modulated host S1P-related genes to reduce S1P levels. In addition, co-administration of S1P with shigellae could induce inflammatory responses in the gut. Here we propose that Shigella species have evasion mechanisms that dampen host inflammatory responses by lowering host S1P levels in the gut of adult mice.

Intestinal anastomotic injury alters spatially defined microbiome composition and function

BACKGROUND

When diseased intestine (i.e., from colon cancer, diverticulitis) requires resection, its reconnection (termed anastomosis) can be complicated by non-healing of the newly joined intestine resulting in spillage of intestinal contents into the abdominal cavity (termed anastomotic leakage). While it is suspected that the intestinal microbiota have the capacity to both accelerate and complicate anastomotic healing, the associated genotypes and functions have not been characterized.

RESULTS

Using 16S rRNA amplicon sequencing of samples collected on the day of surgery (postoperative day 0 (POD0)) and the 6th day following surgery (postoperative day 0 (POD6)), we analyzed the changes in luminal versus tissue-associated microbiota at anastomotic sites created in the colon of rats. Results indicated that anastomotic injury induced significant changes in the anastomotic tissue-associated microbiota with minimal differences in the luminal microbiota. The most striking difference was a 500-fold and 200-fold increase in the relative abundance of Enterococcus and Escherichia/Shigella, respectively. Functional profiling predicted the predominance of bacterial virulence-associated pathways in post-anastomotic tissues, including production of hemolysin, cytolethal toxins, fimbriae, invasins, cytotoxic necrotizing factors, and coccolysin.

CONCLUSION

Taken together, our results suggest that compositional and functional changes accompany anastomotic tissues and may potentially accelerate or complicate anastomotic healing.

Förster resonance energy transfer (FRET) as a tool for dissecting the molecular mechanisms for maturation of the Shigella type III secretion needle tip complex

Förster resonance energy transfer (FRET) provides a powerful tool for monitoring intermolecular interactions and a sensitive technique for studying Å-level protein conformational changes. One system that has particularly benefited from the sensitivity and diversity of FRET measurements is the maturation of the Shigella type III secretion apparatus (T3SA) needle tip complex. The Shigella T3SA delivers effector proteins into intestinal cells to promote bacterial invasion and spread. The T3SA is comprised of a basal body that spans the bacterial envelope and a needle with an exposed tip complex that matures in response to environmental stimuli. FRET measurements demonstrated bile salt binding by the nascent needle tip protein IpaD and also mapped resulting structural changes which led to the recruitment of the translocator IpaB. At the needle tip IpaB acts as a sensor for host cell contact but prior to secretion, it is stored as a heterodimeric complex with the chaperone IpgC. FRET analyses showed that chaperone binding to IpaB’s N-terminal domain causes a conformational change in the latter. These FRET analyses, with other biophysical methods, have been central to understanding T3SA maturation and will be highlighted, focusing on the details of the FRET measurements and the relevance to this particular system.

Intravital models of infection lay the foundation for tissue microbiology

In complex environments, such as those found in the human host, pathogenic bacteria constantly battle the unfavorable conditions imposed by the host response to their presence. During Escherichia coli-induced pyelonephritis, a cascade of events are shown in an intravital animal model to occur in a timely and sequential manner, representing the dynamic interplay between host and pathogen. Today, intravital techniques allow for observing infection in the living host. At resolutions almost on the single-cell level, improved detection methods offer a movie-like description of infection dynamics. Tissue microbiology involves monitoring host-pathogen interaction within the dynamic microecology of infectious sites in the live host. This new field holds great promise for insightful research into microbial disease intervention strategies.

Toxofilin upregulates the host cortical actin cytoskeleton dynamics, facilitating Toxoplasma invasion

Toxoplasma gondii, a human pathogen and a model apicomplexan parasite, actively and rapidly invades host cells. To initiate invasion, the parasite induces the formation of a parasite-cell junction, and progressively propels itself through the junction, inside a newly formed vacuole that encloses the entering parasite. Little is known about how a parasite that is a few microns in diameter overcomes the host cell cortical actin barrier to achieve the remarkably rapid process of internalization (less than a few seconds). Using correlative light and electron microscopy in conjunction with electron tomography and three-dimensional image analysis we identified that toxofilin, an actin-binding protein, secreted by invading parasites correlates with localized sites of disassembly of the host cell actin meshwork. Moreover, quantitative fluorescence speckle microscopy of cells expressing toxofilin showed that toxofilin regulates actin filament disassembly and turnover. Furthermore, Toxoplasma tachyzoites lacking toxofilin, were found to be impaired in cortical actin disassembly and exhibited delayed invasion kinetics. We propose that toxofilin locally upregulates actin turnover thus increasing depolymerization events at the site of entry that in turn loosens the local host cell actin meshwork, facilitating parasite internalization and vacuole folding.

Enteroaggregative Escherichia coli induced increase in intracellular calcium concentration modulates cytoskeletal F-actin rearrangement and bacterial entry in INT-407 cells

BACKGROUND

Enteroaggregative Escherichia coli (EAEC) is an emerging enteric pathogen, associated with cases of acute and persistent diarrhoea worldwide. The pathogenesis of EAEC is yet to be understood. In intestinal epithelium, an increase in [Ca²⁺](i) has been attributed due to the action of different enteric pathogens. EAEC was shown to increase [Ca²⁺](i) in HEp-2 cells.The present study was undertaken to investigate the effect of EAEC induced increase in [Ca²⁺](i) oncultured human intestinal epithelial cells.

METHODS

INT-407 cells were infected with EAEC (T8 strain) in the absence and presence of dantrolene (inhibitor of release of Ca²⁺ from intracellular stores)/verapamil (L-type Ca²⁺ channel blocker)/BAPTAAM (Ca²⁺ chelator)/U73122 (PLC inhibitor)/Cytochalasin-D (inhibitor of actin polymerization). [Ca²⁺](i) was estimated using Fura-2/AM. Cytoskeletal rearrangement was assessed by F-actin staining using TRITC-phalloidin. The invasiveness of EAEC-T8 to INT-407 cells was checked by electron microscopy and invasion assay.

RESULTS

A significant increase in [Ca²⁺](i) was observed in EAEC-T8 infected INT-407 cells, which was reduced in presence of dantrolene/verapamil/U73122. EAEC-T8 could induce cytoskeletal F-actin polymerization in INT-407 cells and was found to be invasive in nature. The cytoskeletal rearrangement as well as invasion of EAEC-T8 was attenuated in presence of U73122/dantrolene/BAPTA-AM/verapamil/cytochalasin D.

CONCLUSIONS

EAEC induced increase in [Ca²⁺](i) seems to play a major role in host cytoskeletal F-actin rearrangements leading to invasion of the organism.

GENERAL SIGNIFICANCE

Our study undoubtedly will lead to an improved understanding of EAEC-pathogenesis.

The rickettsia surface cell antigen 4 applies mimicry to bind to and activate vinculin

Pathogenic Rickettsia species cause high morbidity and mortality, especially R. prowazekii, the causative agent of typhus. Like many intracellular pathogens, Rickettsia exploit the cytoskeleton to enter and spread within the host cell. Here we report that the cell surface antigen sca4 of Rickettsia co-localizes with vinculin in cells at sites of focal adhesions in sca4-transfected cells and that sca4 binds to and activates vinculin through two vinculin binding sites (VBSs) that are conserved across all Rickettsia. Remarkably, this occurs through molecular mimicry of the vinculin-talin interaction that is also seen with the IpaA invasin of the intracellular pathogen Shigella, where binding of these VBSs to the vinculin seven-helix bundle head domain (Vh1) displaces intramolecular interactions with the vinculin tail domain that normally clamp vinculin in an inactive state. Finally, the vinculin·sca4-VBS crystal structures reveal that vinculin adopts a new conformation when bound to the C-terminal VBS of sca4. Collectively, our data define the mechanism by which sca4 activates vinculin and interacts with the actin cytoskeleton, and they suggest important roles for vinculin in Rickettsia pathogenesis.

Candida albicans interactions with epithelial cells and mucosal immunity

Candida albicans interactions with epithelial cells are critical for commensal growth, fungal pathogenicity and host defence. This review will outline our current understanding of C. albicans-epithelial interactions and will discuss how this may lead to the induction of a protective mucosal immune response.

Cytoskeleton-modulating effectors of enteropathogenic and enterohemorrhagic Escherichia coli: a case for EspB as an intrinsically less-ordered effector

Enterohemorrhagic and enteropathogenic Escherichia coli produce various effector proteins that are directly injected into the host-cell cytosol through the type III secretion system. E. coli secreted protein (Esp)B is one such effector protein, and affects host-cell morphology by reorganizing actin networks. Unlike most globular proteins that have well-ordered, rigid structures, the structures of type III secretion system effectors from pathogenic Gram-negative bacteria, including EspB, are often less well-ordered. This minireview focuses on the functional relationship between the structural properties of these proteins and their roles in type III secretion system-associated pathogenesis.

Carbon metabolism of intracellular bacterial pathogens and possible links to virulence

New technologies such as high-throughput methods and 13C-isotopologue-profiling analysis are beginning to provide us with insight into the in vivo metabolism of microorganisms, especially in the host cell compartments that are colonized by intracellular bacterial pathogens. In this Review, we discuss the recent progress made in determining the major carbon sources and metabolic pathways used by model intracellular bacterial pathogens that replicate either in the cytosol or in vacuoles of infected host cells. Furthermore, we highlight the possible links between intracellular carbon metabolism and the expression of virulence genes.

Entry of Burkholderia organisms into respiratory epithelium: CFTR, microfilament and microtubule dependence

BACKGROUND

The pathogenesis of infection with Burkholderia cepacia complex (Bcc) organisms may be linked to its capacity to invade respiratory epithelium.

METHODS

An antibiotic exclusion assay was used to study B. dolosa AU4459 and B. cenocepacia J2315 invasion into wild-type (WT) and CFTR-deficient respiratory epithelial cells. Inhibitors were used to evaluate Bcc invasion dependency on host microtubule (mt) and microfilament (mf) systems.

RESULTS

B. dolosa entered WT-CFTR cells with 5-fold greater efficiency than CFTR deficient cells (25% vs 5%, respectively). Invasion dropped to <0.5% after either mf or mt inhibition. B. cenocepacia entered WT (0.05%) and CFTR-deficient cells (0.07%) with similarly low efficiencies, which significantly decreased with either mf or mt inhibition (0.008% and 0.002%, respectively).

CONCLUSION

B. dolosa and B. cenocepacia enter respiratory epithelial cells in a mf and mt dependent fashion. Mutated CFTR leads to less internalization of B. dolosa, but not B. cenocepacia.

Identifying infection-associated genes of Candida albicans in the postgenomic era

The human pathogenic yeast Candida albicans can cause an unusually broad range of infections reflecting a remarkable potential to adapt to various microniches within the human host. The exceptional adaptability of C. albicans is mediated by rapid alterations in gene expression in response to various environmental stimuli and this transcriptional flexibility can be monitored with tools such as microarrays. Using such technology it is possible to (1) capture a genome-wide portrait of the transcriptome that mirrors the environmental conditions, (2) identify known genes, signalling pathways and transcription factors involved in pathogenesis, (3) identify new patterns of gene expression and (4) identify previously uncharacterized genes that may be associated with infection. In this review, we describe the molecular dissection of three distinct stages of infections, covering both superficial and invasive disease, using in vitro, ex vivo and in vivo infection models and microarrays.

Unc119 protects from Shigella infection by inhibiting the Abl family kinases

Background

Bacteria engage cell surface receptors and intracellular signaling molecules to enter the cell. Unc119 is an adaptor protein, which interacts with receptors and tyrosine kinases. Its role in bacterial invasion of cells is unknown.

Methodology/Principal Findings

We used biochemical, molecular and cell biology approaches to identify the binding partners of Unc119, and to study the effect of Unc119 on Abl family kinases and Shigella infection. We employed loss-of-function and gain-in-function approaches to study the effect of Unc119 in a mouse model of pulmonary shigellosis. Unc119 interacts with Abl family kinases and inhibits their kinase activity. As a consequence, it inhibits Crk phosphorylation, which is essential for Shigella infection. Unc119 co-localizes with Crk and Shigella in infected cells. Shigella infectivity increases in Unc119-deficient epithelial and macrophage cells. In a mouse model of shigellosis cell-permeable TAT-Unc119 inhibits Shigella infection. Conversely, Unc119 knockdown in vivo results in enhanced bacterial invasion and increased lethality. Unc119 is an inducible protein. Its expression is upregulated by probacteria and bacterial products such as lipopolysacharide and sodium butyrate. The latter inhibits Shigella infection in mouse lungs but is ineffective in Unc119 deficiency.

Conclusions

Unc119 inhibits signaling pathways that are used by Shigella to enter the cell. As a consequence it provides partial but significant protection from Shigella infections. Unc119 induction in vivo boosts host defense against infections.

The C-terminus of IpaC is required for effector activities related to Shigella invasion of host cells

Invasion plasmid antigen C (IpaC) is secreted by the Shigella flexneri type III secretion system (TTSS) as an essential trigger of epithelial cell invasion. At the molecular level, IpaC possesses a distinct functional organization. The IpaC C-terminal region between amino acids 319 and 345 is predicted to form a coiled-coil structure. Such alpha-helical motifs appear to be a recurring structural theme among TTSS components. Together with IpaB, this IpaC region is also required for the formation of translocon pores in target cell membranes. In contrast, mutations within the C-terminal tail of IpaC (defined by residues 345-363) have no effect on contact hemolysis (a putative measure of translocon pore formation), but they can contribute significantly to IpaC's ability to trigger S. flexneri entry into cultured cells. Here we describe the molecular dissection of the IpaC C-terminus and how changes in this region affect selected virulence-related activities. IpaC invasion function requires its immediate C-terminus and this general region may be involved in its ability to trigger actin nucleation. In contrast, IpaC could not be shown to interact directly with Cdc42, a host GTPase closely tied to Shigella invasion.

Host cell actin remodeling in response to Trypanosoma cruzi: trypomastigote versus amastigote entry

Trypanosoma cruzi is the protozoan parasite that causes Chagas' disease, a highly prevalent vector-borne disease in Latin America. Chagas' disease is a major public health problem in endemic regions with an estimated 18 million people are infected with T. cruzi and another 100 million at risk (http://www.who.int/ctd/chagas/disease.htm). During its life cycle, T. cruzi alternates between triatomine insect vectors and mammalian hosts. While feeding on host's blood, infected triatomines release in their feces highly motile and infective metacyclic trypomastigotes that may initiate infection. Metacyclic trypomastigotes promptly invade host cells (including gastric mucosa) and once free in the cytoplasm, differentiate into amastigotes that replicate by binary fission. Just before disruption of the parasite-laden cell, amastigotes differentiate back into trypomastigotes which are then released into the tissue spaces and access the circulation. Circulating trypomastigotes that disseminate the infection in the mammalian host may be taken up by feeding triatomines and may also transform, extracellularly, into amastigote-like forms. Unlike their intracellular counterparts, these amastigote-like forms, henceforth called amastigotes, are capable of infecting host cells. Studies in which the mechanisms of amastigote invasion of host cells have been compared to metacyclic trypomastigote entry have revealed interesting differences regarding the involvement of the target cell actin microfilament system.

Connexons and cell adhesion: a romantic phase

Recent evidence indicates, that gap junction forming proteins do not only contribute to intercellular communication (Kanno and Saffitz in Cardiovasc Pathol 10:169–177, 2001; Saez et al. in Physiol Rev 83:1359–1400, 2003), ion homeostasis and volume control (Goldberg et al. in J Biol Chem 277:36725–36730, 2002; Saez et al. in Physiol Rev 83:1359–1400, 2003). They also serve biological functions in a mechanical sense, supporting adherent connections between neighbouring cells of epithelial and non-epithelial tissues (Clair et al. in Exp Cell Res 314:1250–1265, 2008; Shaw et al. in Cell 128:547–560, 2007), where they stabilize migratory pathways in the developing central nervous system (Elias et al. in Nature 448:901–907, 2007; Malatesta et al. in Development 127:5253–5263, 2000; Noctor et al. in Nature 409:714–720, 2001; Rakic in Brain Res 33:471–476, 1971; J Comp Neurol 145:61–83 1972; Science 241:170–176, 1988), or mediate polarized movements and directionality of neural crest cells during organogenesis (Kirby and Waldo in Circ Res 77:211–215, 1995; Xu et al. in Development 133:3629–3639, 2006). Since, most data describing adhesive properties of gap junctions delt with connexin 43 (Cx43) (Beardslee et al. in Circ Res 83:629–635, 1998), we will focus our brief review on this isoform.

Muramylpeptide shedding modulates cell sensing of Shigella flexneri

Bacterial infections trigger the activation of innate immunity through the interaction of pathogen-associated molecular patterns (PAMPs) with pattern recognition molecules (PRMs). The nucleotide-binding oligomerization domain (Nod) proteins are intracellular PRMs that recognize muramylpeptides contained in peptidoglycan (PGN) of bacteria. It is still unclear how Nod1 physically interacts with PGN, a structure internal to the Gram-negative bacterial envelope. To contribute to the understanding of this process, we demonstrate that, like Escherichia coli, Bordetella pertussis and Neisseria gonorrheae, the Gram-negative pathogen Shigella spontaneously releases PGN fragments and that this process can be increased by inactivating either ampG or mppA, genes involved in PGN recycling. Both Shigella mutants, but especially the strain carrying the mppA deletion, trigger Nod1-mediated NF-kappaB activation to a greater extent than the wild-type strain. Likewise, muramylpeptides spontaneously shed by Shigella are able per se to trigger a Nod1-mediated response consistent with the relative amount. Finally, we found that qualitative changes in muramylpeptide shedding can alter in vivo host responses to Shigella infection. Our findings support the idea that muramylpeptides released by pathogens during infection could modulate the immune response through Nod proteins and thereby influence the outcome of disease.

IpaD is localized at the tip of the Shigella flexneri type III secretion apparatus

Type III secretion (T3S) systems are used by numerous Gram-negative pathogenic bacteria to inject virulence proteins into animal and plant host cells. The core of the T3S apparatus, known as the needle complex, is composed of a basal body transversing both bacterial membranes and a needle protruding above the bacterial surface. In Shigella flexneri, IpaD is required to inhibit the activity of the T3S apparatus prior to contact of bacteria with host and has been proposed to assist translocation of bacterial proteins into host cells. We investigated the localization of IpaD by electron microscopy analysis of cross-linked bacteria and mildly purified needle complexes. This analysis revealed the presence of a distinct density at the needle tip. A combination of single particle analysis, immuno-labeling and biochemical analysis, demonstrated that IpaD forms part of the structure at the needle tip. Anti-IpaD antibodies were shown to block entry of bacteria into epithelial cells.

Shigella applies molecular mimicry to subvert vinculin and invade host cells

Shigella flexneri, the causative agent of bacillary dysentery, injects invasin proteins through a type III secretion apparatus upon contacting the host cell, which triggers pathogen internalization. The invasin IpaA is essential for S. flexneri pathogenesis and binds to the cytoskeletal protein vinculin to facilitate host cell entry. We report that IpaA harbors two vinculin-binding sites (VBSs) within its C-terminal domain that bind to and activate vinculin in a mutually exclusive fashion. Only the highest affinity C-terminal IpaA VBS is necessary for efficient entry and cell–cell spread of S. flexneri, whereas the lower affinity VBS appears to contribute to vinculin recruitment at entry foci of the pathogen. Finally, the crystal structures of vinculin in complex with the VBSs of IpaA reveal the mechanism by which IpaA subverts vinculin's functions, where S. flexneri utilizes a remarkable level of molecular mimicry of the talin–vinculin interaction to activate vinculin. Mimicry of vinculin's interactions may therefore be a general mechanism applied by pathogens to infect the host cell.

A first global analysis of plasmid encoded proteins in the ACLAME database

Many plasmids are mobile genetic elements (MGEs) and, as other members of that group of DNA entities, their genomes display a mosaic and combinatorial structure, making their classification extremely difficult. As other MGEs, plasmids play a major role in horizontal transfer of genetic materials and genome reorganization. Yet, the full impact of such phenomenon on major properties of the host cell, such as pathogenicity, the ability to use new carbon sources or resistance to antibiotics, remains to be fully assessed. More and more complete plasmid genome sequences are available. However, in the absence of standards for storing plasmid sequence data and annotating genes and gene products on sequenced plasmid genomes, the resulting information remains rather limited. Using 503 sequenced plasmids organized in the ACLAME database, we discuss how, by structuring information on the genomes, their host and the proteins they code for, one can gain access to either global or more detailed analysis of the plasmid sequence information, as illustrated by a network representation of the relationships between plasmids.

High Vaccine Efficacy against Shigellosis of Recombinant NoninvasiveShigellaMutant That ExpressesYersiniaInvasin

Live attenuated Shigella vaccines elicit protective immune responses, but involve a potential risk of inducing a strong inflammatory reaction. The bacterial invasiveness that is crucial for Ag delivery causes inflammatory destruction of infected epithelial cells and proinflammatory cell death of infected macrophages. In this study, the noninvasive Shigella mutant DeltaipaB was equipped with Yersinia invasin protein, which has been shown to mediate bacterial invasion and targeting to M cells located in follicle-associated epithelium. Invasin-expressing DeltaipaB (DeltaipaB/inv) was internalized into epithelial cells and retained in the intraphagosomal space. DeltaipaB/inv did not induce necrotic cell death of infected macrophages nor cause symptomatic damage after intranasal vaccination of mice. DeltaipaB/inv was safer and more effective than the conventional live vaccine, DeltavirG. Infection by DeltaipaB/inv caused polymorphonuclear neutrophil infiltration in the lung, but did not induce production of large amounts of proinflammatory cytokines. We concluded that the low experimental morbidity and high vaccine efficacy of DeltaipaB/inv are primarily based on high protective immune responses, which may be enhanced by the polymorphonuclear neutrophil infiltration unaccompanied by tissue injury.

The discovery of SycO highlights a new function for type III secretion effector chaperones

Bacterial injectisomes deliver effector proteins straight into the cytosol of eukaryotic cells (type III secretion, T3S). Many effectors are associated with a specific chaperone that remains inside the bacterium when the effector is delivered. The structure of such chaperones and the way they interact with their substrate is well characterized but their main function remains elusive. Here, we describe and characterize SycO, a new chaperone for the Yersinia effector kinase YopO. The chaperone-binding domain (CBD) within YopO coincides with the membrane localization domain (MLD) targeting YopO to the host cell membrane. The CBD/MLD causes intrabacterial YopO insolubility and the binding of SycO prevents this insolubility but not folding and activity of the kinase. Similarly, SycE masks the MLD of YopE and SycT covers an aggregation-prone domain of YopT, presumably corresponding to its MLD. Thus, SycO, SycE and most likely SycT mask, inside the bacterium, a domain needed for proper localization of their cognate effector in the host cell. We propose that covering an MLD might be an essential function of T3S effector chaperones.

Interactions between effector proteins of the Pseudomonas aeruginosa type III secretion system do not significantly affect several measures of disease severity in mammals

The effector proteins of the type III secretion systems of many bacterial pathogens act in a coordinated manner to subvert host cells and facilitate the development and progression of disease. It is unclear whether interactions between the type-III-secreted proteins of Pseudomonas aeruginosa result in similar effects on the disease process. We have previously characterized the contributions to pathogenesis of the type-III-secreted proteins ExoS, ExoT and ExoU when secreted individually. In this study, we extend our prior work to determine whether these proteins have greater than expected effects on virulence when secreted in combination. In vitro cytotoxicity and anti-internalization activities were not enhanced when effector proteins were secreted in combinations rather than alone. Likewise in a mouse model of pneumonia, bacterial burden in the lungs, dissemination and mortality attributable to ExoS, ExoT and ExoU were not synergistically increased when combinations of these effector proteins were secreted. Because of the absence of an appreciable synergistic increase in virulence when multiple effector proteins were secreted in combination, we conclude that any cooperation between ExoS, ExoT and ExoU does not translate into a synergistically significant enhancement of disease severity as measured by these assays.

ShiBASE: an integrated database for comparative genomics of Shigella

Among the major enteric bacterial pathogens, Shigella is found to display extreme genome diversity and dynamics, which imposes a challenge in comparative genomic studies. To facilitate further studies in this area, we have constructed an integrated online database, ShiBASE (),which contains Shigella genomic sequences of four species and additional comparative genomic hybridization (CGH) data of 43 serotypes. ShiBASE offers online comparative analysis on DNA sequences, gene orders, metabolic pathways and virulence factors. In addition, ShiBASE has a newly developed online comparative visualization service, Shi-align, which enables the alignment of any query sequence with the reference genome sequences.

Role of Protein Phosphorylation on Serine/Threonine and Tyrosine in the Virulence of Bacterial Pathogens

Bacterial pathogens have developed a diversity of strategies to interact with host cells, manipulate their behaviors, and thus to survive and propagate. During the process of pathogenesis, phosphorylation of proteins on hydroxyl amino acids (serine, threonine, tyrosine) occurs at different stages, including cell-cell interaction and adherence, translocation of bacterial effectors into host cells, and changes in host cellular structure and function induced by infection. The phosphorylation reactions are catalyzed in a reversible fashion by specific protein kinases and phosphatases that belong to either the invading bacterial cells or the infected eukaryotic host cells. Among the various virulence factors involved in bacterial pathogenesis, special attention has been paid recently to the cell wall components, exopolysaccharides. A major breakthrough has been made by showing the existence of a biological link between the activity of certain protein-tyrosine kinases/phosphatases and the production and/or transport of surface polysaccharides. In addition, genetic studies have revealed a key role played by some serine/threonine kinases in pathogenesis. Considering the structural organization and membrane topology of these different kinases, it can be envisaged that they operate as one-component systems in signal transduction pathways, in the form of single proteins containing input and output domains on the same polypeptide chain. From a general standpoint, the demonstration of a direct relationship between protein phosphorylation on serine/threonine/tyrosine and bacterial virulence represents a novel concept of great importance in deciphering the molecular and cellular mechanisms that underlie pathogenesis.

Abl kinases regulate actin comet tail elongation via an N-WASP-dependent pathway

Microbial pathogens have evolved diverse strategies to modulate the host cell cytoskeleton to achieve a productive infection and have proven instrumental for unraveling the molecular machinery that regulates actin polymerization. Here we uncover a mechanism for Shigella flexneri-induced actin comet tail elongation that links Abl family kinases to N-WASP-dependent actin polymerization. We show that the Abl kinases are required for Shigella actin comet tail formation, maximal intracellular motility, and cell-to-cell spread. Abl phosphorylates N-WASP, a host cell protein required for actin comet tail formation, and mutation of the Abl phosphorylation sites on N-WASP impairs comet tail elongation. Furthermore, we show that defective comet tail formation in cells lacking Abl kinases is rescued by activated forms of N-WASP. These data demonstrate for the first time that the Abl kinases play a role in the intracellular motility and intercellular dissemination of Shigella and uncover a new role for Abl kinases in the regulation of pathogen motility.

Shigellaspp. and enteroinvasiveEscherichia colipathogenicity factors

Bacteria of Shigella spp. (S. boydii, S. dysenteriae, S. flexneri and S. sonnei) and enteroinvasive Escherichia coli (EIEC) are responsible for shigellosis in humans, a disease characterized by the destruction of the colonic mucosa that is induced upon bacterial invasion. Shigella spp. and EIEC strains contain a virulence plasmid of approximately 220 kb that encodes determinants for entry into epithelial cells and dissemination from cell to cell. This review presents the current model on mechanisms of invasion of the colonic epithelium by these bacteria and focuses on their pathogenicity factors, particularly the virulence plasmid-encoded type III secretion system.

Quantification of bacterial invasion into adherent cells by flow cytometry

Quantification of invasive, intracellular bacteria is critical in many areas of cellular microbiology and immunology. We describe a novel and fast approach to determine invasion of bacterial pathogens in adherent cell types such as epithelial cells or fibroblasts based on flow cytometry. Using the CEACAM-mediated uptake of Opa-expressing Neisseria gonorrhoeae as a well-characterized model of bacterial invasion, we demonstrate that the flow cytometry-based method yields results comparable to a standard antibiotic protection assay. Furthermore, the quantification of intracellular bacteria by the novel approach is not biased by intracellular killing of the microbes and correctly discriminates between cell-associated extracellular and bona fide intracellular bacteria. As flow cytometry-based quantification is also applicable to other pathogen-host interactions such as the integrin-mediated internalization of Staphylococcus aureus, this approach provides a fast and convenient alternative for the quantification of bacterial uptake and should be particularly useful in elucidating the molecular mechanisms of pathogen-triggered host cell invasion.

Bacteria-host-cell interactions at the plasma membrane: stories on actin cytoskeleton subversion

Exploitation of the host-cell actin cytoskeleton is pivotal for many microbial pathogens to enter cells, to disseminate within and between infected tissues, to prevent their uptake by phagocytic cells, or to promote intimate attachment to the cell surface. To accomplish this, these pathogens have evolved common as well as unique strategies to modulate actin dynamics at the plasma membrane, which will be discussed here, exemplified by a number of well-studied bacterial pathogens.

Identification of Mycobacterium avium genes that affect invasion of the intestinal epithelium

Invasion of intestinal mucosa of the host by Mycobacterium avium is a critical step in pathogenesis and likely involves several different bacterial proteins, lipids, glycoproteins, and/or glycolipids. Through the screening of an M. avium genomic library in Mycobacterium smegmatis, we have identified a number of M. avium genes that are associated with increased invasion of mucosal epithelial cells. In order to further investigate these genes, we cloned six of them into a plasmid downstream of a strong mycobacterial promoter (L5 mycobacterial phage promoter), resulting in constitutive expression. Bacteria were then evaluated for increased expression and examined for invasion of HT-29 intestinal epithelial cells. The genes identified encode proteins that are similar to (i) M. tuberculosis coenzyme A carboxylase, (ii) M. tuberculosis membrane proteins of unknown function, (iii) M. tuberculosis FadE20, (iv) a Mycobacterium paratuberculosis surface protein, and (v) M. tuberculosis cyclopropane fatty acyl-phopholipid synthase. The constitutive expression of these genes confers to M. avium the ability to invade HT-29 intestinal epithelial cells with a severalfold increase in efficiency compared to both the wild-type M. avium and M. avium containing the vector alone. Using the murine intestinal ligated loop model, it was observed that the constitutive expression of M. avium proteins has a modest impact on the ability to enter the intestinal mucosa when compared with the wild-type control, suggesting that under in vivo conditions these genes are expressed at higher levels. Evaluation of the expression of these invasion-related genes indicated that under conditions similar to the intestinal lumen environment, the genes identified are upregulated. These data suggest that invasion of the intestinal mucosa is an event that requires the participation of several bacterial factors and the expression of the genes that encode them is less observed under standard laboratory growth conditions.

Cortactin: an Achilles' heel of the actin cytoskeleton targeted by pathogens

Cortactin is an actin-binding protein and a central regulator of the actin cytoskeleton. Importantly, cortactin is also a common target exploited by microbes during infection. Its involvement in disease development is exemplified by a variety of pathogenic processes, such as pedestal formation [enteropathogenic and enterohaemorrhagic Escherichia coli (EPEC and EHEC)], invasion (Shigella, Neisseria, Rickettsia, Chlamydia, Staphylococcus and Cryptosporidium), actin-based motility (Listeria, Shigella and vaccinia virus) and cell scattering (Helicobacter). Recent progress turns our attention to how cortactin function can be regulated by serine and tyrosine phosphorylation. This has an important impact on how pathogens abuse cortactin to modulate the architecture of the host actin cytoskeleton.

Important bacterial gastrointestinal pathogens in children: a pathogenesis perspective

This article focuses on the five most common bacterial enteropathogens of the developed world--Helicobacter pylori, Escherichia coli, Shigella, Salmonella, and Campylobacter--from the perspective of how they cause disease and how they relate to each other. Basic and recurring themes of bacterial pathogenesis, including mechanisms of entry, methods of adherence, sites of cellular injury, role of toxins, and how pathogens acquire particular virulence traits (and antimicrobial resistance), are discussed.