SipA, SopA, SopB, SopD, and SopE2 contribute to Salmonella enterica serotype typhimurium invasion of epithelial cells

Salmonella translocates across an in vitro M cell model independently of SPI-1 and SPI-2

We have used an in vitro model of intestinal M cells to examine the mechanisms by which Salmonella enterica translocates across these specialized cells, which constitute a primary site of infection of the mammalian host. S. enterica can invade cultured cells by deploying a type III secretion system (TTSS) encoded within Salmonella pathogenicity island 1 (SPI-1) to translocate effector proteins into the host cell cytoplasm that trigger cellular responses, including prominent cytoskeletal rearrangements. After Salmonella enters the host cell, a second TTSS encoded in SPI-2 modulates intracellular trafficking and enables the bacteria to replicate within a modified vacuolar compartment. Within the host intestine, specialized antigen-sampling M cells, which reside in the epithelium overlying lymphoid tissues in the gut, are a preferential site of Salmonella invasion. The mechanisms of infection of M cells remain poorly defined and it is not known whether either SPI-1 or SPI-2 is required for infection of these cells. To address these questions we have employed an in vitro M cell model involving co-culture of polarized Caco-2 intestinal epithelial cells with Raji B cells. S. enterica serovar Typhimurium translocated across Caco-2/Raji co-cultures to a much greater extent than they cross native Caco-2 cell monolayers. Salmonella translocation was greatly reduced by heat treatment or fixation, suggesting that processes distinct from the sampling of inert particles are the main determinants of bacterial translocation. Translocation across both mono-cultured and co-cultured Caco-2 cells was partially inhibited by treatment with the dynamin inhibitor dynasore, but resistant to EIPA, an inhibitor of macropinocytosis. There was no difference between the abilities of wild-type Salmonella Typhimurium and mutants lacking multiple SPI-1 effectors to translocate across the M cell model, although the SPI-1 effector mutants were somewhat attenuated for translocation across native Caco-2 layers. There was also no difference between wild-type and SPI-2 mutants in M cell translocation. Together these data suggest that that SPI-1 and SPI-2 are dispensable for rapid M cell translocation and that infection at these specialized epithelial sites involves distinctive mechanisms that are not reliably modelled using conventional cell culture infection models.

Pseudogene accumulation in the evolutionary histories of Salmonella enterica serovars Paratyphi A and Typhi

BACKGROUND

Of the > 2000 serovars of Salmonella enterica subspecies I, most cause self-limiting gastrointestinal disease in a wide range of mammalian hosts. However, S. enterica serovars Typhi and Paratyphi A are restricted to the human host and cause the similar systemic diseases typhoid and paratyphoid fever. Genome sequence similarity between Paratyphi A and Typhi has been attributed to convergent evolution via relatively recent recombination of a quarter of their genomes. The accumulation of pseudogenes is a key feature of these and other host-adapted pathogens, and overlapping pseudogene complements are evident in Paratyphi A and Typhi.

RESULTS

We report the 4.5 Mbp genome of a clinical isolate of Paratyphi A, strain AKU_12601, completely sequenced using capillary techniques and subsequently checked using Illumina/Solexa resequencing. Comparison with the published genome of Paratyphi A ATCC9150 revealed the two are collinear and highly similar, with 188 single nucleotide polymorphisms and 39 insertions/deletions. A comparative analysis of pseudogene complements of these and two finished Typhi genomes (CT18, Ty2) identified several pseudogenes that had been overlooked in prior genome annotations of one or both serovars, and identified 66 pseudogenes shared between serovars. By determining whether each shared and serovar-specific pseudogene had been recombined between Paratyphi A and Typhi, we found evidence that most pseudogenes have accumulated after the recombination between serovars. We also divided pseudogenes into relative-time groups: ancestral pseudogenes inherited from a common ancestor, pseudogenes recombined between serovars which likely arose between initial divergence and later recombination, serovar-specific pseudogenes arising after recombination but prior to the last evolutionary bottlenecks in each population, and more recent strain-specific pseudogenes.

CONCLUSION

Recombination and pseudogene-formation have been important mechanisms of genetic convergence between Paratyphi A and Typhi, with most pseudogenes arising independently after extensive recombination between the serovars. The recombination events, along with divergence of and within each serovar, provide a relative time scale for pseudogene-forming mutations, affording rare insights into the progression of functional gene loss associated with host adaptation in Salmonella.

Impaired synthesis and secretion of SopA in Salmonella Typhimurium dam mutants

DNA adenine methylation regulates virulence gene expression in certain bacteria, including Salmonella Typhimurium. The aim of this study was to investigate the involvement of DNA adenine methylase (Dam) methylation in the expression and secretion of the SPI-1 effector protein SopA. For this purpose, SopA-FLAG-tagged wild-type and dam strains of Salmonella Typhimurium were constructed. The expression and secretion of SopA were determined in bacterial culture and in intracellular bacteria recovered from infected HEp-2 epithelial cells. Bacterial culture supernatants and pellets were used to investigate secreted proteins and cell-associated proteins, respectively. Western blot and quantitative reverse transcriptase PCR analysis showed that the dam mutant expresses lower levels of SopA than the wild-type strain. Interestingly, the strain lacking Dam synthesizes SopA under nonpermissive conditions (28 degrees C). In addition, SopA secretion was drastically impaired in the dam mutant. In vivo experiments showed that the intracellular Salmonella dam mutant synthesizes SopA although in lower amounts than the wild-type strain. Taken together, our results suggest that Dam methylation modulates the expression and secretion of SopA in Salmonella Typhimurium.

Porcine in vitro and in vivo models to assess the virulence of Salmonella enterica serovar Typhimurium for pigs

Salmonella Typhimurium infections in pigs pose an important human health hazard. One promising control measure is the development of live attenuated vaccine strains using defined knockout mutants. Preferably, screening of candidate knockout vaccine strains for attenuation should first be done in models allowing testing of a large number of strains. Thereafter, a limited number of selected strains should be further characterized in an experimental infection model in pigs. The aim of the present study was to develop such models. The invasive and proliferative characteristics of S. Typhimurium were assessed in both a non-polarized and a polarized porcine intestinal epithelial cell line. Neutrophils obtained from porcine blood were used to study the capacity of Salmonella to withstand killing by these phagocytes. The ability to induce an intestinal inflammatory response was investigated in a terminal intestinal loop model. The systemic phase of infection was mimicked by studying the uptake and intracellular survival of S. Typhimurium in porcine pulmonary alveolar macrophages and peripheral blood monocytes. These models should allow screening for attenuated strains. For further characterization, an experimental infection model was established, providing extensive data on the course of an oral infection and the optimal time points for colonization (day 5 postinoculation [pi]) and persistency (days 21–28 pi) in pigs. In conclusion, screening for virulence of S. Typhimurium strains with subsequent confirmation for a subset of strains in a well-defined experimental infection model would significantly reduce the number of experimental pigs required.

Salmonella enterica Typhimurium SipA induces CXC-chemokine expression through p38MAPK and JUN pathways

The role of Salmonella typhimurium type III secretion system (T3SS-1)-translocated proteins in chemokines' expression and protein phosphorylation was investigated in HeLa cells. Infection of HeLa cells with S. typhimurium activated IL-8 and GRO-alpha expression at higher levels than infection with a S. typhimurium sipAsopABDE2 mutant, confirming that T3SS-1-secreted proteins are required to fully induce chemokine expression in HeLa cells. A S. typhimurium sipAsopABDE2 mutant complemented with sipA or a strain carrying a chromosomal copy of sipA (sopABDE2 mutant) activated chemokines at significantly higher levels than a S. typhimurium sipAsopABDE2 mutant. However, extracellular addition of recombinant SipA failed to induce IL-8 expression. Phosphorylation analyses revealed that S. typhimurium induced a twofold increase in the phosphorylation of B23, CREB1, ERK1, JUN, p38MAPK, and NR1. JUN and p38MAPK were phosphorylated by S. typhimurium carrying a chromosomal copy of sipA (sopABDE2 mutant) while none was more than twofold phosphorylated in cells infected with the S. typhimurium sipAsopABDE2 mutant. Treating cells with JUN and p38MAPK inhibitors significantly decreased IL-8 expression in sopABDE2 mutant infected cells. These data indicate that S. typhimurium SipA induces expression of CXC chemokines through phosphorylation of IL-8-transcription regulatory proteins, JUN and p38MAK.

The potassium transporter Trk and external potassium modulate Salmonella enterica protein secretion and virulence

Potassium (K(+)) is the most abundant intracellular cation and is essential for many physiological functions of all living organisms; however, its role in the pathogenesis of human pathogens is not well understood. In this study, we characterized the functions of the bacterial Trk K(+) transport system and external K(+) in the pathogenesis of Salmonella enterica, a major food-borne bacterial pathogen. Here we report that Trk is important for Salmonella to invade and grow inside epithelial cells. It is also necessary for the full virulence of Salmonella in an animal infection model. Analysis of proteins of Salmonella indicated that Trk is involved in the expression and secretion of effector proteins of the type III secretion system (TTSS) encoded by Salmonella pathogenicity island 1 (SPI1) that were previously shown to be necessary for Salmonella invasion. In addition to the role of the Trk transporter in the pathogenesis of Salmonella, we discovered that external K(+) modulates the pathogenic properties of Salmonella by increasing the expression and secretion of effector proteins of the SPI1-encoded TTSS and by enhancing epithelial cell invasion. Our studies demonstrated that K(+) is actively involved in the pathogenesis of Salmonella and indicated that Salmonella may take advantage of the high K(+) content inside host cells and in the intestinal fluid during diarrhea to become more virulent.

Interaction of Salmonella enterica serovar Typhi with cultured epithelial cells: roles of surface structures in adhesion and invasion

In this study we investigate the ability of Salmonella enterica serovar Typhi (S. Typhi) surface structures to influence invasion and adhesion in epithelial cell assay systems. In general, S. Typhi was found to be less adherent, invasive and cytotoxic than S. enterica serovar Typhimurium (S. Typhimurium). Culture conditions had little effect on adhesion of S. Typhi to cultured cells but had a marked influence on invasion. In contrast, bacterial growth conditions did not influence S. Typhi apical invasion of polarized cells. The levels of S. Typhi, but not S. Typhimurium, invasion were increased by application of bacteria to the basolateral surface of polarized cells. Expression of virulence (Vi) capsule by S. Typhi resulted in a modest reduction in adhesion, but profoundly reduced levels of invasion of non-polarized cells. However, Vi capsule expression had no affect on invasion of the apical or basolateral surfaces of polarized cells. Mutation of the staA, tcfA or pilS genes did not affect invasion or adhesion in either the presence or the absence of Vi capsule.

The PagN protein of Salmonella enterica serovar Typhimurium is an adhesin and invasin

BACKGROUND

The pagN gene of Salmonella enterica serovar Typhimurium is a PhoP-regulated gene that is up-regulated during growth within macrophages and in vivo in murine models of infection. The PagN protein displays similarity to the Hek and Tia invasins/adhesins of Escherichia coli. Thus far no function has been ascribed to the PagN protein.

RESULTS

Here we show that the outer membrane located PagN protein mediates agglutination of red blood cells and that this can be masked by LPS. When expressed in Escherichia coli the PagN protein supports adhesion to and invasion of mammalian cells in a manner that is dependent on cytoskeletal rearrangements. S. enterica sv Typhimurium pagN mutants display a reduction in adhesion to and invasion of epithelial cells. Finally, we demonstrate that over-expression of PagN in a SPI-1 mutant can partially compensate for the lack of a functional invasasome.

CONCLUSION

PagN is an outer membrane protein that may contribute to the virulence of S. Typhimurium. This protein is a haemagglutinin and contributes to the adherence to mammalian cells. In addition, PagN can mediate high-level invasion of CHO-K1 cells. Previously,pagN mutants have been shown to be less competitive in vivo and thus this may be due to their lessened ability to interact with mammalian cells. Finally PagN can be added to an ever-growing repertoire of factors that contribute to the pathogenesis of Salmonella.

Stably integrated luxCDABE for assessment of Salmonella invasion kinetics

Salmonella Typhimurium is a common cause of gastroenteritis in humans and also localizes to neoplastic tumors in animals. Invasion of specific eukaryotic cells is a key mechanism of Salmonella interactions with host tissues. Early stages of gastrointestinal cell invasion are mediated by a Salmonella type III secretion system, powered by the adenosine triphosphatase invC. The aim of this work was to characterize the invC dependence of invasion kinetics into disparate eukaryotic cells traditionally used as models of gut epithelium or neoplasms. Thus, a nondestructive real-time assay was developed to report eukaryotic cell invasion kinetics using lux+ Salmonella that contain chromosomally integrated luxCDABE genes. Bioluminescence-based invasion assays using lux+ Salmonella exhibited inoculum dose-response correlation, distinguished invasion-competent from invasion-incompetent Salmonella, and discriminated relative Salmonella invasiveness in accordance with environmental conditions that induce invasion gene expression. In standard gentamicin protection assays, bioluminescence from lux+ Salmonella correlated with recovery of colony-forming units of internalized bacteria and could be visualized by bioluminescence microscopy. Furthermore, this assay distinguished invasion-competent from invasion-incompetent bacteria independent of gentamicin treatment in real time. Bioluminescence reported Salmonella invasion of disparate eukaryotic cell lines, including neoplastic melanoma, colon adenocarcinoma, and glioma cell lines used in animal models of malignancy. In each case, Salmonella invasion of eukaryotic cells was invC dependent.

Role for myosin II in regulating positioning of Salmonella-containing vacuoles and intracellular replication

Salmonella enterica serovar Typhimurium grows within host cells in a permissive compartment termed the Salmonella-containing vacuole (SCV). These bacteria use two distinct type III secretion systems (T3SS) to deliver virulence proteins (effectors) into cells. Effectors secreted by the Salmonella pathogenicity island 1 (SPI-1)-encoded T3SS mediate invasion and early SCV maturation steps, while those secreted by the SPI-2 T3SS affect the SCV at later stages postinfection. Some SPI-2 effectors modulate microtubule motor activity on the SCV. Here, we show that the actin-based motor myosin II also affects SCV dynamics during infection. Following invasion, myosin II is required for SCV positioning near the nucleus of host cells. Later, myosin II counteracts the activities of the SPI-2 effectors PipB2 and SseJ to maintain SCV positioning and stability, respectively. Myosin II activity was required for maximal bacterial growth in macrophages. Rho kinase activity was required for SCV positioning. The effector SopB, a known activator of Rho GTPases, was found to be required for SCV positioning, and transfection of cells with SopB was sufficient to induce myosin II phosphorylation. These studies reveal a novel role for myosin II in controlling SCV dynamics during infection and suggest that SopB activates myosin II.

Campylobacter jejuni CsrA mediates oxidative stress responses, biofilm formation, and host cell invasion

The putative global posttranscriptional regulator csrA was mutated in Campylobacter jejuni 81-176. The csrA mutant was attenuated in surviving oxidative stress. CsrA also contributed to biofilm formation and adherence to and invasion of INT407 intestinal epithelial cells, suggesting a regulatory role for CsrA in C. jejuni pathogenesis.

Salmonella enterica serovar Senftenberg human clinical isolates lacking SPI-1

Nontyphoidal Salmonella species cause gastrointestinal disease worldwide. The prevailing theory of Salmonella enteropathogenesis is that bacterial invasion of the intestinal epithelium is essential for virulence and that this requires the virulence-associated genomic region Salmonella pathogenicity island 1 (SPI-1). Recent studies of Salmonella enterica infection models have demonstrated that enterocolitis and diarrhea in mice and cows can occur independently of SPI-1. In this study, we sought to confirm whether two S. enterica serovar Senftenberg clinical isolates lacked genes essential for SPI-1 function. Two clinical strains were isolated and identified as being S. enterica serovar Senftenberg from four stool samples from a food-borne disease outbreak affecting seven individuals in Shenzhen, Guangdong Province, China, using conventional methods, pulsed-field gel electrophoresis and multilocus sequence typing. The possibility of coinfection with other potential bacteria or usual viruses was excluded. Two isolates were analyzed for the presence of invA, sipA, ssaR, sifA, and sopE2 by PCR and Southern blotting and were then assayed for the presence of SPI-1 by PCR and long-range PCR for fhlA-hilA, hilA-spaP, and spaP-invH and Southern blot analysis. A long-range PCR fragment from fhlA to mutS covering the 5' and 3' flanks of SPI-1 was also amplified from the two clinical isolates and sequenced. In addition, the two clinical isolates were assayed for enteroinvasiveness in vitro. Murine infection models were also examined. Biochemical tests and serotyping confirmed that the two clinical isolates are S. enterica serovar Senftenberg. However, they lacked genes critical for SPI-1 function but contained SPI-2 genes and were attenuated for the invasion of cultured intestinal epithelial cells. In conclusion, clinical S. enterica serovar Senftenberg strains isolated from a food-borne disease outbreak lack the invasion-associated locus SPI-1, indicating that SPI-1 is not essential for human gastroenteritis.

Salmonellae interplay with host cells

Salmonellae are important causes of enteric diseases in all vertebrates. Characterization of the molecular mechanisms that underpin the interactions of salmonellae with their animal hosts has advanced greatly over the past decade, mainly through the study of Salmonella enterica serovar Typhimurium in tissue culture and animal models of infection. Knowledge of these bacterial processes and host responses has painted a dynamic and complex picture of the interaction between salmonellae and animal cells. This Review focuses on the molecular mechanisms of these host-pathogen interactions, in terms of their context, significance and future perspectives.

Pathogenesis of enteric Salmonella infections

PURPOSE OF REVIEW

Gastrointestinal disease caused by Salmonella species leads to significant morbidity and mortality worldwide. The use of various animal models has greatly advanced understanding of Salmonella pathogenesis at intestinal and systemic sites. This review will emphasize recent advances in the understanding of intestinal Salmonella infections.

RECENT FINDINGS

Recent research has focused on bacterial products and the host pathogen recognition receptors involved in the activation of immune pathways. In particular, activation of Toll-like receptor 5 and Ipaf by Salmonella flagellin has been a major finding. The discovery of cryptopatches as novel lymphoid follicles and the characterization of intestinal dendritic cell populations have been examined in the context of Salmonella infections. The development and use of the streptomycin pretreated mouse model of enterocolitis has allowed researchers to probe the host factors contributing to intestinal immunopathology. Furthermore, the analysis of microbiota in Salmonella infections has provided new insights regarding the role of inflammation in gastrointestinal diseases. In addition, the contributions of specific Salmonella type 3-secreted effectors to the establishment and modulation of inflammation have been further refined.

SUMMARY

New advances in animal models have allowed researchers to further define the contribution of specific bacterial and host factors involved in Salmonella-induced enterocolitis.

SipA, SopA, SopB, SopD and SopE2 effector proteins of Salmonella enterica serovar Typhimurium are synthesized at late stages of infection in mice

Salmonella pathogenicity island (SPI)-1 is essential for invasion of non-phagocytic cells, whereas SPI-2 is required for intracellular survival and proliferation in phagocytes. Some SPI-1 effectors, however, are induced upon invasion of both phagocytic and non-phagocytic cells, suggesting that they may also be required post-invasion. In the present work, the presence was analysed of SipA, SopA, SopB, SopD and SopE2 effector proteins of Salmonella enterica serovar Typhimurium in vitro and in vivo during murine salmonellosis. Tagged (3xFLAG) strains of S. enterica serovar Typhimurium were inoculated intraperitoneally or intragastrically to BALB/c mice and recovered from the spleen and mesenteric lymph nodes of moribund mice. Tagged proteins were detected by SDS-PAGE and immunoblotting with anti-FLAG antibodies. In vitro experiments showed that SPI-1 effector proteins SipA, SopA, SopB, SopD and SopE2 were secreted under SPI-1 conditions. Interestingly, it was found that S. enterica serovar Typhimurium continued to synthesize SipA, SopB, SopD and SopE2 in colonized organs for several days, regardless of the route of inoculation. Together, these results indicate that SPI-1 effector proteins may participate in the late stages of Salmonella infection in mice.

Type III secretion systems and disease

Type III secretion systems (T3SSs) are complex bacterial structures that provide gram-negative pathogens with a unique virulence mechanism enabling them to inject bacterial effector proteins directly into the host cell cytoplasm, bypassing the extracellular milieu. Although the effector proteins vary among different T3SS pathogens, common pathogenic mechanisms emerge, including interference with the host cell cytoskeleton to promote attachment and invasion, interference with cellular trafficking processes, cytotoxicity and barrier dysfunction, and immune system subversion. The activity of the T3SSs correlates closely with infection progression and outcome, both in animal models and in human infection. Therefore, to facilitate patient care and improve outcomes, it is important to understand the T3SS-mediated virulence processes and to target T3SSs in therapeutic and prophylactic development efforts.

The Vi-capsule prevents Toll-like receptor 4 recognition of Salmonella

The viaB locus enables Salmonella enterica serotype Typhi to reduce Toll-like receptor (TLR) dependent cytokine production in tissue culture models. This DNA region contains genes involved in the regulation (tviA), biosynthesis (tviBCDE) and export (vexABCDE) of the Vi capsule. Expression of the Vi capsule in S. Typhimurium, but not expression of the TviA regulatory protein, reduced tumour necrosis factor-alpha (TNF-alpha) and IL-6 production by murine bone-marrow derived macrophages. Production of TNF-alpha and IL-6 was dependent on expression of TLR4 as stimulation of macrophages from TLR4(-/-) mice with S. Typhimurium did not result in expression of these cytokines. Intraperitoneal infection of mice with S. Typhimurium induced expression of TNF-alpha and inducible nitric oxide synthase (iNOS) in the liver. Introduction of the cloned viaB region into S. Typhimurium reduced TNF-alpha and iNOS expression to levels observed after infection with a S. Typhimurium msbB mutant. In contrast, no differences in TNF-alpha expression between the S. Typhimurium wild type and strains expressing the Vi-capsule or carrying a mutation in msbB were observed after infection of TLR4(-/-) mice. We conclude that the Vi capsule prevents both in vitro and in vivo recognition of S. Typhimurium lipopolysaccharide by TLR4.

A small non-coding RNA of the invasion gene island (SPI-1) represses outer membrane protein synthesis from the Salmonella core genome

The Salmonella pathogenicity island (SPI-1) encodes approximately 35 proteins involved in assembly of a type III secretion system (T3SS) which endows Salmonella with the ability to invade eukaryotic cells. We have discovered a novel SPI-1 gene, invR, which expresses an abundant small non-coding RNA (sRNA). The invR gene, which we identified in a global search for new Salmonella sRNA genes, is activated by the major SPI-1 transcription factor, HilD, under conditions that favour host cell invasion. The RNA chaperone, Hfq, is essential for the in vivo stability of the approximately 80 nt InvR RNA. Hfq binds InvR with high affinity in vitro, and InvR co-immunoprecipitates with FLAG epitope-tagged Hfq in Salmonella extracts. Surprisingly, deletion/overexpression of invR revealed no phenotype in SPI-1 regulation. In contrast, we find that InvR represses the synthesis of the abundant OmpD porin encoded by the Salmonella core genome. As invR is conserved in the early branching Salmonella bongori, we speculate that porin repression by InvR may have aided successful establishment of the SPI-1 T3SS after horizontal acquisition in the Salmonella lineage. This study identifies the first regulatory RNA of an enterobacterial pathogenicity island, and new roles for Hfq and HilD in SPI-1 gene expression.

The capsule encoding the viaB locus reduces interleukin-17 expression and mucosal innate responses in the bovine intestinal mucosa during infection with Salmonella enterica serotype Typhi

The viaB locus contains genes for the biosynthesis and export of the Vi capsular antigen of Salmonella enterica serotype Typhi. Wild-type serotype Typhi induces less CXC chemokine production in tissue culture models than does an isogenic viaB mutant. Here we investigated the in vivo relevance of these observations by determining whether the presence of the viaB region prevents inflammation in two animal models of gastroenteritis. Unlike S. enterica serotype Typhimurium, serotype Typhi or a serotype Typhi viaB mutant did not elicit marked inflammatory changes in the streptomycin-pretreated mouse model. In contrast, infection of bovine ligated ileal loops with a serotype Typhi viaB mutant resulted in more fluid accumulation and higher expression of the chemokine growth-related oncogene alpha (GROalpha) and interleukin-17 (IL-17) than did infection with the serotype Typhi wild type. There was a marked upregulation of IL-17 expression in both the bovine ligated ileal loop model and the streptomycin-pretreated mouse model, suggesting that this cytokine is an important component of the inflammatory response to infection with Salmonella serotypes. Introduction of the cloned viaB region into serotype Typhimurium resulted in a significant reduction of GROalpha and IL-17 expression and in reduced fluid secretion. Our data support the idea that the viaB region plays a role in reducing intestinal inflammation in vivo.

SopD acts cooperatively with SopB during Salmonella enterica serovar Typhimurium invasion

The intracellular bacterial pathogen, Salmonella enterica serovar Typhimurium (S. typhimurium), causes disease in a variety of hosts. To invade and replicate in host cells, these bacteria subvert host molecular machinery using bacterial proteins, called effectors, which they translocate into host cells using specialized protein delivery systems. One of these effectors, SopD, contributes to gastroenteritis, systemic virulence and persistence of S. typhimurium in animal models of infection. Recently, SopD has been implicated in invasion of polarized epithelial cells and here we investigate the features of SopD-mediated invasion. We show that SopD plays a role in membrane fission and macropinosome formation during S. typhimurium invasion, events previously shown to be mediated by the SopB effector. We further demonstrate that SopD acts cooperatively with SopB to promote these events during invasion. Using live cell imaging we show that a SopD-GFP fusion does not localize to HeLa cell cytosol as previously described, but instead is membrane associated. Upon S. typhimurium infection of these cells, SopD-GFP is recruited to the invasion site, and this recruitment required the phosphatase activity of SopB. Our findings demonstrate a role for SopD in manipulation of host-cell membrane during S. typhimurium invasion and reveal the nature of its cooperative action with SopB.

IQGAP1 regulates Salmonella invasion through interactions with actin, Rac1, and Cdc42

To infect host cells, Salmonella utilizes an intricate system to manipulate the actin cytoskeleton and promote bacterial uptake. Proteins injected into the host cell by Salmonella activate the Rho GTPases, Rac1 and Cdc42, to induce actin polymerization. Following uptake, a different set of proteins inactivates Rac1 and Cdc42, returning the cytoskeleton to normal. Although the signaling pathways allowing Salmonella to invade host cells are beginning to be understood, many of the contributing factors remain to be elucidated. IQGAP1 is a multidomain protein that influences numerous cellular functions, including modulation of Rac1/Cdc42 signaling and actin polymerization. Here, we report that IQGAP1 regulates Salmonella invasion. Through its interaction with actin, IQGAP1 co-localizes with Rac1, Cdc42, and actin at sites of bacterial uptake, whereas infection promotes the interaction of IQGAP1 with both Rac1 and Cdc42. Knockdown of IQGAP1 significantly reduces Salmonella invasion and abrogates activation of Cdc42 and Rac1 by Salmonella. Overexpression of IQGAP1 significantly increases the ability of Salmonella to enter host cells and required interaction with both actin and Cdc42/Rac1. Together, these data identify IQGAP1 as a novel regulator of Salmonella invasion.

Delineation of the Salmonella enterica serovar Typhimurium HilA regulon through genome-wide location and transcript analysis

The Salmonella enterica serovar Typhimurium HilA protein is the key regulator for the invasion of epithelial cells. By a combination of genome-wide location and transcript analysis, the HilA-dependent regulon has been delineated. Under invasion-inducing conditions, HilA binds to most of the known target genes and a number of new target genes. The sopB, sopE, and sopA genes, encoding effector proteins secreted by the type III secretion system on Salmonella pathogenicity island 1 (SPI-1), were identified as being both bound by HilA and differentially regulated in an HilA mutant. This suggests a cooperative role for HilA and InvF in the regulation of SPI-1-secreted effectors. Also, siiA, the first gene of SPI-4, is both bound by HilA and differentially regulated in an HilA mutant, thus linking this pathogenicity island to the invasion key regulator. Finally, the interactions of HilA with the SPI-2 secretion system gene ssaH and the flagellar gene flhD imply a repressor function for HilA under invasion-inducing conditions.

Applications of cell imaging in Salmonella research

Salmonella enterica is a Gram-negative enteropathogen that can cause localized infections, typically resulting in gastroenteritis, or systemic infection, e.g., typhoid fever, in both humans and warm-blooded animals. Understanding the mechanisms by which Salmonella induce disease has been the focus of intensive research. This has revealed that Salmonella invasion requires dynamic cross-talk between the microbe and host cells, in which bacterial adherence rapidly leads to a complex sequence of cellular responses initiated by proteins translocated into the host cell by a type III secretion system (T3SS). Once these Salmonella-induced responses have resulted in bacterial invasion, proteins translocated by a second T3SS initiate further modulation of cellular activities to enable survival and replication of the invading pathogen. These processes contribute to Salmonella entry into the host and the clinical symptoms of gastrointestinal and systemic infection. Elucidation of the complex and highly dynamic pathogen-host interactions ultimately requires analysis at the level of single cells and single infection events. To achieve this goal, researchers have applied a diverse range of microscopical methods to examine Salmonella infection in models ranging from whole animal to isolated cells and simple eukaryotic organisms. For example, electron microscopy and confocal microscopy can reveal the juxtaposition of Salmonella, its products, and cellular components at high resolution. Simple light microscopy (LM) can also be used to investigate the interaction of bacteria with host cells and has advantages for live cell imaging, which enables detailed analysis of the dynamics of infection and cellular responses. Here we review the use of imaging techniques in Salmonella research and compare the capabilities of different classes of microscope to address specific types of research question. We also provide protocols and notes on several LM techniques routinely used in our own research.

Intestinal innate immunity and the pathogenesis of Salmonella enteritis

Acute gastroenteritis caused by Salmonella typhimurium infection is a clinical problem with significant public health impact. The availability of several experimental models of this condition has allowed detailed investigation of the cellular and molecular interactions involved in its pathogenesis. Such studies have shed light on the roles played by bacterial virulence factors and host innate immune mechanisms in the development of intestinal inflammation.

Salmonella, the host and disease: a brief review

Salmonella species cause substantial morbidity, mortality and burden of disease globally. Infections with Salmonella species cause multiple clinical syndromes. Central to the pathophysiology of all human salmonelloses is the induction of a strong host innate immune/inflammatory response. Whether this ultimately reflects an adaptive advantage to the host or pathogen is not clear. However, it is evident that both the host and pathogen have evolved mechanisms of triggering host responses that are detrimental to the other. In this review, we explore some of the host and pathogenic mechanisms mobilized in the two predominant clinical syndromes associated with infection with Salmonella enterica species: enterocolitis and typhoid.

From insects to human hosts: Identification of major genomic differences between entomopathogenic strains of Photorhabdus and the emerging human pathogen Photorhabdus asymbiotica

Pathogenic bacteria of the genus Photorhabdus are naturally found in symbiotic association with soil entomopathogenic nematodes, and are of increasing economic interest in view of their potential for the development of novel biopesticides. This bipartite natural system is currently used for the biological control of crop pests in several countries. However, an increasing number of Photorhabdus strains have recently been isolated from human clinical specimens in both the United States and Australia, associated with locally invasive soft tissue infections and disseminated bacteraemia. In view of their growing use in biological control, which increases the potential rate of exposure of humans to these pathogens, we decided to undertake a comparative study of the genomic differences between insect and human pathogenic strains of Photorhabdus, in an attempt to understand the genetic mechanisms involved in the apparent change of host specificity, presumably responsible for their recently acquired capacity to infect humans. The data presented here demonstrates that major genomic differences exist between strains of Photorhabdus exhibiting virulence against insects or humans. Several individual genes, coding for virulence factors, were isolated and shown to be specific to the Photorhabdus asymbiotica human pathogens. One of these genes, sopB, encoding a host cell invasion factor translocated via the type III secretion system, has been cloned and the comparison of its genomic context in different pathogens strongly indicates that horizontal gene transfer is implicated in the acquisition of these virulence factors specific to the human pathogens. The precise role of this and other virulence factors identified here in the pathogenicity of P. asymbiotica towards humans is currently under investigation.

Salmonella Typhimurium SPI-1 genes promote intestinal but not tonsillar colonization in pigs

Salmonella Pathogenicity Island 1 (SPI-1) genes are indispensable for virulence of Salmonella Typhimurium in several animal species. The role of SPI-1 in the pathogenesis of Salmonella Typhimurium infections of pigs, however, is not well described. The interactions of a porcine Salmonella Typhimurium field strain and its isogenic mutants with disruptions in the SPI-1 genes hilA, sipA and sipB with porcine intestinal epithelial cells were characterized in vitro and in a ligated intestinal loop model in pigs. HilA and SipB were essential in the invasion of porcine intestinal epithelial cells in vitro. A sipA mutant was impaired for invasion using a polarized cell line, but fully invasive in a non-polarized cell line. All SPI-1 mutants induced a significant decrease in influx of neutrophils in the porcine intestinal loop model compared with the wild type strain. Pigs were orally inoculated with 10(8) colony forming units of both the wild type Salmonella Typhimurium strain and its isogenic sipB::kan mutant strain. The sipB mutant strain was significantly impaired to invade the intestinal, but not the tonsillar tissue, one day after inoculation and was unable to efficiently colonize the intestines and the GALT, but not the tonsils, 3 days after inoculation. This study shows that SPI-1 plays a crucial role in the invasion and colonization of the porcine gut and in the induction of influx of neutrophils towards the intestinal lumen, but not in the colonization of the tonsils.

The inflammation‐associatedSalmonellaSopA is a HECT‐like E3 ubiquitin ligase

Salmonella translocate a group of type III effectors into the host cells to induce entry, promote survival and cause intestinal inflammation. Although the biochemical and cellular mechanisms of how bacterial effectors function inside host cells remain largely unknown, studies have indicated that a likely strategy is to exploit host cellular pathways through functional mimicry. We report here that SopA, a Salmonella type III effector, mimics the mammalian HECT E3 ubiquitin ligase. SopA preferentially uses the host UbcH5a, UbcH5c and UbcH7 as E2s, which are involved in inflammation. Both the wild-type SopA and the mutant SopAC753S were expressed and translocated at similar levels during the infection of HeLa cells. A Salmonella strain expressing a catalytically incompetent SopAC753S mutant had reduced Salmonella-induced polymorphonuclear leukocytes transepithelial migration. We speculate that SopA ubiquitinate bacterial/host proteins involved in Salmonella-induced intestinal inflammation.

Transcription modulation of Salmonella enterica serovar Typhimurium promoters by sub-MIC levels of rifampin

Promoter-lux fusions that showed rifampin-modulated transcription were identified from a Salmonella enterica serovar Typhimurium 14028 reporter library. The transformation of a subset of fusions into mutants that lacked one of six global regulatory proteins or were rifampin resistant showed that transcription modulation was independent of the global regulators, promoter specific, and dependent on the interaction of rifampin with RNA polymerase.

Salmonella enterica serovar Typhimurium effectors SopB, SopE, SopE2 and SipA disrupt tight junction structure and function

Salmonella enterica serovar Typhimurium is a major cause of human gastroenteritis. Infection of epithelial monolayers by S. Typhimurium disrupts tight junctions that normally maintain the intestinal barrier and regulate cell polarity. Tight junction disruption is dependent upon the Salmonella pathogenicity island-1 (SPI-1) type 3 secretion system but the specific effectors involved have not been identified. In this study we demonstrate that SopB, SopE, SopE2 and SipA are the SPI-1-secreted effectors responsible for disruption of tight junction structure and function. Tight junction disruption by S. Typhimurium was prevented by inhibiting host protein geranylgeranylation but was not dependent on host protein synthesis or secretion of host-derived products. Unlike wild-type S. Typhimurium, DeltasopB, DeltasopE/E2, DeltasipA, or DeltasipA/sopB mutants, DeltasopB/E/E2 and DeltasipA/sopE/E2 mutants were unable to increase the permeability of polarized epithelial monolayers, did not disrupt the distribution or levels of ZO-1 and occludin, and did not alter cell polarity. These data suggest that SPI-1-secreted effectors utilize their ability to stimulate Rho family GTPases to disrupt tight junction structure and function.

Neutrophil influx during non-typhoidal salmonellosis: who is in the driver's seat?

A massive neutrophil influx in the intestine is the histopathological hallmark of Salmonella enterica serovar Typhimurium-induced enterocolitis in humans. Two major hypotheses on the mechanism leading to neutrophil infiltration in the intestinal mucosa have emerged. One hypothesis suggests that S. enterica serovar Typhimurium takes an active role in triggering this host response by injecting proteins, termed effectors, into the host cell cytosol which induce a proinflammatory gene expression profile in the intestinal epithelium. The second hypothesis suggests a more passive role for the pathogen by proposing that bacterial invasion stimulates the innate pathways of inflammation because the pathogen-associated molecular patterns of S. enterica serovar Typhimurium are recognized by pathogen recognition receptors on cells in the lamina propria. A review of the current literature reveals that, while pathogen recognition receptors are clearly involved in eliciting neutrophil influx during S. enterica serovar Typhimurium infection, a direct contribution of effectors in triggering proinflammatory host cell responses cannot currently be ruled out.

Three-dimensional organotypic models of human colonic epithelium to study the early stages of enteric salmonellosis

In vitro cell culture models used to study how Salmonella initiates disease at the intestinal epithelium would benefit from the recognition that organs and tissues function in a three-dimensional (3-D) environment and that this spatial context is necessary for development of cultures that more realistically resemble in vivo tissues/organs. Our aim was to establish and characterize biologically meaningful 3-D models of human colonic epithelium and apply them to study the early stages of enteric salmonellosis. The human colonic cell line HT-29 was cultured in 3-D and characterized by immunohistochemistry, histology, and scanning electron microscopy. Wild-type Salmonella typhimurium and an isogenic SPI-1 type three secretion system (TTSS) mutant derivative (invA) were used to compare the interactions with 3-D cells and monolayers in adherence/invasion, tissue pathology, and cytokine expression studies. The results showed that 3-D culture enhanced many characteristics normally associated with fully differentiated, functional intestinal epithelia in vivo, including better organization of junctional, extracellular matrix, and brush-border proteins, and highly localized mucin production. Wild-type Salmonella demonstrated increased adherence, but significantly lower invasion for 3-D cells. Interestingly, the SPI-I TTSS mutant showed wild-type ability to invade into the 3-D cells but did not cause significant structural changes to these cells. Moreover, 3-D cells produced less interleukin-8 before and after Salmonella infection. These results suggest that 3-D cultures of human colonic epithelium provide valuable alternative models to study human enteric salmonellosis with potential for novel insight into Salmonella pathogenesis.

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.

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

Knowledge about the origin and identity of the microbial products recognized by the innate immune system is important for understanding the pathogenesis of inflammatory diseases. We investigated the potential role of Salmonella enterica serotype Typhimurium fimbriae as pathogen-associated molecular patterns (PAMPs) that may stimulate innate pathways of inflammation. We screened a panel of 11 mutants, each carrying a deletion of a different fimbrial operon, for their enteropathogenicity using the calf model of human gastroenteritis. One mutant (csgBA) was attenuated in its ability to elicit fluid accumulation and GROalpha mRNA expression in bovine ligated ileal loops. The mechanism by which thin curled fimbriae encoded by the csg genes contribute to inflammation was further investigated using tissue culture. The S. Typhimurium csgBA mutant induced significantly less IL-8 production than the wild type in human macrophage-like cells. Purified thin curled fimbriae induced IL-8 expression in human embryonic kidney (HEK293) cells transfected with Toll-like receptor (TLR) 2/CD14 but not in cells transfected with TLR5, TLR4/MD2/CD14 or TLR11. Fusion proteins between the major fimbrial subunit of thin curled fimbriae (CsgA) and glutathione-S-transferase (GST) elicited IL-8 production in HEK293 cells transfected with TLR2/CD14. Proteinase K treatment abrogated IL-8 production elicited in these cells by GST-CsgA, but not by synthetic lipoprotein. GST-CsgA elicited more IL-6 production than GST in bone marrow-derived macrophages from TLR2+/+ mice, while there was no difference in IL-6 secretion between GST-CsgA and GST in macrophages from TLR2-/- mice. These data suggested that CsgA is a PAMP that is recognized by TLR2.

Type III secretion: what's in a name?

The term 'type III secretion' has seen widespread use. However, problems persist in nomenclature. We propose that the standard abbreviation for this kind of secretion should be 'T3S' and that 'type III secretion system' should be abbreviated to 'T3SS'. There is also a need for a new terminology to distinguish flagellar and non-flagellar type III secretion systems that reflects their common evolutionary ancestry but does not obscure their distinctive features. Finally, the use of the term 'type III secretion' to cover cytolysin-mediated translocation is to be deprecated because an authentic type III secretion system has already been described in gram-positive bacteria, namely the flagellar protein export apparatus.

Virulence of broad- and narrow-host-range Salmonella enterica serovars in the streptomycin-pretreated mouse model

Salmonella enterica subspecies I serovars are common bacterial pathogens causing diseases ranging from enterocolitis to systemic infections. Some serovars are adapted to specific hosts, whereas others have a broad host range. The molecular mechanisms defining the virulence characteristics and the host range of a given S. enterica serovar are unknown. Streptomycin pretreated mice provide a surrogate host model for studying molecular aspects of the intestinal inflammation (colitis) caused by serovar Typhimurium (S. Hapfelmeier and W. D. Hardt, Trends Microbiol. 13:497-503, 2005). Here, we studied whether this animal model is also useful for studying other S. enterica subspecies I serovars. All three tested strains of the broad-host-range serovar Enteritidis (125109, 5496/98, and 832/99) caused pronounced colitis and systemic infection in streptomycin pretreated mice. Different levels of virulence were observed among three tested strains of the host-adapted serovar Dublin (SARB13, SD2229, and SD3246). Several strains of host restricted serovars were also studied. Two serovar Pullorum strains (X3543 and 449/87) caused intermediate levels of colitis. No intestinal inflammation was observed upon infection with three different serovar Paratyphi A strains (SARB42, 2804/96, and 5314/98) and one serovar Gallinarum strain (X3796). A second serovar Gallinarum strain (287/91) was highly virulent and caused severe colitis. This strain awaits future analysis. In conclusion, the streptomycin pretreated mouse model can provide an additional tool to study virulence factors (i.e., those involved in enteropathogenesis) of various S. enterica subspecies I serovars. Five of these strains (125109, 2229, 287/91, 449/87, and SARB42) are subject of Salmonella genome sequencing projects. The streptomycin pretreated mouse model may be useful for testing hypotheses derived from this genomic data.

Host restriction of Salmonella enterica serotype Typhi is not caused by functional alteration of SipA, SopB, or SopD

Salmonella enterica serotype Typhi is a strictly human adapted pathogen that does not cause disease in nonprimate vertebrate hosts, while Salmonella enterica serotype Typhimurium is a broad-host-range pathogen. Serotype Typhi lacks some of the proteins (effectors) exported by the invasion-associated type III secretion system that are required by serotype Typhimurium for eliciting fluid secretion and inflammation in bovine ligated ileal loops. We investigated whether the remaining serotype Typhi effectors implicated in enteropathogenicity (SipA, SopB, and SopD) are functionally exchangeable with their serotype Typhimurium homologues. Serotype Typhi elicited fluid accumulation in bovine ligated ileal loops at levels similar to those elicited by a noninvasive serotype Typhimurium strain (the sipA sopABDE2 mutant) or by sterile culture medium. However, introduction of the cloned serotype Typhi sipA, sopB, and sopD genes complemented the ability of a serotype Typhimurium sipA sopABDE2 mutant to elicit fluid secretion in bovine ligated ileal loops. Introduction of the cloned serotype Typhi sipA, sopB, and sopD genes increased the invasiveness of a serotype Typhimurium sipA sopABDE2 mutant for human colon carcinoma epithelial (HT-29 and T84) cells and bovine kidney (MDBK) cells. Translational fusions between the mature TEM-1 beta-lactamase reporter and SipA or SopD demonstrated that serotype Typhi translocates these effectors into host cells. We conclude that the inability of serotype Typhi to cause fluid accumulation in bovine ligated ileal loops is not caused by a functional alteration of its SipA, SopB, and SopD effector proteins with respect to their serotype Typhimurium homologues.

The amino-terminal non-catalytic region of Salmonella typhimurium SigD affects actin organization in yeast and mammalian cells

The internalization of Salmonella into epithelial cells relies on the function of bacterial proteins which are injected into the cell by a specialized type III secretion system. Such bacterial effectors interfere with host cell signalling and induce local cytoskeletal rearrangements. One of such effectors is SigD/SopB, which shares homology with mammalian inositol phosphatases. We made use of the Saccharomyces cerevisiae model for elucidating new aspects of SigD function. Endogenous expression of SigD in yeast caused severe growth inhibition. Surprisingly, sigD alleles mutated in the catalytic site or even deleted for the whole C-terminal phosphatase domain still inhibited yeast growth by inducing loss of actin polarization and precluding the budding process. Accordingly, when expressed in HeLa cells, the same sigD alleles lost the ability of depleting phosphatidylinositol 4,5-bisphosphate from the plasma membrane, but still caused disappearance of actin fibres and loss of adherence. We delineate a region of 25 amino acids (residues 118-142) that is necessary for the effect of SigD on actin in HeLa cells. Our data indicate that SigD exerts a toxic effect linked to its N-terminal region and independent of its phosphatase activity.

Intracellular Voyeurism: Examining the Modulation of Host Cell Activities bySalmonella enterica Serovar Typhimurium

Salmonella spp. can infect host cells by gaining entry through phagocytosis or by inducing host cell membrane ruffling that facilitates bacterial uptake. With its wide host range, Salmonella enterica serovar Typhimurium has proven to be an important model organism for studying intracellular bacterial pathogenesis. Upon entry into host cells, serovar Typhimurium typically resides within a membrane-bound compartment termed the Salmonella-containing vacuole (SCV). From the SCV, serovar Typhimurium can inject several effector proteins that subvert many normal host cell systems, including endocytic trafficking, cytoskeletal rearrangements, lipid signaling and distribution, and innate and adaptive host defenses. The study of these intracellular events has been made possible through the use of various imaging techniques, ranging from classic methods of transmission electron microscopy to advanced livecell fluorescence confocal microscopy. In addition, DNA microarrays have now been used to provide a "snapshot" of global gene expression in serovar Typhimurium residing within the infected host cell. This review describes key aspects of Salmonella-induced subversion of host cell activities, providing examples of imaging that have been used to elucidate these events. Serovar Typhimurium engages specific host cell machinery from initial contact with the host cell to replication within the SCV. This continuous interaction with the host cell has likely contributed to the extensive arsenal that serovar Typhimurium now possesses, including two type III secretion systems, a range of ammunition in the form of TTSS effectors, and a complex genetic regulatory network that coordinates the expression of hundreds of virulence factors.

Salmonella enterica serotype Typhimurium MisL is an intestinal colonization factor that binds fibronectin

MisL is an autotransporter protein encoded by Salmonella pathogenicity island 3 (SPI3). To investigate the role of MisL in Salmonella enterica serotype Typhimurium (S. Typhimurium) pathogenesis, we characterized its function during infection of mice and identified a host receptor for this adhesin. In a mouse model of S. Typhimurium intestinal persistence, a misL mutant was shed with the faeces in significantly lower numbers than the wild type and was impaired in its ability to colonize the cecum. Previous studies have implicated binding of extracellular matrix proteins as a possible mechanism for S. Typhimurium intestinal persistence. A gluthathione-S-transferase (GST) fusion protein to the MisL passenger domain (GST-MisL(29-281)) was constructed to investigate binding to extracellular matrix proteins. In a solid-phase binding assay the purified GST-MisL(29-281) fusion protein bound to fibronectin and collagen IV, but not to collagen I. MisL expression was not detected by Western blot in S. Typhimurium grown under standard laboratory conditions. However, when expression of the cloned misL gene was driven by the Escherichia coli arabinose promoter, MisL could be detected in the S. Typhimurium outer membrane by Western blot and on the bacterial cell surface by flow cytometry. Expression of MisL enabled S. Typhimurium to bind fibronectin to its cell surface, resulting in attachment to fibronectin-coated glass slides and in increased invasiveness for human epithelial cells derived from colonic carcinoma (T84 cells). These data identify MisL as an extracellular matrix adhesin involved in intestinal colonization.