Citrobacter rodentium espB is necessary for signal transduction and for infection of laboratory mice

Butyrate Supplementation at High Concentrations Alters Enteric Bacterial Communities and Reduces Intestinal Inflammation in Mice Infected with Citrobacter rodentium

The study findings provide evidence that administration of butyrate in a dose-dependent manner can improve the weight gain of infected mice, enhance clearance of the infection, reduce inflammation through altered cytokine expression, and enhance tissue repair and mucus secretion. Moreover, butyrate treatment also affected the abundance of bacterial populations in both noninflamed and inflamed intestines. Notably, this investigation provides foundational information that can be used to determine the effects of prebiotics and other functional foods on the production of butyrate by enteric bacteria and their impact on intestinal health and host well-being.

Butyrate is a short-chain fatty acid by-product of the microbial fermentation of dietary fermentable materials in the large intestine; it is the main energy source for enterocyte regeneration, modulates the enteric microbial community, and contributes to increasing host health via mechanisms that are relatively poorly defined. Limited research has examined the therapeutic potential of butyrate using models of enteric inflammation incited by pathogenic organisms. We used Citrobacter rodentium to incite acute Th1/Th17 inflammation to ascertain the impact of butyrate on the host-microbiota relationship. Rectal administration of 140 mM butyrate to mice increased fecal concentrations of butyrate and increased food consumption and weight gain in mice infected with C. rodentium. Histological scores of colonic inflammation were lower in infected mice administered 140 mM butyrate. Expression of Il10, Tgfβ, and Muc2 was elevated in noninfected mice administered butyrate in comparison to mice not administered butyrate. Infected mice administered butyrate displayed elevated expression of genes necessary for pathogen clearance (i.e., Il17A and Il1β) and of genes involved in epithelial barrier repair and restoration (i.e., Relmβ, Tff3, and Myd88). Butyrate supplemented to inflamed colons increased the abundances of Proteobacteria and Lachnospiraceae and reduced the abundance of Clostridiaceae species. Mice with enteritis that were administered butyrate also exhibited an increased abundance of mucus-associated bacteria. In summary, rectal administration of butyrate increased feed consumption and weight gain, ameliorated C. rodentium-induced cell injury through enhanced expression of immune regulation and tissue repair mechanisms, and increased the abundance of butyrate-producing bacteria in mice with enteritis.

IMPORTANCE The study findings provide evidence that administration of butyrate in a dose-dependent manner can increase weight gain in infected mice, enhance clearance of the infection, reduce inflammation through altered cytokine expression, and enhance tissue repair and mucus secretion. Moreover, butyrate treatment also affected the abundance of bacterial populations in both noninflamed and inflamed intestines. Notably, this investigation provides foundational information that can be used to determine the effects of prebiotics and other functional foods on the production of butyrate by enteric bacteria and their impact on intestinal health and host well-being.

Impacts of resistant starch and wheat bran consumption on enteric inflammation in relation to colonic bacterial community structures and short-chain fatty acid concentrations in mice

Background

Identifying the connection among diet, the intestinal microbiome, and host health is currently an area of intensive research, but the potential of dietary fiber (DF) consumption to ameliorate intestinal inflammation has not been extensively studied. We examined the impacts of the DFs, wheat bran (WB) and resistant starch (RS) on host enteric health. A murine model of acute Th1/Th17 colitis (i.e. incited by Citrobacter rodentium) was used.

Results

Diets enriched with RS increased weight gain in mice inoculated with C. rodentium compared to mice consuming a conventional control (CN) diet. Short-chain fatty acid (SCFA) quantities in the cecum and distal colon were higher in mice consuming DFs, and these mice exhibited higher butyrate concentrations in the distal colon during inflammation. Histopathologic scores of inflammation in the proximal colon on day 14 post-inoculation (p.i.) (peak infection) and 21 p.i. (late infection) were lower in mice consuming DF-enriched diets compared to the CN diet. Consumption of WB reduced the expression of Th1/Th17 cytokines. As well, the expression of bacterial recognition and response genes such as Relmβ, RegIIIγ, and Tlr4 increased in mice consuming the RS-enriched diets. Furthermore, each diet generated a region-specific bacterial community, suggesting a link between selection for specific bacterial communities, SCFA concentrations, and inflammation in the murine colon.

Conclusions

Collectively, data indicated that the consumption of DF-rich diets ameliorates the effects of C. rodentium-induced enteritis by modifying the host microbiota to increase SCFA production, and bacterial recognition and response mechanisms to promote host health.

Electronic supplementary material

The online version of this article (doi:10.1186/s13099-016-0149-6) contains supplementary material, which is available to authorized users.

The Serine Protease Autotransporter Pic Modulates Citrobacter rodentium Pathogenesis and Its Innate Recognition by the Host

Bacterial pathogens produce a number of autotransporters that possess diverse functions. These include the family of serine protease autotransporters of Enterobacteriaceae (SPATEs) produced by enteric pathogens such as Shigella flexneri and enteroaggregative Escherichia coli. Of these SPATEs, one termed "protein involved in colonization," or Pic, has been shown to possess mucinase activity in vitro, but to date, its role in in vivo enteric pathogenesis is unknown. Testing a pic null (ΔpicC) mutant in Citrobacter rodentium, a natural mouse pathogen, found that the C. rodentium ΔpicC strain was impaired in its ability to degrade mucin in vitro compared to the wild type. Upon infection of mice, the ΔpicC mutant exhibited a hypervirulent phenotype with dramatically heavier pathogen burdens found in intestinal crypts. ΔpicC mutant-infected mice suffered greater barrier disruption and more severe colitis and weight loss, necessitating their euthanization between 10 and 14 days postinfection. Notably, the virulence of the ΔpicC mutant was normalized when the picC gene was restored; however, a PicC point mutant causing loss of mucinase activity did not replicate the ΔpicC phenotype. Exploring other aspects of PicC function, the ΔpicC mutant was found to aggregate to higher levels in vivo than wild-type C. rodentium. Moreover, unlike the wild type, the C. rodentium ΔpicC mutant had a red, dry, and rough (RDAR) morphology in vitro and showed increased activation of the innate receptor Toll-like receptor 2 (TLR2). Interestingly, the C. rodentium ΔpicC mutant caused a degree of pathology similar to that of wild-type C. rodentium when infecting TLR2-deficient mice, showing that despite its mucinase activity, PicC's major role in vivo may be to limit C. rodentium's stimulation of the host's innate immune system.

Active Transport of Phosphorylated Carbohydrates Promotes Intestinal Colonization and Transmission of a Bacterial Pathogen

Efficient acquisition of extracellular nutrients is essential for bacterial pathogenesis, however the identities and mechanisms for transport of many of these substrates remain unclear. Here, we investigate the predicted iron-binding transporter AfuABC and its role in bacterial pathogenesis in vivo. By crystallographic, biophysical and in vivo approaches, we show that AfuABC is in fact a cyclic hexose/heptose-phosphate transporter with high selectivity and specificity for a set of ubiquitous metabolites (glucose-6-phosphate, fructose-6-phosphate and sedoheptulose-7-phosphate). AfuABC is conserved across a wide range of bacterial genera, including the enteric pathogens EHEC O157:H7 and its murine-specific relative Citrobacter rodentium, where it lies adjacent to genes implicated in sugar sensing and acquisition. C. rodentium ΔafuA was significantly impaired in an in vivo murine competitive assay as well as its ability to transmit infection from an afflicted to a naïve murine host. Sugar-phosphates were present in normal and infected intestinal mucus and stool samples, indicating that these metabolites are available within the intestinal lumen for enteric bacteria to import during infection. Our study shows that AfuABC-dependent uptake of sugar-phosphates plays a critical role during enteric bacterial infection and uncovers previously unrecognized roles for these metabolites as important contributors to successful pathogenesis.

Essentially all Gram-negative pathogens are reliant on specific transport machineries termed binding protein-dependent transporters (BPDTs) to transport solutes such as amino acids, sugars and metal ions across their membranes. In this study we investigated AfuABC, a predicted iron-transporting BPDT found in many bacterial pathogens. We show by structural and functional approaches that AfuABC is not an iron transporter. Instead, AfuABC is a trio of proteins that bind and transport sugar-phosphates such as glucose-6-phosphate (G6P). In doing so, we present the first structural solution of a G6P-specific transport protein and add to the few known unique machineries for sugar-phosphate uptake by bacteria. Furthermore, we show that AfuABC is required by the intestinal pathogen C. rodentium to effectively transmit between mice and re-establish infection, leading us to propose that the transport of sugar-phosphates is an important part of general bacterial pathogenesis.

CD40 Ligand Deficient C57BL/6 Mouse Is a Potential Surrogate Model of Human X-Linked Hyper IgM (X-HIGM) Syndrome for Characterizing Immune Responses against Pathogens

Individuals with X-HIGM syndrome fail to express functional CD40 ligand; consequently they cannot mount effective protective antibody responses against pathogenic bacteria. We evaluated, compared, and characterized the humoral immune response of wild type (WT) and C57-CD40L deficient (C57-CD40L^−/−) mice infected with Citrobacter rodentium. Basal serum isotype levels were similar for IgM and IgG3 among mice, while total IgG and IgG2b concentrations were significantly lower in C57-CD40L^−/− mice compared with WT. Essentially IgG1 and IgG2c levels were detectable only in WT mice. C57-CD40L^−/− animals, orally inoculated with 2 × 10⁹ CFU, presented several clinical manifestations since the second week of infection and eventually died. In contrast at this time point no clinical manifestations were observed among C57-CD40L^−/− mice infected with 1 × 10⁷ CFU. Infection was subclinical in WT mice inoculated with either bacterial dose. The serum samples from infected mice (1 × 10⁷ CFU), collected at day 14 after infection, had similar C. rodentium-specific IgM titres. Although C57-CD40L^−/− animals had lower IgG and IgG2b titres than WT mice, C57-CD40L^−/− mice sera displayed complement-mediated bactericidal activity against C. rodentium. C. rodentium-infected C57-CD40L^−/− mice are capable of producing antibodies that are protective. C57-CD40L^−/− mouse is a useful surrogate model of X-HIGM syndrome for studying immune responses elicited against pathogens.

Psidium guajava leaf extract prevents intestinal colonization of Citrobacter rodentium in the mouse model

Diarrheal diseases are the second highest cause of mortality of children under 5 years worldwide. There is a continuous search for developing a cost-effective treatment for diarrhea as the present ones are facing challenges. Medicinal plants can be explored further as an alternative treatment for diarrhea. Psidium guajava leaves have been used as an antidiarrheal globally. Citrobacter rodentium, a common mouse pathogen, is known to mimic the pathogenecity of enteropathogenic and enterohemorrhagic E. coli. It can thus present an effective model to study infectious diarrhea. In the present study, the P. guajava leaf extract was tested for its efficacy in treating infectious diarrhea using a C. rodentium mouse model. The mice in the test group (treated with P. guajava leaf extract) showed quicker clearance of infection as compared with the control group. The bacterial load in the fecal sample of the mice in the test group was high on Day 4 as compared with that in the control group, suggesting a flush out of the bacteria. In the test group, 6/7 (85.71%) mice showed clearance of infection by Day 19. The control group continued to show infection till Day 29. P. guajava leaf extract thus has the potential for use in the treatment of infectious diarrhea.

Biology and Diseases of Mice

Today’s laboratory mouse, Mus musculus, has its origins as the ‘house mouse’ of North America and Europe. Beginning with mice bred by mouse fanciers, laboratory stocks (outbred) derived from M. musculus musculus from eastern Europe and M. m. domesticus from western Europe were developed into inbred strains. Since the mid-1980s, additional strains have been developed from Asian mice (M. m. castaneus from Thailand and M. m. molossinus from Japan) and from M. spretus which originated from the western Mediterranean region.

Recent progress in engineering human-associated microbiomes

Recent progress in molecular biology and genetics opens up the possibility of engineering a variety of biological systems, from single-cellular to multicellular organisms. The consortia of microbes that reside on the human body, the human-associated microbiota, are particularly interesting as targets for forward engineering and manipulation due to their relevance in health and disease. New technologies in analysis and perturbation of the human microbiota will lead to better diagnostic and therapeutic strategies against diseases of microbial origin or pathogenesis. Here, we discuss recent advances that are bringing us closer to realizing the true potential of an engineered human-associated microbial community.

In vitro and in vivo model systems for studying enteropathogenic Escherichia coli infections

Enteropathogenic Escherichia coli (EPEC) and enterohemorrhagic E. coli (EHEC) belong to a group of bacteria known as attaching and effacing (A/E) pathogens that cause disease by adhering to the lumenal surfaces of their host's intestinal epithelium. EPEC and EHEC are major causes of infectious diarrhea that result in significant childhood morbidity and mortality worldwide. Recent advances in in vitro and in vivo modeling of these pathogens have contributed to our knowledge of how EPEC and EHEC attach to host cells and subvert host-cell signaling pathways to promote infection and cause disease. A more detailed understanding of how these pathogenic microbes infect their hosts and how the host responds to infection could ultimately lead to new therapeutic strategies to help control these significant enteric pathogens.

Mutations that impact the enteropathogenic Escherichia coli Cpx envelope stress response attenuate virulence in Galleria mellonella

In this paper, we show that the larvae of the greater wax moth, Galleria mellonella, can be used as a model to study enteropathogenic Escherichia coli (EPEC) virulence. G. mellonella larvae are killed after infection with EPEC type strain E2348/69 but not by an attenuated derivative that expresses diminished levels of the major virulence determinants or by a mutant specifically defective in type III secretion (T3S). Infecting EPEC inhabit the larval hemocoel only briefly and then become localized to melanized capsules, where they remain extracellular. Previously, it was shown that mutations affecting the Cpx envelope stress response lead to diminished expression of the bundle-forming pilus (BFP) and the type III secretion system (T3SS). We demonstrate that mutations that activate the Cpx pathway have a dramatic effect on the ability of the bacterium to establish a lethal infection, and this is correlated with an inability to grow in vivo. Infection with all E. coli strains led to increased expression of the antimicrobial peptides (AMPs) gloverin and cecropin, although strain- and AMP-specific differences were observed, suggesting that the G. mellonella host perceives attenuated strains and Cpx mutants in unique manners. Overall, this study shows that G. mellonella is an economical, alternative infection model for the preliminary study of EPEC host-pathogen interactions, and that induction of the Cpx envelope stress response leads to defects in virulence.

Allele- and tir-independent functions of intimin in diverse animal infection models

Upon binding to intestinal epithelial cells, enterohemorrhagic Escherichia coli (EHEC), enteropathogenic E. coli (EPEC), and Citrobacter rodentium trigger formation of actin pedestals beneath bound bacteria. Pedestal formation has been associated with enhanced colonization, and requires intimin, an adhesin that binds to the bacterial effector translocated intimin receptor (Tir), which is translocated to the host cell membrane and promotes bacterial adherence and pedestal formation. Intimin has been suggested to also promote cell adhesion by binding one or more host receptors, and allelic differences in intimin have been associated with differences in tissue and host specificity. We assessed the function of EHEC, EPEC, or C. rodentium intimin, or a set of intimin derivatives with varying Tir-binding abilities in animal models of infection. We found that EPEC and EHEC intimin were functionally indistinguishable during infection of gnotobiotic piglets by EHEC, and that EPEC, EHEC, and C. rodentium intimin were functionally indistinguishable during infection of C57BL/6 mice by C. rodentium. A derivative of EHEC intimin that bound Tir but did not promote robust pedestal formation on cultured cells was unable to promote C. rodentium colonization of conventional mice, indicating that the ability to trigger actin assembly, not simply to bind Tir, is required for intimin-mediated intestinal colonization. Interestingly, streptomycin pre-treatment of mice eliminated the requirement for Tir but not intimin during colonization, and intimin derivatives that were defective in Tir-binding still promoted colonization of these mice. These results indicate that EPEC, EHEC, and C. rodentium intimin are functionally interchangeable during infection of gnotobiotic piglets or conventional C57BL/6 mice, and that whereas the ability to trigger Tir-mediated pedestal formation is essential for colonization of conventional mice, intimin provides a Tir-independent activity during colonization of streptomycin pre-treated mice.

The impact of the microbiota on the pathogenesis of IBD: lessons from mouse infection models

Inflammatory bowel disease (IBD), including Crohn's disease and ulcerative colitis, is a major human health problem. The bacteria that live in the gut play an important part in the pathogenesis of IBD. However, owing to the complexity of the gut microbiota, our understanding of the roles of commensal and pathogenic bacteria in establishing a healthy intestinal barrier and in its disruption is evolving only slowly. In recent years, mouse models of intestinal inflammatory disorders based on defined bacterial infections have been used intensively to dissect the roles of individual bacterial species and specific bacterial components in the pathogenesis of IBD. In this Review, we focus on the impact of pathogenic and commensal bacteria on IBD-like pathogenesis in mouse infection models and summarize important recent developments.

Impaired innate immune response and enhanced pathology during Citrobacter rodentium infection in mice lacking functional P-selectin

The selectin family of adhesion molecules mediates recruitment of immune cells to sites of inflammation which is critical for host resistance against infection. To characterize the role of selectins in host defence against Citrobacter rodentium infection, wild-type (WT) mice and mice lacking P-selectin glycoprotein ligand-1 (PSGL-1), P-, E- and L-selectin were infected using a Citrobacter-induced colitis model. Infected mice lacking PSGL-1 or P-selectin showed a more pronounced morbidity associated with higher bacterial load, elevated IL-12 p70, TNF-alpha, IFN-gamma, MCP-1 and IL-6 production, more severe inflammation and surprisingly higher leucocyte infiltration in the guts than WT control. Recruitment of neutrophils and macrophages and caecal inflammation were drastically reduced in infected P-selectin knockout mice receiving blocking monoclonal antibodies to ICAM-1 or LFA-1, indicating that these adhesion molecules may compensate for the loss of selectins in leucocyte recruitment. Furthermore, the adaptive immune response in mice lacking PSGL-1 or P-selectin remained functional since these infected mice were capable of eradicating the bacteria and being protected upon re-challenge with C. rodentium. These data demonstrate a definitive phenotypic impairment of innate response in mice lacking PSGL-1 or P-selectin, and suggest that these adhesion molecules are important in host innate immune response against Citrobacter infection.

Resistance of chemokine receptor 6-deficient mice to Yersinia enterocolitica infection: evidence of defective M-cell formation in vivo

M cells, specialized cells within Peyer's patches (PPs), are reduced in number in chemokine receptor 6 (CCR6)-deficient mice. The pathogenic microorganism Yersinia enterocolitica exploits M cells for the purpose of mucosal tissue invasion exclusively through PPs. The aim of this study was to evaluate the course of yersiniosis in CCR6-deficient mice and to investigate whether these mice might be used as an in vivo model to determine M-cell function. After oral challenge with Y. enterocolitica, control mice suffered from lethal septic infection whereas CCR6-deficient mice showed very limited symptoms of infection. Immunohistochemical analysis demonstrated PP invasion by Y. enterocolitica in control mice whereas no bacteria could be found in CCR6-deficient mice. In addition, a significant induction of proinflammatory cytokines could be found in control mice whereas proinflammatory cytokine levels in CCR6-deficient mice remained unchanged. In contrast, intraperitoneal infection resulted in severe systemic yersiniosis in both mouse groups. Abrogated oral Y. enterocolitica infection in CCR6-deficient mice demonstrates the importance of CCR6 expression in the physiological and pathological immune responses generated within PPs by influencing M-cell differentiation, underscoring the important role of M cells in the process of microbial uptake. CCR6-deficient mice may therefore represent a suitable model for the study of M-cell function in vivo.

The enteropathogenic E. coli effector EspB facilitates microvillus effacing and antiphagocytosis by inhibiting myosin function

Enteropathogenic Escherichia coli (EPEC) destroys intestinal microvilli and suppresses phagocytosis by injecting effectors into infected cells through a type III secretion system (TTSS). EspB, a component of the TTSS, is also injected into the cytoplasm of host cells. However, the physiological functions of EspB within the host cell cytoplasm remain unclear. We show that EspB binds to myosins, which are a superfamily of proteins that interact with actin filaments and mediate essential cellular processes, including microvillus formation and phagocytosis. EspB inhibits the interaction of myosins with actin, and an EspB mutant that lacks the myosin-binding region maintained its TTSS function but could not induce microvillus effacing or suppress phagocytosis. Moreover, the myosin-binding region of EspB is essential for Citrobacter rodentium, an EPEC-related murine pathogen, to efficiently infect mice. These results suggest that EspB inhibits myosin functions and thereby facilitates efficient infection by EPEC.

Saccharomyces boulardii ameliorates Citrobacter rodentium-induced colitis through actions on bacterial virulence factors

Saccharomyces boulardii has received increasing attention as a probiotic effective in the prevention and treatment of infectious and inflammatory bowel diseases. The aim of this study was to examine the ameliorating effects of S. boulardii on Citrobacter rodentium colitis in vivo and identify potential mechanisms of action. C57BL/6 mice received 2.5 x 10(8) C. rodentium by gavage on day 0, followed by S. boulardii (25 mg; 5 x 10(8) live cells) gavaged twice daily from day 2 to day 9. Animal weights were monitored until death on day 10. Colons were removed and assessed for epithelial barrier function, histology, and myeloperoxidase activity. Bacterial epithelial attachment and type III secreted proteins translocated intimin receptor Tir (the receptor for bacterial intimin) and EspB (a translocation apparatus protein) required for bacterial virulence were assayed. In infected mice, S. boulardii treatment significantly attenuated weight loss, ameliorated crypt hyperplasia (234.7 +/- 7.2 vs. 297.8 +/- 17.6 microm) and histological damage score (0.67 +/- 0.67 vs. 4.75 +/- 0.75), reduced myeloperoxidase activity (2.1 +/- 0.4 vs. 4.7 +/- 0.9 U/mg), and attenuated increased mannitol flux (17.2 +/- 5.0 vs. 31.2 +/- 8.2 nm.cm(-2).h(-1)). The ameliorating effects of S. boulardii were associated with significantly reduced numbers of mucosal adherent C. rodentium, a marked reduction in Tir protein secretion and translocation into mouse colonocytes, and a striking reduction in EspB expression and secretion. We conclude that S. boulardii maintained colonic epithelial barrier integrity and ameliorated inflammatory sequelae associated with C. rodentium infection by attenuating C. rodentium adherence to host epithelial cells through putative actions on the type III secretion system.

Development of fatal colitis in FVB mice infected with Citrobacter rodentium

Citrobacter rodentium is the causative agent of transmissible murine colonic hyperplasia. The disease is characterized by severe but temporary epithelial hyperplasia with limited inflammation in the descending colon of adult mice on a variety of genetic backgrounds. The natural history of infection with this murine pathogen has been characterized in outbred Swiss Webster (SW) mice but not in the cognate inbred FVB strain. In contrast to subclinical infection in SW mice, 12-week-old FVB mice developed overt disease with significant weight loss and mortality beginning by 9 days postinoculation (dpi). By 21 dpi, more than 75% of infected FVB mice died or had to be euthanized, whereas no mortality developed in SW mice. Mortality in FVB mice was fully prevented by fluid therapy. Fecal shedding of bacteria was similar in both groups through 9 dpi; however, a slight but significant delay in bacterial clearance was observed in FVB mice by 12 to 18 dpi. SW mice developed hyperplasia with minimal inflammation in the descending colon. FVB mice developed epithelial cell hyperproliferation, severe inflammation with erosions and ulcers, and epithelial atypia by 6 dpi in the descending colon. In the majority of surviving FVB mice, colonic lesions, including epithelial atypia, were reversible, although a small percentage (5 to 7%) exhibited chronic colitis through 7 months postinoculation. The existence of susceptible and resistant lines of mice with similar genetic backgrounds will facilitate the identification of host factors responsible for the outcome of infection and may lead to the development of novel strategies for preventing and treating infectious colitis.

Analysis of the function of enteropathogenic Escherichia coli EspB by random mutagenesis

Enteropathogenic Escherichia coli (EPEC) is an important cause of infantile diarrhea, especially in developing countries. EspB, a key virulence factor of EPEC, is required for the attaching and effacing effect characteristic of EPEC and enterohemorrhagic E. coli and has been posited to play several functions in the process of infection. Attaching and effacing activity is associated with the accumulation of filamentous actin beneath adherent bacteria as measured in the fluorescence actin staining (FAS) test. To determine whether different domains of EspB are responsible for different functions, 42 plasmids carrying mutated espB were introduced into an espB deletion mutant. Two major groups of espB mutants were identified. One group of 17 mutants exhibited positive FAS results and normal levels of hemolytic activity. Another group of 22 mutants exhibited negative FAS results and low levels of hemolytic activity. Three mutants were exceptional. One mutant was FAS positive but had significantly reduced hemolytic activity. Conversely, a second mutant was FAS negative but had full hemolytic activity. A third mutant had a significantly reduced FAS level compared to the wild type but full hemolytic activity. The results of EspF and Tir translocation assays confirmed that FAS-negative insertions disrupt effector translocation and mutants with FAS-positive insertions retain protein translocation activity. These results suggest that EspB has distinct domain functions involved in effector translocation that can be distinguished from its role as a component of the translocation pore.

Citrobacter rodentium of mice and man

The major classes of enteric bacteria harbour a conserved core genomic structure, common to both commensal and pathogenic strains, that is most likely optimized to a life style involving colonization of the host intestine and transmission via the environment. In pathogenic bacteria this core genome framework is decorated with novel genetic islands that are often associated with adaptive phenotypes such as virulence. This classical genome organization is well illustrated by a group of extracellular enteric pathogens, which includes enteropathogenic Escherichia coli (EPEC), enterohaemorrhagic E. coli (EHEC) and Citrobacter rodentium, all of which use attaching and effacing (A/E) lesion formation as a major mechanism of tissue targeting and infection. Both EHEC and EPEC are poorly pathogenic in mice but infect humans and domestic animals. In contrast, C. rodentium is a natural mouse pathogen that is related to E. coli, hence providing an excellent in vivo model for A/E lesion forming pathogens. C. rodentium also provides a model of infections that are mainly restricted to the lumen of the intestine. The mechanism's by which the immune system deals with such infections has become a topic of great interest in recent years. Here we review the literature of C. rodentium from its emergence in the mid-1960s to the most contemporary reports of colonization, pathogenesis, transmission and immunity.

Characterization of two non-locus of enterocyte effacement-encoded type III-translocated effectors, NleC and NleD, in attaching and effacing pathogens

Intestinal colonization by enteropathogenic and enterohemorrhagic Escherichia coli requires the locus of enterocyte effacement-encoded type III secretion system. We report that NleC and NleD are translocated into host cells via this system. Deletion mutants induced attaching and effacing lesions in vitro, while infection of calves or lambs showed that neither gene was required for colonization.

Attaching-effacing bacteria in animals

Enteric bacteria with a demonstrable or potential ability to form attaching-effacing lesions, so-called attaching-effacing (AE) bacteria, have been found in the intestinal tracts of a wide variety of warm-blooded animal species, including man. In some host species, for example cattle, pigs, rabbits and human beings, attaching-effacing Escherichia coli (AEEC) have an established role as enteropathogens. In other host species, AE bacteria are of less certain significance. With continuing advances in the detection and typing of AE strains, the importance of these bacteria for many hosts is likely to become clearer. The pathogenic effects of AE bacteria result from adhesion to the intestinal mucosa by a variety of mechanisms, culminating in the formation of the characteristic intimate adhesion of the AE lesion. The ability to induce AE lesions is mediated by the co-ordinated expression of some 40 bacterial genes organized within a so-called pathogenicity island, known as the "Locus for Enterocyte Effacement". It is also believed that the production of bacterial toxins, principally Vero toxins, is a significant virulence factor for some AEEC strains. Recent areas of research into AE bacteria include: the use of Citrobacter rodentium to model human AEEC disease; quorum-sensing mechanisms used by AEEC to modulate virulence gene expression; and the potential role of adhesion in the persistent colonization of the intestine by AE bacteria. This review of AE bacteria covers their molecular biology, their occurrence in various animal species, and the diagnosis, pathology and clinical aspects of animal diseases with which they are associated. Reference is made to human pathogens where appropriate. The focus is mainly on natural colonization and disease, but complementary experimental data are also included.

Mast cells limit systemic bacterial dissemination but not colitis in response to Citrobacter rodentium

Enteropathogenic Escherichia coli and enterohemorrhagic E. coli cause an inflammatory colitis in human patients characterized by neutrophil infiltration, proinflammatory cytokine expression, and crypt hyperplasia. Citrobacter rodentium causes a similar colitis in mice and serves as a model for enteropathogenic E. coli infection in humans. C. rodentium induces systemic T-cell-dependent antibody production that facilitates clearance of the bacteria and protects the host from reinfection. The role of innate immune cells in infectious colitis, however, is less well understood. In this study, we have determined the role of mast cells in the inflammatory response and disease induced by C. rodentium. Mice deficient in mast cells exhibit more severe colonic histopathology and have a higher mortality rate following infection with C. rodentium than do wild-type animals. Despite unimpaired neutrophil recruitment and lymphocyte activation, mast cell-deficient mice have a disseminated infection evident in crucial organ systems that contributes to sepsis. Importantly, mast cells also have the capacity to directly kill C. rodentium. Together, these results suggest that mast cells protect the host from systemic infection by reducing the bacterial load and preventing dissemination of the bacterium from the colon.

Citrobacter rodentium lifA/efa1 is essential for colonic colonization and crypt cell hyperplasia in vivo

Previously, we have identified a large gene (lifA, for lymphocyte inhibitory factor A) in enteropathogenic Escherichia coli (EPEC) encoding a protein termed lymphostatin that suppresses cytokine expression in vitro. This protein also functions as an adhesion factor for enterohemorrhagic E. coli (EHEC) and Shiga toxin-producing E. coli and is alternatively known as efa1 (EHEC factor for adherence 1). The lifA/efa1 gene is also present in Citrobacter rodentium, an enteric pathogen that causes a disease termed transmissible murine colonic hyperplasia (TMCH), which induces colitis and massive crypt cell proliferation, in mice. To determine if lifA/efa1 is required for C. rodentium-induced colonic pathology in vivo, three in-frame mutations were generated, disrupting the glycosyltransferase (GlM12) and protease (PrMC31) motifs and a domain in between that does not encode any known activity (EID3). In contrast to infection with wild-type C. rodentium, that with any of the lifA/efa1 mutant strains did not induce weight loss or TMCH. Enteric infection with motif mutants GlM12 and PrM31 resulted in significantly reduced colonization counts during the entire 20-day course of infection. In contrast, EID3 was indistinguishable from the wild type during the initial colonic colonization, but cleared rapidly after day 8 of the infection. The colonic epithelium of all infected mice displayed increased epithelial regeneration. However, significantly increased regeneration was observed by day 20 only in mice infected with the wild-type in comparison to those infected with lifA/efa1 mutant EID3. In summary, lifA/efa1 is a critical gene outside the locus for enterocyte effacement that regulates bacterial colonization, crypt cell proliferation, and epithelial cell regeneration.

Mouse model of enteropathogenic Escherichia coli infection

Enteropathogenic Escherichia coli (EPEC) is an important cause of diarrhea in humans. EPEC infection of cultured intestinal epithelial cells induces attaching and effacing (A/E) lesions, alters intestinal ion transport, increases paracellular permeability, and stimulates inflammation. The lack of a small-animal model has restricted in vivo studies examining EPEC-host interactions. The aim of this study was to characterize the C57BL/6J mouse as a model of EPEC infection. We have shown that EPEC can adhere to and colonize the intestinal epithelium of C57BL/6J mice. Animal weight and water intake were not altered during 10 days of EPEC infection. The proximal colon of infected mice contained semisolid stool, with stool pellets forming only in the distal colon. In contrast, the entire colon of control mice contained formed stool. Microvillous effacement and actin rearrangement, characteristic of A/E lesions, were seen in the intestine of infected mice but not control mice. Histological assessment revealed increased numbers of lamina propria neutrophils with occasional crypt abscesses, intraepithelial lymphocytes, and goblet cells in the intestine of EPEC-infected mice. Altogether, these data suggest that the C57BL/6J mouse is susceptible to infection by EPEC and will provide a suitable in vivo model for studying the consequences of EPEC infection.

Organ specificity, colonization and clearance dynamics in vivo following oral challenges with the murine pathogen Citrobacter rodentium

Citrobacter rodentium belongs to a family of human and animal enteric pathogens that includes the clinically significant enterohaemorrhagic Escherichia coli (EHEC) and enteropathogenic E. coli (EPEC). These pathogens use attaching and effacing (A/E) lesions to colonize the host gastrointestinal tract. In this study we have used bioluminescence imaging (BLI) to investigate the organ specificity, dynamics of colonization and clearance of mice by C. rodentium in situ and in real time. The bioluminescent C. rodentium derivative, strain ICC180, expresses the luxCDABE operon from the entemopathogenic nematode symbiont Photorhabdus luminescens and light levels accurately reflect bacterial numbers both in vitro and in vivo. We have demonstrated that primary colonization of the mouse by C. rodentium takes place within the caecum, specifically within the specialized patch of lymphoid tissue known as the caecal patch. Following colonization of the caecum C. rodentium established a colonic infection. Clearance of C. rodentium ICC180 parallels the colonization dynamics, i.e. the caecum was first to be cleared followed by the colon. A bioluminescent eae (encoding the outer membrane adhesin intimin) C. rodentium mutant failed to establish long-term colonization, although low levels of bacteria could be recovered for up to 3 days post challenge from the caecum.

Initiation and resolution of mucosal inflammation

Antigens entering the body through the mucosal surface are screened by a highly developed immune system comprised not only of traditional lymphoid cells but also epithelial cells, fibroblasts, and antigen-presenting cells (APCs). For example, in the intestinal tract, gut-associated lymphoid tissue (GALT) is tolerant to the approx 400 separate commensal strains residing mainly in the colon, but also retains the capacity to detect and remove virulent bacteria before they infect systemically. This review summarizes recent work characterizing the molecular mechanisms involved in acute and chronic intestinal inflammation. We will also describe a natural murine pathogen, Citrobacter rodentium, which is being used to explore the host response to enteric pathogens and the resulting immunopathology.

Mutation of toxB and a truncated version of the efa-1 gene in Escherichia coli O157:H7 influences the expression and secretion of locus of enterocyte effacement-encoded proteins but not intestinal colonization in calves or sheep

Enterohemorrhagic Escherichia coli (EHEC) strains comprise a broad group of bacteria, some of which cause attaching and effacing (AE) lesions and enteritis in humans and animals. Non-O157:H7 EHEC strains contain the gene efa-1 (referred to in previous publications as efa1), which influences adherence to cultured epithelial cells. An almost identical gene in enteropathogenic E. coli (lifA) mediates the inhibition of lymphocyte proliferation and proinflammatory cytokine synthesis. We have shown previously that significantly lower numbers of EHEC O5 and O111 efa-1 mutants are shed in feces following experimental infection in calves and that these mutants exhibit reduced adherence to intestinal epithelia compared with isogenic wild-type strains. E. coli O157:H7 strains lack efa-1 but encode a homolog on the pO157 plasmid (toxB/l7095) and contain a truncated version of the efa-1 gene (efa-1'/z4332 in O island 122 of the EDL933 chromosome). Here we report that E. coli O157:H7 toxB and efa-1' single and double mutants exhibit reduced adherence to cultured epithelial cells and show reduced expression and secretion of proteins encoded by the locus of enterocyte effacement (LEE), which plays a key role in the host-cell interactions of EHEC. The activity of LEE1, LEE4, and LEE5 promoters was not significantly altered in E. coli O157:H7 strains harboring toxB or efa-1' mutations, indicating that the effect on the expression of LEE-encoded secreted proteins occurs at a posttranscriptional level. Despite affecting type III secretion, mutation of toxB and efa-1' did not significantly affect the course of fecal shedding of E. coli O157:H7 following experimental inoculation of 10- to 14-day-old calves or 6-week-old sheep. Mutation of tir caused a significant reduction in fecal shedding of E. coli O157:H7 in calves, indicating that the formation of AE lesions is important for colonization of the bovine intestine.

Alpha 1-antitrypsin binds to and interferes with functionality of EspB from atypical and typical enteropathogenic Escherichia coli strains

Enteropathogenic Escherichia coli (EPEC), including diffusely adhering atypical E. coli, strains use a type III secretion system to deliver effector proteins into the membrane and cytoplasm of infected cells. The E. coli secreted proteins A, B, and D (EspA, EspB, and EspD) are required for the formation of the characteristic attaching and effacing (A/E) lesions. EspB and EspD are thought to form a translocation pore in the host cell membrane through which effector proteins are injected into the host cytosol. Besides its function in pore formation, EspB has been found in the cytosol and implicated to function as an effector protein. We screened for putative host cell proteins interacting with EspB of atypical EPEC strains and identified alpha(1)-antitrypsin (AAT) as a binding partner for EspB. AAT binds to EspB in pull-down and overlay experiments and also to EspD in overlay experiments. In agreement with the role of EspB and EspD in pore formation, EPEC-mediated hemolysis of red blood cells is strongly reduced by AAT in a concentration-dependent manner, indicating that AAT interferes with type III secretion by inhibiting the formation of the translocation pore. This is further supported by a decreased actin polymerization after infection of HeLa or CaCo-2 cells with EPEC in the presence of physiologically relevant concentrations of AAT. In this study, we identify AAT as a new binding partner for EspB and EspD, suggesting a previously unappreciated role for AAT in host cell defense against EPEC infections and potentially also against other bacterial pathogens.

Clearance of Citrobacter rodentium requires B cells but not secretory immunoglobulin A (IgA) or IgM antibodies

Citrobacter rodentium, a murine model pathogen for human enteropathogenic Escherichia coli, predominantly colonizes the lumen and mucosal surface of the colon and cecum and causes crypt hyperplasia and mucosal inflammation. Mice infected with C. rodentium develop a secretory immunoglobulin A (IgA) response, but the role of B cells or secretory antibodies in host defense is unknown. To address this question, we conducted oral C. rodentium infections in mice lacking B cells, IgA, secreted IgM, polymeric Ig receptor (pIgR), or J chain. Normal mice showed peak bacterial numbers in colon and feces at 1 week and bacterial eradication after 3 to 4 weeks. B-cell-deficient mice were equally susceptible initially but could not control infection subsequently. Tissue responses showed marked differences, as infection of normal mice was accompanied by transient crypt hyperplasia and mucosal inflammation in the colon and cecum at 2 but not 6 weeks, whereas B-cell-deficient mice had few mucosal changes at 2 weeks but severe epithelial hyperplasia with ulcerations and mucosal inflammation at 6 weeks. The functions of B cells were not mediated by secretory antibodies, since mice lacking IgA or secreted IgM or proteins required for their transport into the lumen, pIgR or J chain, cleared C. rodentium normally. Nonetheless, systemic administration of immune sera reduced bacterial numbers significantly in normal and pIgR-deficient mice, and depletion of IgG abrogated this effect. These results indicate that host defense against C. rodentium depends on B cells and IgG antibodies but does not require production or transepithelial transport of IgA or secreted IgM.

Identification of a novel Citrobacter rodentium type III secreted protein, EspI, and roles of this and other secreted proteins in infection

Citrobacter rodentium is a member of a group of pathogens that colonize the lumen of the host gastrointestinal tract via attaching and effacing (A/E) lesion formation. C. rodentium, which causes transmissible colonic hyperplasia in mice, is used as an in vivo model system for the clinically significant A/E pathogens enterohemorrhagic and enteropathogenic Escherichia coli. These bacteria all contain a pathogenicity island called the locus of enterocyte effacement (LEE), which encodes a type III secretion system that is designed to deliver effector proteins into eukaryotic host cells. These effectors are involved in the subversion of host eukaryotic cell functions to the benefit of the bacterium. In this study we used mutant strains to determine the effects of the C. rodentium LEE-encoded effectors EspF, EspG, EspH, and Map on virulence in the mouse model. In addition, we identified a novel secreted protein, EspI encoded outside the LEE, whose secretion is also dependent on a functional type III secretion system. Mutant strains with each of the effectors investigated were found to be outcompeted by wild-type bacteria in mixed-infection experiments in vivo, although the effects of EspF and EspH were only subtle. In single-infection experiments, we found that EspF, EspG, and EspH are not required for efficient colonization of the mouse colon or for the production of hyperplasia. In contrast, strains producing EspI and Map had significant colonization defects and resulted in dramatically reduced levels of hyperplasia, and they exhibited very different growth dynamics in mice than the wild-type strain exhibited.

Critical Role of T Cell-Dependent Serum Antibody, but Not the Gut-Associated Lymphoid Tissue, for Surviving Acute Mucosal Infection withCitrobacter rodentium, an Attaching and Effacing Pathogen

Citrobacter rodentium uses virulence factors similar to the enteropathogenic Escherichia coli to produce attaching and effacing lesions in the distal colon of mice. We used this infection model to determine components of adaptive immunity needed to survive infection. During acute infection, wild-type mice develop breaks across infected epithelial surfaces but resolve infection. Surprisingly, mice markedly deficient in mucosal lymphocyte populations from beta(7) integrin deficiency resolve infection, as do CD8alpha-/- or TCR-delta-/- mice. In contrast, CD4-/- or TCR-beta-/- mice develop polymicrobial sepsis and end-organ damage, and succumb during acute infection, despite epithelial damage similar to wild-type mice. B cell-deficient (MuMT-/-) or B and T cell-deficient (recombinase-activating gene 2-/-) mice develop severe pathology in colon and internal organs, and deteriorate rapidly during acute infection. Surviving mice develop robust Citrobacter-specific serum IgM responses during acute infection, whereas mice that succumb do not. However, CD4-/- mice receiving serum Igs from infected wild-type mice survive and clear the infection. Our data show that survival of apparently self-limited and luminal mucosal infections requires a systemic T cell-dependent Ab response against bacteria that enter through damaged mucosa. These findings have implications for understanding host defense against mucosal infections, including the pathogenesis of these diseases in immunocompromised populations.

Lactoferrin impairs type III secretory system function in enteropathogenic Escherichia coli

Enteropathogenic Escherichia coli (EPEC) is an important cause of infant diarrhea in developing countries. EPEC uses a type III secretory system to deliver effector proteins into the host cell. These proteins cause the characteristic attaching and effacing lesion on enterocytes. Lactoferrin, a glycoprotein present in human milk, inhibits EPEC adherence to mammalian cells. To determine the effect of lactoferrin on the initial host cell attachment step that is mediated by the type III secretory system, we focused on EPEC-induced actin polymerization in HEp2 cells, on the hemolytic activity, and on measurement of E. coli secreted proteins A, B, and D (EspABD). Lactoferrin blocked EPEC-mediated actin polymerization in HEp2 cells and blocked EPEC-induced hemolysis. The mechanism of this inhibition was lactoferrin-mediated degradation of secreted proteins necessary for bacterial contact and pore formation, particularly EspB. The proteolytic effect of lactoferrin was prevented by serine protease inhibitors. This disruption of the type III secretory system implies that lactoferrin could provide broad cross protection against the enteropathogens that share this mechanism.

Identification of a novel type IV pilus gene cluster required for gastrointestinal colonization of Citrobacter rodentium

Citrobacter rodentium is used as an in vivo model system for clinically significant enteric pathogens such as enterohaemorrhagic Escherichia coli (EHEC) and enteropathogenic E. coli (EPEC). These pathogens all colonize the lumen side of the host gastrointestinal tract via attaching and effacing (A/E) lesion formation. In order to identify genes required for the colonization of A/E-forming pathogens, a library of signature-tagged transposon mutants of C. rodentium was constructed and screened in mice. Of the 576 mutants tested, 14 were attenuated in their ability to colonize the descending colon. Of these, eight mapped to the locus of enterocyte effacement (LEE), which is required for the formation of A/E lesions, underlying the importance of this mechanism for pathogenesis. Another mutant, P5H2, was found to have a transposon insertion in an open reading frame that has strong similarity to type IV pilus nucleotide-binding proteins. The region flanking the transposon insertion was sequenced, identifying a cluster of 12 genes that encode the first described pilus of C. rodentium (named colonization factor Citrobacter, CFC). The proteins encoded by cfc genes have identity to proteins of the type IV COF pilus of enterotoxigenic E. coli (ETEC), the toxin co-regulated pilus of Vibrio cholerae and the bundle-forming pilus of EPEC. A non-polar mutation in cfcI, complementation of this strain with wild-type cfcI and complementation of strain P5H2 with wild-type cfcH confirmed that these genes are required for colonization of the gastrointestinal tract by C. rodentium. Thus, CFC provides a convenient model to study type IV pilus-mediated pathogen-host interactions under physiological conditions in the natural colonic environment.

Citrobacter rodentium translocated intimin receptor (Tir) is an essential virulence factor needed for actin condensation, intestinal colonization and colonic hyperplasia in mice

Citrobacter rodentium infection of mice serves as a relevant small animal model to study enterohaemorrhagic Escherichia coli (EHEC) and enteropathogenic E. coli (EPEC) infections in man. Enteropathogenic E. coli and EHEC translocate Tir into the host cytoplasmic membrane, where it serves as the receptor for the bacterial adhesin intimin and plays a central role in actin condensation beneath the adherent bacterium. In this report, we examined the function of C. rodentium Tir both in vitro and in vivo. Similar to EPEC, C. rodentium Tir is tyrosine phosphorylated and is essential for actin condensation. Citrobacter Tir and EPEC Tir are functionally interchangeable and both require tyrosine phosphorylation to mediate actin rearrangements. In contrast, Citrobacter Tir supports actin nucleation in EHEC independent of tyrosine phosphorylation, while EHEC Tir cannot replace Citrobacter Tir for this function. This indicates that C. rodentium and EPEC use an actin nucleating mechanism different from EHEC. We also found that Tir is expressed and translocated into mouse enterocytes in vivo by C. rodentium during infections. This represents the first direct demonstration of a type III effector translocated in vivo into a natural host by any pathogen. In addition, we showed that Tir, but not its tyrosine phosphorylation, is essential for C. rodentium to colonize the large bowel and induce attaching/effacing (A/E) lesions and colonic hyperplasia in mice, and that both EPEC Tir and EHEC Tir can substitute for Citrobacter Tir for these activities in vivo. These results thus demonstrate that Tir is an essential virulence factor in this infection model. The data also show that the function of Tir tyrosine phosphorylation and its subsequent actin nucleating activity are not essential for C. rodentium colonization of the mouse gut nor for inducing A/E lesions and colonic hyperplasia, thereby uncoupling colonization and disease from actin condensation for this A/E pathogen.

CesD2 of enteropathogenic Escherichia coli is a second chaperone for the type III secretion translocator protein EspD

Enteropathogenic Escherichia coli (EPEC) and enterohemorrhagic E. coli are extracellular pathogens that employ a type III secretion system to export translocator and effector proteins, proteins which facilitates colonization of the mucosal surface of the intestine via formation of attaching and effacing (A/E) lesions. The genes encoding the proteins for A/E lesion formation are located on a pathogenicity island, termed the locus of enterocyte effacement (LEE), which contains eae encoding intimin as well as the type III secretion system and effector genes. Many type III secreted proteins are stabilized and maintained in a secretion-competent conformation in the bacterial cytosol by specific chaperone proteins. Three type III chaperones have been described thus far within the EPEC LEE region: CesD, for the translocator proteins EspB and EspD; CesT, for the effector proteins Tir and Map; and CesF, for EspF. In this study we report the characterization of CesD2 (previously Orf27), a second LEE-encoded chaperone for EspD. We show specific CesD2-EspD protein interaction which appears to be necessary for proper EspD secretion in vitro and pathogenesis in vivo as demonstrated in the A/E-lesion-forming mouse pathogen Citrobacter rodentium.

Prophylactic feeding of Lactobacillus acidophilus NCFM to mice attenuates overt colonic hyperplasia

The objective of this project was to determine if the probiotic Lactobacillus acidophilus NCFM would protect mice from developing transmissible murine colonic hyperplasia (TMCH) caused by Citrobacter rodentium. Our hypothesis was that the oral administration of L. acidophilus NCFM to mice would mitigate colonic hyperplasia and modulate the host immune response. A concurrent administration (CA) study was performed by feeding mice phosphate-buffered saline (PBS), C. rodentium only, L. acidophilus NCFM only, or C. rodentium and NCFM concurrently on the same day. The mice in the CA study were not protected by the probiotic, since their mean colon sample weights (0.109 g) were significantly higher than those of the PBS controls (0.0774 g), and the hematoxylin and eosin-stained samples showed histological changes typically associated with TMCH. A prophylactic feeding (PF) study was performed by orally feeding mice PBS or NCFM once daily for 20 consecutive days; in addition, on day 7, mice were challenged with either PBS or C. rodentium. Mice in the PF study were protected when they consumed the probiotic prior to the pathogen challenge, since their mean colon sample weights (0.0812 g) were not significantly higher than those of the controls (0.0753 g). The hematoxylin and eosin-stained samples appeared similar to the control samples, and the intestinal interleukin (IL)-15 and gamma interferon (IFN-gamma) mRNA levels were reduced. L. acidophilus NCFM did attenuate overt colonic hyperplasia when fed to mice prior to challenge with C. rodentium. The mouse model used in this study enabled us to investigate the efficacy of the L. acidophilus NCFM in preventing gastrointestinal disease and is a valid model for future probiotic research.

Modulation of inducible nitric oxide synthase expression by the attaching and effacing bacterial pathogen citrobacter rodentium in infected mice

Citrobacter rodentium belongs to the attaching and effacing family of enteric bacterial pathogens that includes both enteropathogenic and enterohemorrhagic Escherichia coli. These bacteria infect their hosts by colonizing the intestinal mucosal surface and intimately attaching to underlying epithelial cells. The abilities of these pathogens to exploit the cytoskeleton and signaling pathways of host cells are well documented, but their interactions with the host's antimicrobial defenses, such as inducible nitric oxide synthase (iNOS), are poorly understood. To address this issue, we infected mice with C. rodentium and found that iNOS mRNA expression in the colon significantly increased during infection. Immunostaining identified epithelial cells as the major source for immunoreactive iNOS. Finding that nitric oxide (NO) donors were bacteriostatic for C. rodentium in vitro, we examined whether iNOS expression contributed to host defense by infecting iNOS-deficient mice. Loss of iNOS expression caused a small but significant delay in bacterial clearance without affecting tissue pathology. Finally, immunofluorescence staining was used to determine if iNOS expression was localized to infected cells by staining for the C. rodentium virulence factor, translocated intimin receptor (Tir), as well as iNOS. Interestingly, while more than 85% of uninfected epithelial cells expressed iNOS, fewer than 15% of infected (Tir-positive) cells expressed detectable iNOS. These results demonstrate that both iNOS and intestinal epithelial cells play an active role in host defense during C. rodentium infection. However, the selective expression of iNOS by uninfected but not infected cells suggests that this pathogen has developed mechanisms to locally limit its exposure to host-derived NO.

LsaA, an antigen involved in cell attachment and invasion, is expressed by Lawsonia intracellularis during infection in vitro and in vivo

Lawsonia intracellularis has been identified recently as the etiological agent of proliferative enteropathies, which are characterized by intestinal epithelial hyperplasia and associated moderate immune responses. This disease complex has been reported in a broad range of animals, prevalently in pigs, and L. intracellularis has been linked with ulcerative colitis in humans. L. intracellularis is an obligate intracellular bacterium, and the pathogenic mechanisms used to cause disease are unknown. Using in vitro-grown organisms as a source of genomic DNA, we identified a Lawsonia gene which encodes a surface antigen, LsaA (for Lawsonia surface antigen), associated with attachment to and entry into cells. The deduced amino acid sequence of this protein showed some similarity to members of a novel protein family identified in a number of other bacterial pathogens but for which roles are not fully defined. Transcription of this gene was detected by reverse transcription-PCR in L. intracellularis grown in vitro in IEC18 cells and in bacteria present in ileal tissue from infected animals. Immunohistochemistry with specific monoclonal antibody and immunoblotting with sera from infected animals demonstrated that LsaA protein is synthesized by L. intracellularis during infection. Expression of this gene during infection in vitro and in vivo suggests that this surface antigen is involved during infection, and phenotypic analysis indicated a role during L. intracellularis attachment to and entry into intestinal epithelial cells

Detection of EspB using reversed passive latex agglutination: application to determination of enteropathogenic Escherichia coli

We developed a new practical method to identify enteropathogenic Escherichia coli (EPEC) by detecting the pathogenic factor, EspB. E. coli were cultured in Dulbecco's Modification of Eagle's Medium (DMEM), and EspB was detected in the culture supernatant by reversed passive latex agglutination (RPLA). All 63 E. coli strains harboring the eaeA gene encoding intimin were positive for RPLA, and all 25 strains without the eaeA gene were negative. Among these 63 eaeA-positive strains, 38 Shiga toxin-producing E. coli (STEC) produced Shiga toxin (Stx) under the same culture conditions (DMEM). Subtypes of EspB alpha, beta and gamma were antigenically cross-reactive to each other as determined by RPLA and Western blotting. A kit for Stx detection (RPLA) is commercially available and therefore this RPLA for detection of EspB could be a practical method to define EPEC in both clinical laboratories and the field.

Mice lacking T and B lymphocytes develop transient colitis and crypt hyperplasia yet suffer impaired bacterial clearance during Citrobacter rodentium infection

The bacterial pathogen Citrobacter rodentium belongs to a family of gastrointestinal pathogens that includes enteropathogenic and enterohemorrhagic Escherichia coli and is the causative agent of transmissible colonic hyperplasia in mice. The molecular mechanisms used by these pathogens to colonize host epithelial surfaces and form attaching and effacing (A/E) lesions have undergone intense study. In contrast, little is known about the host's immune response to these infections and its importance in tissue pathology and bacterial clearance. To address these issues, wild-type mice and mice lacking T and B lymphocytes (RAG1 knockout [KO]) were infected with C. rodentium. By day 10 postinfection (p.i.), both wild-type and RAG1 KO mice developed colitis and crypt hyperplasia, and these responses became more exaggerated in wild-type mice over the next 2 weeks, as they cleared the infection. By day 24 p.i., bacterial clearance was complete, and the colitis had subsided; however, crypt heights remained increased. In contrast, inflammatory and crypt hyperplastic responses in the RAG1 KO mice were transient, subsiding after 2 weeks. By day 24 p.i., RAG1 KO mice showed no signs of bacterial clearance and infection was often fatal. Surprisingly, despite remaining heavily infected, tissues from RAG1 KO mice surviving the acute colitis showed few signs of disease. These results thus emphasize the important contribution of the host immune response during infection by A/E bacterial pathogens. While T and/or B lymphocytes are essential for host defense against C. rodentium, they also mediate much of the tissue pathology and disease symptoms that occur during infection.

Structural studies on the O-polysaccharide of the lipopolysaccharide produced by Citrobacter rodentium (ATCC 51459)

Citrobacter rodentium is the etiologic agent of transmissible murine colonic hyperplasia (TMCH) and is the only Citrobacter species known to possess virulence factors homologous to human enteropathogenic and enterohemorrhagic Escherichia coli. Members of this species are considered clonal and represent the only known attaching and effacing bacterial pathogen of mice and thus provides a useful animal model for studying the molecular basis of attaching and effacing pathology. The lipopolysaccharide (LPS) produced by C. rodentium has not been previously studied or its possible role as a virulence factor determined. The structure of the LPS has been undertaken as a first step in an investigation of its possible role in pathogenesis. The structure of C. rodentium (ATCC 51459, prototype TMCH isolate, original biotype 4280, previously designated DBS 100) LPS was determined from composition and methylation analyses, mass spectrometry, and two-dimensional nuclear magnetic resonance spectroscopy. The antigenic O-polysaccharide was found to be a high molecular mass branched polymer of repeating pentasaccharide units composed of 2-acetamido-2-deoxy-D-glucose (D-GlcNAc), d-glucose (D-Glc), and L-rhamnose (L-Rha) in the molar ratio 2 : 2 : 1 linked through phosphate, and has the structure: [structure: see text]

Locus of enterocyte effacement from Citrobacter rodentium: sequence analysis and evidence for horizontal transfer among attaching and effacing pathogens

The family of attaching and effacing (A/E) bacterial pathogens, which includes diarrheagenic enteropathogenic Escherichia coli (EPEC) and enterohemorrhagic E. coli (EHEC), remains a significant threat to human and animal health. These bacteria intimately attach to host intestinal cells, causing the effacement of brush border microvilli. The genes responsible for this phenotype are encoded in a pathogenicity island called the locus of enterocyte effacement (LEE). Citrobacter rodentium is the only known murine A/E pathogen and serves as a small animal model for EPEC and EHEC infections. Here we report the full DNA sequence of C. rodentium LEE and provide a comparative analysis with the published LEEs from EPEC, EHEC, and the rabbit diarrheagenic E. coli strain RDEC-1. Although C. rodentium LEE shows high similarities throughout the entire sequence and shares all 41 open reading frames with the LEE from EPEC, EHEC, and RDEC-1, it is unique in its location of the rorf1 and rorf2/espG genes and the presence of several insertion sequences (IS) and IS remnants. The LEE of EPEC and EHEC is inserted into the selC tRNA gene. In contrast, the Citrobacter LEE is flanked on one side by an operon encoding an ABC transport system, and an IS element and sequences homologous to Shigella plasmid R100 and EHEC pO157 flank the other. The presence of plasmid sequences next to C. rodentium LEE suggests that the prototype LEE resided on a horizontally transferable plasmid. Additional sequence analysis reveals that the 3-kb plasmid in C. rodentium is nearly identical to p9705 in EHEC O157:H7, suggesting that horizontal plasmid transfer among A/E pathogens has occurred. Our results indicate that the LEE has been acquired by C. rodentium and A/E E. coli strains independently during evolution.

Enterohaemorrhagic and enteropathogenic Escherichia coli use a different Tir-based mechanism for pedestal formation

Enterohaemorrhagic Escherichia coli (EHEC) adheres to the host intestinal epithelium, resulting in the formation of actin pedestals beneath adhering bacteria. EHEC and a related pathogen, enteropathogenic E. coli (EPEC), insert a bacterial receptor, Tir, into the host plasma membrane, which is required for pedestal formation. An important difference between EPEC and EHEC Tir is that EPEC but not EHEC Tir is tyrosine phosphorylated once delivered into the host. In this study, we assessed the role of Tir tyrosine phosphorylation in pedestal formation by EPEC and EHEC. In EPEC, pedestal formation is absolutely dependent on Tir tyrosine phosphorylation and is not complemented by EHEC Tir. The protein sequence surrounding EPEC Tir tyrosine 474 is critical for Tir tyrosine phosphorylation and pedestal formation by EPEC. In contrast, Tir tyrosine phosphorylation is not required for pedestal formation by EHEC. EHEC forms pedestals with both wild-type EPEC Tir and the non-tyrosine-phosphorylatable EPEC Tir Y474F. Pedestal formation by EHEC requires the type III delivery of additional EHEC factors into the host cell. These findings highlight differences in the mechanisms of pedestal formation by these closely related pathogens and indicate that EPEC and EHEC modulate different signalling pathways to affect the host actin cytoskeleton.

Comparison of bacteriological, genetic and pathological characters between Escherichia coli O115a,c:K(B) and Citrobacter rodentium

Murine pathogenic Escherichia coli O115a,c:K(B) (MPEC) is the causative agent of mouse megaenteron, the pathology of which resembles that of transmissible murine colonic hyperplasia caused by Citrobacter rodentium. We compared their genetic and pathological features to reveal the relationship between these two bacteria. To evaluate the genetic distances, 16S rDNA genes were sequenced and biochemical reactions were tested. Mouse strain susceptibility tests, using CF1 MPEC-susceptible germfree mice and BALB/cA(Jic) resistant mice were performed. MPEC strains and C. rodentium showed more than 99.6% identity by comparison of 16S rDNA gene sequences. All results from biochemical reactions and the mouse strain susceptibility tests were identical. It is proposed that MPEC should be reclassified as C. rodentium.

Molecular pathogenesis of Citrobacter rodentium and transmissible murine colonic hyperplasia

Here we review the history, clinical significance, pathology and molecular pathogenesis of Citrobacter rodentium, the causative agent of transmissible murine colonic hyperplasia. C. rodentium serves as an important model pathogen for investigating the mechanisms controlling attaching and effacing pathology, epithelial hyperproliferation, and tumor promotion in the distal colon of the mouse.

Gut feelings: enteropathogenic E. coli (EPEC) interactions with the host

Enteropathogenic Escherichia coli (EPEC) is a gram-negative bacterial pathogen that adheres to human intestinal epithelial cells, resulting in watery, persistent diarrhea. It subverts the host cell cytoskeleton, causing a rearrangement of cytoskeletal components into a characteristic pedestal structure underneath adherent bacteria. In contrast to other intracellular pathogens that affect the actin cytoskeleton from inside the host cytoplasm, EPEC remains extracellular and transmits signals through the host cell plasma membrane via direct injection of virulence factors by a "molecular syringe," the bacterial type III secretion system. One injected factor is Tir, which functions as the plasma membrane receptor for EPEC adherence. Tir directly links extracellular EPEC through the epithelial membrane and firmly anchors it to the host cell actin cytoskeleton, thereby initiating pedestal formation. In addition to stimulating actin nucleation and polymerization in the host cell, EPEC activates several other signaling pathways that lead to tight junction disruption, inhibition of phagocytosis, altered ion secretion, and immune responses. This review summarizes recent developments in our understanding of EPEC pathogenesis and discusses similarities and differences between EPEC pedestals, focal contacts, and Listeria monocytogenes actin tails.

Citrobacter rodentium, the causative agent of transmissible murine colonic hyperplasia, exhibits clonality: synonymy of C. rodentium and mouse-pathogenic Escherichia coli

Citrobacter rodentium (formerly Citrobacter freundii biotype 4280 and Citrobacter genomospecies 9) was described on the basis of biochemical characterization and DNA-DNA hybridization data and is the only Citrobacter species known to possess virulence factors homologous to those of the human pathogens enteropathogenic Escherichia coli and enterohemorrhagic E. coli. These virulence factors are encoded on the locus of enterocyte effacement (LEE), a pathogenicity island required for the characteristic attaching and effacing (AE) pathology seen in infection with these three pathogens. C. rodentium, which apparently infects only mice, provides a useful animal model for studying the molecular basis of AE pathology. No work has been done to assess differences in pathogenicity between C. rodentium isolates from diverse sources. Here, we report the examination of 15 C. rodentium isolates using a battery of genetic and biochemical approaches. No differences were observed between the isolates by repetitive-element sequence-based PCR analysis, biochemical analysis, and possession of LEE-specific virulence factors. These data suggest that members of the species are clonal. We further characterized an atypical E. coli strain from Japan called mouse-pathogenic E. coli (MPEC) that, in our hands, caused the same disease as C. rodentium. Applying the same battery of tests, we found that MPEC possesses LEE-encoded virulence factors and is indistinguishable from the previously characterized C. rodentium isolate DBS100. These results demonstrate that MPEC is a misclassified C. rodentium isolate and that members of this species are clonal and represent the only known attaching and effacing bacterial pathogen of mice.