Pathogenic strains of E. coli including enteropathogenic E. coli (EPEC), enterohemorrhagic E. coli (EHEC), enterotoxigenic E. coli (ETEC) are principle causes for diarrhoea in many parts of the globe. Citrobacter rodentium (C. rodentium), a gram negative bacterium, is a murine pathogen that also utilizes type III secretion system and similar virulence factors to EPEC and EHEC and forms comparable attaching/effacing lesions in the intestines as EPEC and EHEC. The infection caused by C. rodentium in mice is usually self-limiting and results in only minor systemic effects with higher mortality in some susceptible mouse strains. All these characteristics have made the bacteria a commonly used model to study host immune responses to pathogenic E. coli infection. In this review, we focus on the impact of virulence factors of the pathogen; different immune components involved in the immune response and summarize their role during C. rodentium infection.
Escherichia coli is a frequent commensal organism of human intestine, often colonizing immediately after birth and usually remaining for decades [
C. rodentium (formerly known as Citrobacter freundii biotype 4280) a non-motile, gram-negative bacteria in the family of Enterobacteriaceae, is recognised as the contributing species of transmissible murine colonic hyperplasia (TMCH) [
Following entry to the body via oral route, C. rodentium colonizes the caecal patch, a type of lymphoid tissue in caecum, from where the bacteria gradually progress to colonise distal colon [
The type III secretion system is programmed by a cluster of genes recognized as locus of enterocyte effacement (LEE), a conserved pathogenicity island consisting of 35.6 kb [
Upon entry to the host cells, Tir is tyrosine phosphorylated, which recruits non-catalytic region of tyrosine kinase adaptor protein Nck [
Besides Tir, LEE encodes several secreted translocators: EspA, EspD, EspB, EspF, EspG, EspH, EspZ, which are entirely translocated into the host cells and are involved in modulating host cytoskeleton leading to the manifestation of disease [
Other than the genes for rorf1 and rorf2/espG and several insertion sequences (IS) and IS remnants, both the LEE of C. rodentium and that of EPEC and EHEC shares all 41 ORFs and the linear gene sequences (
Similar to EPEC and EHEC, C. rodentium infection encompasses three distinct phases: 1) an initial colonization phase specially facilitated by bacterial effector proteins, 2) an acute phase characterized by colonic hyperplasia with the initiation of diarrhoea in severe cases, 3) a convalescent phase manifest as the clearance of bacteria and the prevention of further invasion.
During the first week after inoculation, C. rodentium colonizes the brush border microvilli and higher numbers of bacteria are seen closely adherent to the mucosal epithelial cells (
Most of the existing knowledge on the immune response and its relation with pathology has been expanded using mice with the targeted ablations of various immune components. Innate immune response as well as adaptive immune response appears to control mucosal defence against C. rodentium [
The concept of exploring mice with deficiencies in immune components first came from the study that colonic mucosa of infected mice contained large infiltrates of CD4+ T cells with a helper T cell 1 cytokine response [
mortality. However, depletion of CD8+ T cells or TCRγδ+ T cells did not adversely affect survival of infection and played a minor role in surviving the acute phase of infection. Two studies [
To analyse the potential of secretory antibodies in bacterial clearance, mice with selective deficiencies for IgA, IgM or IgG were used [
Simmons and co-workers investigated C. rodentium infection in IFNγ-deficient and IL-12 deficient mice. IFNγ-deficient mice had higher bacterial numbers and
Mouse models | Effects of ablation of specific adaptive immune components on C. rodentium infection | Refs |
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Mice lacking CD4+ T cells or TCRαβ+ T cells | A survival limit of two weeks and exhibited 100% mortality | [ |
Mice lacking CD8+ T cells or TCRγδ+ T cells | Do not adversely affect survival of infection | [ |
Mice without mature B cells (μMT mice) | Cannot lessen bacterial load or clear bacterial colonization over a prolonged period | [ |
RAG1-deficient mice (both B and T cells are absent) | Develop chronic intestinal colonization and unable to clear infection and die after 3-4 weeks | [ |
IgA- and IgM- deficient mice | Develop effective immunity against a secondary challenge and play a negligible role in controlling C. rodentium infection. | [ |
Mice deficient in IgG antibodies | Lose the ability to develop protective response against secondary challenge. | [ |
IFNγ-deficient mice | Higher bacterial numbers and enhanced mucosal thickening in colons and cannot clear infection until day 28 | [ |
IL-12 deficient mice | Elicit higher bacterial numbers for the first 3 weeks of infection and eventually clear infection by day 35 | [ |
Mice lacking IL-22 | Display systemic bacterial load and enhanced epithelial hyperplasia and mortality range up to 100% within the first two weeks of infection | [ |
Treg deficient mice (DEREG mice) | Diminished bacterial clearance, systemic dissemination of bacteria with compromised Th17 immune response accompanied by less inflammation-associated pathology | [ |
IL-10 ablated mice | Resolve infection earlier than wild-type mice with less infection associated colitis | [ |
enhanced mucosal thickening in their colons and could not clear infection until day 28 [
During infection, CD4+ Th17 cell subsets were particularly amplified in Peyer’s patches (PP) but were unaltered in mesenteric LNs [
Symonds and co-workers demonstrated an elevated FoxP3 mRNA expression in the distal colon at all stages of infection with C. rodentium. Also, C. rodentium infection exhibited an up-regulation of IL17 mRNA expression [
Th22 cells were found to be important in the mucosal anti-microbial host defense against C. rodentium. Basu and co-workers [
There are several innate immune components which perform a vital role in mucosal homeostasis and in antimicrobial immunity. An augmented pathology was observed in mice lacking Toll-like receptor 2 (TLR2) due to an impaired epithelial barrier [
Liu and co-workers demonstrated the biological function of inflammasomes in immune response against C. rodentium. Mice deficient in inflammasome components Nlrp3, Nlrc4, and caspase-1 were hyper susceptible to C. rodentium induced intestinal inflammation due to impaired production of IL-1β and IL-18 [
Mice lacking the p50 subunit of the NF-κB transcription factor, a nuclear factor kappa B, had reduced ability to clear C. rodentium infection [
Mouse models | Effects of ablation of specific innate immune components on C. rodentium infection | Refs |
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Mice lacking Toll-like receptor 2 (TLR2) | An augmented pathology due to an impaired epithelial barrier | [ |
Mice lacking MYD88 | Have greater bacterial loads in colon and peripheral tissues and suffer from severe colitis and death | [ |
IL-1R deficient mice | Increased susceptibility to tissue damage but do not display amplified pathogen burdens in colon. | [ |
Deficiency of TLR4 | Decreased tissue pathology and inflammatory cell infiltration in gut. While the extent of infection is unaffected, dissemination of bacteria through colon is hindered | [ |
Mice deficient in Nlrp3, Nlrc4, and caspase-1 | Hyper susceptible to C. rodentium induced intestinal inflammation. However, exhibit only mild defects and do not die after infection | [ |
IL-1β−/− and IL-18−/− mice | Increased bacterial burdens and severe histopathology. | [ |
Nod2−/− mice | Diminished intestinal clearance to C. rodentium. due to impaired secretion of CCL2 from colonic cells | [ |
Mice lacking the p50 subunit of NF-κB | Reduced ability to clear C. rodentium infection. | [ |
Mice deficient in p38α | A continued bacterial load with no apparent histological lesions, however, fails to recruit CD4+ T cells and impaired chemokines expression. | [ |
Ablation of specific macrophage/monocyte compartment | Neither cell type is essential to trigger immunity | [ |
Mice lacking PSGL-1 and P, E and L-selectin | Mice defective in PSGL-1 and P-selectin suffer morbidity, extensive inflammatory responses and augmented bacterial burden, however, mice defective in either E or L-selectin do not exhibit severe infection | [ |
Mice lacking β7 integrin | Efficiently control infection and clear bacteria 5-6 week after inoculation | [ |
Mice deficient Muc2 | Susceptible to the C. rodentium-induced colitis and display quick weight loss and exhibit 90% mortality | [ |
with noticeable bacterial dissemination, augmented bacterial titre, and deteriorated tissue pathology [
The function of macrophages and monocytes during C. rodentium infection was investigated using ablation of specific macrophage/monocyte compartment during infection. Although neither cell type was essential to trigger immunity, monocytes and macrophages played a role by secreting IL-12, which prompted Th1 polarization and IFN-γ secretion. Thus, monocytes and macrophages contribute in C. rodentium immunity by secreting cytokines that direct T cell polarization [
To outline the function of selectins and their ligands during C. rodentium infection, Kum and co-workers investigated infection in mice lacking PSGL-1, a P-selectin glycoprotein ligand-1 and P, E and L-selectin [
Mice deficient in main intestinal mucin, Muc2, which have an altered intestinal mucus layer, were more susceptible to the C. rodentium-induced colitis and displayed quick weight loss and exhibited about 90% mortality due to a closer interaction of intestinal microbes with the epithelial barrier [
Probiotics, a combination of live microorganisms attenuated infection with C. rodentium in adult mice and provided a protective role in C. rodentium induced death in neonatal mice [
Metronidazole pretreatment augmented exposure to C. rodentium-induced colitis compared to that of untreated mice 6 days postinfection and resulted in a diminished number of Porphyromonadaceae and amplified population of lactobacilli [
A limitation to the study of C. rodentium infection model is the absence of antigen-specific tools with which to characterize the fate and function of the pathogen/antigen-specific response during infection [
EPEC and EHEC are the leading cause of diarrhoea in human, affecting children and adults in both developing and developed countries. C. rodentium is an enteric murine pathogen that mimics virulence factors of human EPEC and EHEC and forms comparable attaching and effacing lesions, as a central mechanism of tissue targeting, virulence factors and infection in mice. As a result of this association with other important inflammatory diseases, and that there are cases of more than a million deaths each year from EPEC and EHEC, the knowledge about the pathophysiology of C. rodentium infections and following infection how the host immune system responds to it is of immense significance to understand its subsequent function during these inflammatory diseases.
This review comprehensively covers the salient features of recent discoveries related to C. rodentium virulence, epithelial hyperplasia, innate and adaptive immune responses, and the pathophysiology of diarrhoea. It is acknowledged that EPEC and EHEC can be modelled efficiently in mice. Murine C. rodentium is a well characterised model of diarrhoeal disease as the molecular, cellular, pathophysiological aspects of the disease have been well studied. Therefore, C. rodentium represents an excellent model in which to study the innate and adaptive immune components. We believe that the advances that have been included in this review will give a comprehensive insight to combat the acute diarrhoeal illness in human. Nevertheless, once again C. rodentium has been proved to be a useful in vivo model for studying pathogenesis of secretory diarrhoeal diseases/gastrointestinal pathogen and for preventive/mucosal vaccinations and therapeutic approaches.
The authors declare no conflict of interest that could be perceived to bias the work.
Rahman, T., Seraj, Md.F. and Islam, Md.M. (2018) Citrobacter rodentium, a Gut Pathogen: The Yin and the Yang of Its Pathophysiology, Immunity and Clinical Manifestation in Mice. Advances in Microbiology, 8, 699-718. https://doi.org/10.4236/aim.2018.89047