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Keywords:

  • adherent-invasive Escherichia coli;
  • IBD;
  • Crohn's disease;
  • ulcerative colitis

Abstract

  1. Top of page
  2. Abstract
  3. E. COLI: FROM COMMENSALISM TO PATHOGENICITY
  4. E. COLI AS POTENT INFLAMMATORY RESPONSE INDUCERS
  5. INCREASED NUMBER OF MUCOSA-ASSOCIATED E. COLI IN PATIENTS WITH IBD
  6. HIGH LEVELS OF ANTI-E. COLI OUTER MEMBRANE PROTEIN C ANTIBODIES IN PATIENTS WITH CD
  7. ADHESIVE PROPERTIES OF CD-ASSOCIATED E. COLI STRAINS
  8. INVASIVE PROPERTIES OF CD-ASSOCIATED E. COLI STRAINS
  9. SURVIVAL AND REPLICATION OF CD-ASSOCIATED E. COLI STRAINS WITHIN MACROPHAGES
  10. PREVALENCE OF ADHERENT-INVASIVE E. COLI IN PATIENTS WITH IBD
  11. CD-ASSOCIATED ADHERENT-INVASIVE E. COLI AND FORMATION OF GRANULOMAS
  12. CONCLUSION
  13. REFERENCES

Increased numbers of mucosa-associated Escherichia coli are observed in both major inflammatory bowel diseases, Crohn's disease (CD) and ulcerative colitis (UC). With the identification of mutations in the NOD2-encoding gene in patients with CD and given the intracellular location of NOD2, the presence of pathogenic invasive bacteria could be the link between innate immune response to invasive bacteria and the development of the inflammation. Adherent-invasive E. coli (AIEC) are isolated from ileal biopsies of 36.4% of patients with ileal involvement of CD. These pathogenic E. coli colonize the intestinal mucosa by adhering to intestinal epithelial cells and are also true invasive pathogens, able to invade intestinal epithelial cells and to replicate intracellularly. AIEC strains also survive and replicate extensively within macrophages without inducing host cell death, and their high replication rates induce the secretion of large amounts of tumor necrosis factor α (TNF-α). There is also evidence suggesting that AIEC is involved in the formation of granulomas. The presence of AIEC is restricted to CD patients. Mucosa-associated E. coli in patients with UC can adhere to intestinal epithelial cells and induce the secretion of IL-8, but there is no evidence that these E. coli strains are invasive.

(Inflamm Bowel Dis 2007)

Several lines of evidence suggest that bacteria play a role in the onset and perpetuation of inflammatory bowel diseases (IBDs).1–5 Intestinal bacteria are essential for the development of intestinal inflammation, being required for the onset of inflammation in numerous knockout models of IBD.6–8 In patients with Crohn's disease (CD), exposure of the terminal ileum postsurgically to luminal contents is associated with increased inflammation, and diversion of the fecal stream is associated with improvement.9 Patients with IBD have higher concentrations of mucosa-associated bacteria than controls.10 Generalized or localized dysbiosis is observed in IBD, corresponding to the presence of low numbers of usual bacteria, high numbers of unusual bacteria, and sometimes a reduction in biodiversity. Studies of luminal bacterial composition in patients with IBD using culture and molecular techniques have reported a decrease in beneficial bacteria such as Bifidobacterium and Lactobacillus species and an increase in pathogenic bacteria such as Bacteroides and Escherichia coli.10–12 Such dysbiosis induce a breakdown in the balance between putative species of “protective” versus “harmful” intestinal bacteria and may promote inflammation.13, 14

E. COLI: FROM COMMENSALISM TO PATHOGENICITY

  1. Top of page
  2. Abstract
  3. E. COLI: FROM COMMENSALISM TO PATHOGENICITY
  4. E. COLI AS POTENT INFLAMMATORY RESPONSE INDUCERS
  5. INCREASED NUMBER OF MUCOSA-ASSOCIATED E. COLI IN PATIENTS WITH IBD
  6. HIGH LEVELS OF ANTI-E. COLI OUTER MEMBRANE PROTEIN C ANTIBODIES IN PATIENTS WITH CD
  7. ADHESIVE PROPERTIES OF CD-ASSOCIATED E. COLI STRAINS
  8. INVASIVE PROPERTIES OF CD-ASSOCIATED E. COLI STRAINS
  9. SURVIVAL AND REPLICATION OF CD-ASSOCIATED E. COLI STRAINS WITHIN MACROPHAGES
  10. PREVALENCE OF ADHERENT-INVASIVE E. COLI IN PATIENTS WITH IBD
  11. CD-ASSOCIATED ADHERENT-INVASIVE E. COLI AND FORMATION OF GRANULOMAS
  12. CONCLUSION
  13. REFERENCES

Escherichia coli colonizes the gastrointestinal tract of human infants within a few hours after birth. As a commensal, E. coli and its mammalian host coexist in good harmony in the mucus layer of the colon and with mutual benefit. It is a successful competitor against anaerobes or facultative anaerobes of the intestinal microflora. Commensal E. coli strains rarely cause disease except in immunocompromised hosts or when the normal gastrointestinal barriers are breached. However, Dr. Escherich suggested from the earliest description of the bacteria that certain E. coli strains can be associated with disease, stating that E. coli could be involved in infections of the intestine and urinary tract in humans. Some E. coli strains have acquired specific virulence factors via DNA horizontal transfer of transposons, plasmids, bacteriophages, and pathogenicity islands. They increase the ability of the bacteria to adapt to new niches and allow them to cause a broad spectrum of diseases. Among the E. coli strains that can cause intestinal disease in humans, there are at least 6 well-characterized classes or pathotypes: enteropathogenic E. coli (EPEC), enterohemorrhagic E. coli (EHEC), enterotoxinogenic E.coli (ETEC), enteroaggregative E. coli (EAEC), enteroinvasive E. coli (EIEC), and diffusely adherent E. coli (DAEC). All these pathogenic E. coli cause disease by a broad range of virulence factors that can affect a wide variety of critical host cell processes, including protein synthesis, signal transduction, cytoskeletal function, cell division, ion secretion, transcription, apoptosis, and mitochondrial function (for review, see Kaper et al15). All these pathogens have to express, in addition to toxins and bacterial effectors affecting eukaryotic processes, numerous fitness and colonization factors that allow the bacteria to adhere to and/or invade host cells.

E. COLI AS POTENT INFLAMMATORY RESPONSE INDUCERS

  1. Top of page
  2. Abstract
  3. E. COLI: FROM COMMENSALISM TO PATHOGENICITY
  4. E. COLI AS POTENT INFLAMMATORY RESPONSE INDUCERS
  5. INCREASED NUMBER OF MUCOSA-ASSOCIATED E. COLI IN PATIENTS WITH IBD
  6. HIGH LEVELS OF ANTI-E. COLI OUTER MEMBRANE PROTEIN C ANTIBODIES IN PATIENTS WITH CD
  7. ADHESIVE PROPERTIES OF CD-ASSOCIATED E. COLI STRAINS
  8. INVASIVE PROPERTIES OF CD-ASSOCIATED E. COLI STRAINS
  9. SURVIVAL AND REPLICATION OF CD-ASSOCIATED E. COLI STRAINS WITHIN MACROPHAGES
  10. PREVALENCE OF ADHERENT-INVASIVE E. COLI IN PATIENTS WITH IBD
  11. CD-ASSOCIATED ADHERENT-INVASIVE E. COLI AND FORMATION OF GRANULOMAS
  12. CONCLUSION
  13. REFERENCES

As a general rule, the interaction between adherent and/or invasive E. coli and host cells induce inflammatory responses. EPEC infection of intestinal epithelial cells induces inflammation by activating several pathways, including PKC and ERK, which lead to NF-κB activation, Il-8 expression upregulation, and transmigration of polymorphonuclear leukocytes.16–19 A recent study showed that EPEC-induced inflammation is a balance between pro- and antiinflammatory proteins.20 Extracellular factors including flagella trigger inflammation,21 and bacterial effectors, such as EspB, secreted via a type III secretion system, attenuate this response.20 Likewise, EHEC interacting with intestinal epithelial cells activates intracellular signaling pathways and an epithelial cell proinflammatory response dependent on Shiga toxin and, to a greater extent, on H7 flagellin.22–26 In fact, for all motile E. coli strains, when flagellin interacts with Toll-like receptor 5 (TLR5) there is a proinflammatory response. In addition, E. coli adhesion to host cells by specific adhesive factors induces enhanced epithelial cell Il-8 production, as reported for EAEC strains via the adhesin AAF or for DAEC strains via the Afa/Dr adhesions.27–29 For EIEC, which differs from Shigella by only a few minor biochemical tests and shares essential virulence factors with them, the invasive phenotype could account for the potent proinflammatory capacity of the bacteria, on the basis of what was reported for Shigella.30 Much of the mucosal inflammation observed during shigellosis is initiated by intracellular sensing of peptidoglycan by cytosolic leucine-rich receptors of the NOD family in epithelial cells.31 This causes activation of the NF-κB and C-Jun terminal kinase pathways and induces expression of IL-8, a chemokine playing a major role in mediating the inflammatory burst and massive infiltration of the mucosa by polymorphonuclear leukocytes. However, this inflammatory response, which is likely to be very harmful for the invading microbes, is regulated by the bacterium itself via type III secretion system-dependent proteins that are injected into host cells and block the transcription of some NF-κB-dependent proinflammatory genes in target cells.32

INCREASED NUMBER OF MUCOSA-ASSOCIATED E. COLI IN PATIENTS WITH IBD

  1. Top of page
  2. Abstract
  3. E. COLI: FROM COMMENSALISM TO PATHOGENICITY
  4. E. COLI AS POTENT INFLAMMATORY RESPONSE INDUCERS
  5. INCREASED NUMBER OF MUCOSA-ASSOCIATED E. COLI IN PATIENTS WITH IBD
  6. HIGH LEVELS OF ANTI-E. COLI OUTER MEMBRANE PROTEIN C ANTIBODIES IN PATIENTS WITH CD
  7. ADHESIVE PROPERTIES OF CD-ASSOCIATED E. COLI STRAINS
  8. INVASIVE PROPERTIES OF CD-ASSOCIATED E. COLI STRAINS
  9. SURVIVAL AND REPLICATION OF CD-ASSOCIATED E. COLI STRAINS WITHIN MACROPHAGES
  10. PREVALENCE OF ADHERENT-INVASIVE E. COLI IN PATIENTS WITH IBD
  11. CD-ASSOCIATED ADHERENT-INVASIVE E. COLI AND FORMATION OF GRANULOMAS
  12. CONCLUSION
  13. REFERENCES

Escherichia coli is the predominant aero-anaerobic Gram-negative species of the normal intestinal flora, where it plays an important role in promoting the stability of the luminal microbial flora and maintaining normal intestinal homeostasis. An immunohistological study searching for pathogenic bacteria or viruses known to cause inflammation or enterocolitis in humans or animals showed that 69% of CD mucosal samples were positively labeled with polyclonal E. coli antibodies.33 The presence of E. coli antigens was also reported in 57% of CD biopsies or resection specimens, and labeling by polyclonal E. coli antibodies was observed in macrophages distributed within the lamina propria, in the germinal centers of mesenteric lymph nodes, and in giant cells along fissures, beneath ulcers, and in granulomas.34 Modifications of luminal bacteria concentrations have been observed in IBD patients. The presence of high concentrations of bacteria forms a biofilm on the surface of the gut mucosa in patients with CD and ulcerative colitis (UC).10 When bacteriologic samples were taken at surgery for CD, E. coli were isolated more frequently from the intestinal serosa and mesenteric nodes of CD patients (27% and 33%, respectively) than in controls.35, 36 Increased numbers of mucosa-associated E. coli are observed in CD and UC.10–12, 37–40 In addition, rectal mucosa-associated E. coli counts were higher in active than in inactive UC and CD and than in controls, and clusters of E. coli were observed in the lamina propria in UC and CD specimens but not in controls.41

HIGH LEVELS OF ANTI-E. COLI OUTER MEMBRANE PROTEIN C ANTIBODIES IN PATIENTS WITH CD

  1. Top of page
  2. Abstract
  3. E. COLI: FROM COMMENSALISM TO PATHOGENICITY
  4. E. COLI AS POTENT INFLAMMATORY RESPONSE INDUCERS
  5. INCREASED NUMBER OF MUCOSA-ASSOCIATED E. COLI IN PATIENTS WITH IBD
  6. HIGH LEVELS OF ANTI-E. COLI OUTER MEMBRANE PROTEIN C ANTIBODIES IN PATIENTS WITH CD
  7. ADHESIVE PROPERTIES OF CD-ASSOCIATED E. COLI STRAINS
  8. INVASIVE PROPERTIES OF CD-ASSOCIATED E. COLI STRAINS
  9. SURVIVAL AND REPLICATION OF CD-ASSOCIATED E. COLI STRAINS WITHIN MACROPHAGES
  10. PREVALENCE OF ADHERENT-INVASIVE E. COLI IN PATIENTS WITH IBD
  11. CD-ASSOCIATED ADHERENT-INVASIVE E. COLI AND FORMATION OF GRANULOMAS
  12. CONCLUSION
  13. REFERENCES

Serological findings showed that antibody titers raised against E. coli were higher in CD patients than in controls.42, 43 Abnormal reactivity to microbial components is observed in patients with IBD, and anti-E. coli outer membrane porin C (anti-OmpC) antibodies have drawn attention for their unusually high coincidence with CD.44, 45 High levels of antibodies against E. coli OmpC are present in 37%–55% of patients with CD, 2%–11% of patients with UC, but in less than 5% of individuals without any IBD.46–48 Reactivity to E. coli OmpC is associated with increased severity of CD, which is characterized by small bowel involvement, frequent disease progression, longer disease duration, and a greater need for intestinal surgery.45 The high expression of anti-OmpC in CD patients and relatively low expression in UC patients has made anti-OmpC a specific serologic marker of CD. It is noteworthy that, in CD-associated E. coli strains, increased expression of OmpC protein was observed under conditions of high osmolarity similar to those in the gastrointestinal tract.49 Interestingly, the role of OmpC in CD-associated adherent and invasive E. coli is crucial for bacterial adhesion to and invasion of intestinal epithelial cells.49 This OmpC-mediated regulation is observed in CD-associated E. coli strains, but not in the nonpathogenic E. coli K-12 strain. This could indicate that CD-associated E. coli have evolved from nonpathogenic to pathogenic bacteria by elaborating intestinal environment adaptation mechanisms for which high osmolarity constitutes a key signal for activating the expression of virulence genes.

ADHESIVE PROPERTIES OF CD-ASSOCIATED E. COLI STRAINS

  1. Top of page
  2. Abstract
  3. E. COLI: FROM COMMENSALISM TO PATHOGENICITY
  4. E. COLI AS POTENT INFLAMMATORY RESPONSE INDUCERS
  5. INCREASED NUMBER OF MUCOSA-ASSOCIATED E. COLI IN PATIENTS WITH IBD
  6. HIGH LEVELS OF ANTI-E. COLI OUTER MEMBRANE PROTEIN C ANTIBODIES IN PATIENTS WITH CD
  7. ADHESIVE PROPERTIES OF CD-ASSOCIATED E. COLI STRAINS
  8. INVASIVE PROPERTIES OF CD-ASSOCIATED E. COLI STRAINS
  9. SURVIVAL AND REPLICATION OF CD-ASSOCIATED E. COLI STRAINS WITHIN MACROPHAGES
  10. PREVALENCE OF ADHERENT-INVASIVE E. COLI IN PATIENTS WITH IBD
  11. CD-ASSOCIATED ADHERENT-INVASIVE E. COLI AND FORMATION OF GRANULOMAS
  12. CONCLUSION
  13. REFERENCES

A correlation between intestinal colonization by E. coli and bacterial adhesion of CD-associated E. coli strains to intestinal epithelial cells has been observed.12, 37 Bacterial adhesion to intestinal epithelial cells is the first step in the pathogenicity of many organisms involved in infectious diseases of the gut. Adhesion enables the bacteria to colonize the mucosa and to resist mechanical removal from the intestine. Studies on the adherence properties of E. coli in IBD have yielded the general conclusion that IBD-associated E. coli strains are able to adhere to various human cells or cell lines. It was reported that adhesive E. coli were isolated from 62% of patients with CD and 68% with UC but from only 6% of normal controls.50 Another independent study reported that 86% of isolates of E. coli from IBD patients were adhesive compared with 27% from patients with infective diarrhea and none from controls.51 Recently, Kotlowski et al38 reported that E. coli with adherence factors were more likely to be associated with UC and CD tissues than with those from controls. The comparison of adhesive properties of E. coli strains isolated from the ileum of CD patients and controls revealed that around 80% of E. coli strains associated with the ileal mucosa of CD patients were adherent versus only 30% of those isolated from controls.12 CD-associated E. coli preferentially adhered to differentiated Caco-2 cells, which represent a mature intestinal cell model. This is consistent with the finding that in patients with CD, crypt epithelial cells, which correspond to immature cells, are rarely involved in early lesions.52

INVASIVE PROPERTIES OF CD-ASSOCIATED E. COLI STRAINS

  1. Top of page
  2. Abstract
  3. E. COLI: FROM COMMENSALISM TO PATHOGENICITY
  4. E. COLI AS POTENT INFLAMMATORY RESPONSE INDUCERS
  5. INCREASED NUMBER OF MUCOSA-ASSOCIATED E. COLI IN PATIENTS WITH IBD
  6. HIGH LEVELS OF ANTI-E. COLI OUTER MEMBRANE PROTEIN C ANTIBODIES IN PATIENTS WITH CD
  7. ADHESIVE PROPERTIES OF CD-ASSOCIATED E. COLI STRAINS
  8. INVASIVE PROPERTIES OF CD-ASSOCIATED E. COLI STRAINS
  9. SURVIVAL AND REPLICATION OF CD-ASSOCIATED E. COLI STRAINS WITHIN MACROPHAGES
  10. PREVALENCE OF ADHERENT-INVASIVE E. COLI IN PATIENTS WITH IBD
  11. CD-ASSOCIATED ADHERENT-INVASIVE E. COLI AND FORMATION OF GRANULOMAS
  12. CONCLUSION
  13. REFERENCES

Primary lesions of CD occur mainly in Peyer's patches,53 as seen in early intestinal lesions induced by entero-invasive pathogens such as Shigella and Salmonella. This supports a putative role for invasive pathogen(s) in the initiation of CD lesions. Indeed, the aphthous ulcer, a necrosis of M-cells of Peyer's lymphoid follicles, is recognized as the earliest lesion of CD.54 Such ulcers occur in shigellosis, salmonellosis, and yersinial enterocolitis, in which invasiveness is an essential virulence factor of the bacteria involved in the disease. Three independent studies have reported the presence of intramucosal E. coli in IBD patients or mucosa-associated E. coli with invasive properties. Invasive bacteria were found in 29%–36% of CD patients, 12%–19% of UC patients, and 3%–9% of controls.37, 55, 56 The invasive process of strain LF82 isolated from a chronic lesion of a CD patient has been extensively studied. This strain efficiently invades a wide range of human epithelial cell lines, including HEp-2 cells, and the intestinal cell lines Intestine-407, Caco-2, and HCT-8.57 The invasion processes of most invasive bacteria, including Shigella flexneri,58, 59 EIEC,60Yersinia enterocolitica,59 and Listeria monocytogenes61 are actin microfilament- but not microtubule-dependent. In contrast, the uptake of LF82, and of the invasive E. coli strains isolated from CD patients, is dependent on both functioning host cell actin microfilaments and microtubules.57 Electron microscopic examination of LF82-infected epithelial cells revealed a macropinocytosis-like process of entry, characterized by elongation of the membrane extensions that surrounded the bacteria at the sites of contact between the entering bacteria and the epithelial cells. Strain LF82 survives and replicates in the host cell cytoplasm after lysis of the endocytic vacuole. The invasive process of LF82 is unique, since it does not possess any of the known genetic invasive determinants described for enteroinvasive, enteropathogenic, and enterotoxigenic E. coli and Shigella strains. The virulence factors of strain LF82 that play a role in its invasive ability are type 1 pili inducing membrane extension,62 flagella conferring bacteria motility and regulating type 1 pili expression,63, 64 outer membrane vesicles delivering bacterial effectors to host cells,65 and the outer membrane protein OmpC, which regulates the expression of most virulence factors.49 Interestingly, outer membrane vesicles of Helicobacter pylori and Pseudomonas aeruginosa were reported to induce proinflammatory responses66, 67 and bacterial flagellin interacting with TLR5 can activate an innate immune response.

SURVIVAL AND REPLICATION OF CD-ASSOCIATED E. COLI STRAINS WITHIN MACROPHAGES

  1. Top of page
  2. Abstract
  3. E. COLI: FROM COMMENSALISM TO PATHOGENICITY
  4. E. COLI AS POTENT INFLAMMATORY RESPONSE INDUCERS
  5. INCREASED NUMBER OF MUCOSA-ASSOCIATED E. COLI IN PATIENTS WITH IBD
  6. HIGH LEVELS OF ANTI-E. COLI OUTER MEMBRANE PROTEIN C ANTIBODIES IN PATIENTS WITH CD
  7. ADHESIVE PROPERTIES OF CD-ASSOCIATED E. COLI STRAINS
  8. INVASIVE PROPERTIES OF CD-ASSOCIATED E. COLI STRAINS
  9. SURVIVAL AND REPLICATION OF CD-ASSOCIATED E. COLI STRAINS WITHIN MACROPHAGES
  10. PREVALENCE OF ADHERENT-INVASIVE E. COLI IN PATIENTS WITH IBD
  11. CD-ASSOCIATED ADHERENT-INVASIVE E. COLI AND FORMATION OF GRANULOMAS
  12. CONCLUSION
  13. REFERENCES

The search for infectious agents likely to cause CD has focused mainly on intracellular pathogens that have evolved to resist phagocytosis and to persist within macrophages, and which may be involved in chronic antigenic stimulation and T cell and macrophage activation. Invasive E. coli strains isolated from CD patients are able to survive and to replicate extensively within murine macrophages in a large vacuole.68 The behavior of the CD-associated invasive E. coli within macrophages is different from that of other invasive bacteria. Whereas most invasive bacteria induce cell death of infected macrophages (for review, see Navarre and Zychlinsky69), no necrosis or apoptosis of macrophages infected with CD-associated invasive E. coli was observed even after 24 hours postinfection.68 Moreover, in contrast to many pathogens that escape from the normal endocytic pathway or infiltrate autophagy, CD-associated invasive E. coli are taken up by macrophages within phagosomes, which mature without diverting from the classical endocytic pathway, and which share features with phagolysosomes.70 To survive and replicate in the harsh environment encountered inside these compartments, including acid pH and proteolytic activity of cathepsin D, bacteria have elaborated adaptation mechanisms in which acidity constitutes a key signal for activating the expression of virulence genes.71 Macrophages infected with CD-associated invasive E. coli release large amounts of tumor necrosis factor α (TNF-α).68 As TNF-α is transcribed and translated de novo after macrophage stimulation, the synthesis of this cytokine demonstrates that macrophages are still active even with numerous intracellular bacteria. Continuous activation and TNF-α release are due to the sustained multiplication of intracellular bacteria within the phagosomes.

PREVALENCE OF ADHERENT-INVASIVE E. COLI IN PATIENTS WITH IBD

  1. Top of page
  2. Abstract
  3. E. COLI: FROM COMMENSALISM TO PATHOGENICITY
  4. E. COLI AS POTENT INFLAMMATORY RESPONSE INDUCERS
  5. INCREASED NUMBER OF MUCOSA-ASSOCIATED E. COLI IN PATIENTS WITH IBD
  6. HIGH LEVELS OF ANTI-E. COLI OUTER MEMBRANE PROTEIN C ANTIBODIES IN PATIENTS WITH CD
  7. ADHESIVE PROPERTIES OF CD-ASSOCIATED E. COLI STRAINS
  8. INVASIVE PROPERTIES OF CD-ASSOCIATED E. COLI STRAINS
  9. SURVIVAL AND REPLICATION OF CD-ASSOCIATED E. COLI STRAINS WITHIN MACROPHAGES
  10. PREVALENCE OF ADHERENT-INVASIVE E. COLI IN PATIENTS WITH IBD
  11. CD-ASSOCIATED ADHERENT-INVASIVE E. COLI AND FORMATION OF GRANULOMAS
  12. CONCLUSION
  13. REFERENCES

On the basis of the pathogenic traits of CD-associated E. coli, a new potentially pathogenic group of E. coli was designated AIEC, for Adherent-Invasive E. coli.57 The criteria for inclusion in the group are: 1) an ability to adhere to and to invade intestinal epithelial cells with a macropinocytosis-like process of entry dependent on actin microfilaments and microtubules recruitment; 2) the ability to survive and to replicate extensively in large vacuoles within macrophages without triggering host cell death; and 3) the ability to induce the release of large amounts of TNF-α by infected macrophages. Adherent-invasive E. coli strains were found to be highly associated with ileal mucosa in CD patients.56 Such pathogenic strains are isolated from ileal specimens of 36.4% of CD patients versus 6% of controls. In colonic specimens, AIEC strains were found in 3.7% of CD patients, 0% of UC patients, and 1.9% of controls. AIEC strains are preferentially found in early recurrent CD lesions after surgery, thus pleading for a possible role in the initiation of inflammation and not only as secondary invaders. Our studies56 together with the study of Martin et al37 support a central role for mucosa-associated adherent-invasive E. coli in the pathogenesis of CD since the translocation of these pathogenic bacteria through the intestinal mucosa could be a crucial step in the propagation of the inflammatory process.

CD-ASSOCIATED ADHERENT-INVASIVE E. COLI AND FORMATION OF GRANULOMAS

  1. Top of page
  2. Abstract
  3. E. COLI: FROM COMMENSALISM TO PATHOGENICITY
  4. E. COLI AS POTENT INFLAMMATORY RESPONSE INDUCERS
  5. INCREASED NUMBER OF MUCOSA-ASSOCIATED E. COLI IN PATIENTS WITH IBD
  6. HIGH LEVELS OF ANTI-E. COLI OUTER MEMBRANE PROTEIN C ANTIBODIES IN PATIENTS WITH CD
  7. ADHESIVE PROPERTIES OF CD-ASSOCIATED E. COLI STRAINS
  8. INVASIVE PROPERTIES OF CD-ASSOCIATED E. COLI STRAINS
  9. SURVIVAL AND REPLICATION OF CD-ASSOCIATED E. COLI STRAINS WITHIN MACROPHAGES
  10. PREVALENCE OF ADHERENT-INVASIVE E. COLI IN PATIENTS WITH IBD
  11. CD-ASSOCIATED ADHERENT-INVASIVE E. COLI AND FORMATION OF GRANULOMAS
  12. CONCLUSION
  13. REFERENCES

The histological characteristics of CD include the presence of aphthous ulcers of the mucosa and epithelioid granulomas. Such granulomas are associated with several infectious diseases involving Mycobacterium tuberculosis, Salmonella spp., Shigella spp., Yersinia enterocolitica, and other pathogenic bacteria able to enter and survive within host cells.72 Like tuberculous granulomas, well-circumscribed granulomas develop during CD by the accumulation of lymphocytes and macrophages, the latter maturing to form epithelioid cells. There is various documented evidence suggesting that E. coli can be involved in the formation of granulomas: 1) E. coli antigens are present in CD granulomas33, 34; 2) E. coli DNA is detected in 80% of microdissected granulomas from CD patients73; and 3) diseases involving granulomatous responses to E. coli have been reported in animals, such as the granulomatous colitis of the Boxer dog or Hjarre's disease in chicken and turkeys.74 Indeed, E. coli was isolated from all granuloma-containing tissue biopsies analyzed from Boxer dogs with colitis.75 Mucoid E. coli has been isolated from tuberculoid lesions of the caeca and liver of chicken and turkeys suffering from Hjarre's disease. In addition, intramuscular inoculation of pure bacterial cultures or triturated diseased tissues reproduced the disease.74, 76, 77 A comparison of invasive E. coli strains isolated from the tissues of Boxer dogs with granulomatous colitis and the AIEC reference strain LF82 showed that all strains had similar invasion mechanisms for entering epithelial cells and behave similarly within macrophages.78 Given the behavior of AIEC strains within macrophages, it was decided therefore to investigate whether AIEC LF82, like the tuberculous bacillus, which also persists within macrophages, induces the formation of granulomas using the in vitro model of human granulomas recently developed by Dr. Puissegur in Dr. Altare's laboratory.79 AIEC LF82 induces aggregation of infected macrophages, fusion of some of them to form multinucleated giant cells, and subsequent recruitment of lymphocytes. These cell aggregates are very similar to the early stages of epithelioid granulomas.80

CONCLUSION

  1. Top of page
  2. Abstract
  3. E. COLI: FROM COMMENSALISM TO PATHOGENICITY
  4. E. COLI AS POTENT INFLAMMATORY RESPONSE INDUCERS
  5. INCREASED NUMBER OF MUCOSA-ASSOCIATED E. COLI IN PATIENTS WITH IBD
  6. HIGH LEVELS OF ANTI-E. COLI OUTER MEMBRANE PROTEIN C ANTIBODIES IN PATIENTS WITH CD
  7. ADHESIVE PROPERTIES OF CD-ASSOCIATED E. COLI STRAINS
  8. INVASIVE PROPERTIES OF CD-ASSOCIATED E. COLI STRAINS
  9. SURVIVAL AND REPLICATION OF CD-ASSOCIATED E. COLI STRAINS WITHIN MACROPHAGES
  10. PREVALENCE OF ADHERENT-INVASIVE E. COLI IN PATIENTS WITH IBD
  11. CD-ASSOCIATED ADHERENT-INVASIVE E. COLI AND FORMATION OF GRANULOMAS
  12. CONCLUSION
  13. REFERENCES

The etiopathogenesis of ileal CD seems to be intimately linked to the presence of AIEC. The high prevalence of AIEC in patients with ileal involvement of CD could be a first step in the establishment of a modified Koch's postulate, which needs to take into account the genetic susceptibility of the host.37, 56 Human genetic studies that have identified mutations in the NOD2/CARD15 encoding gene associated with the severity of the ileal form of CD may provide the link between the innate immune response to invasive bacteria and the development of CD.81, 82 The expression of functional NOD2 might play a role in the clearance of bacteria, even if they are virulent, such as AIEC. Indeed, intestinal epithelial cells that do not express NOD2 or that express the NOD2/CARD15 3020insC CD-associated variant are unable to contain intracellular bacterial replication.83, 84 Ileal CD could result from AIEC abnormally colonizing the gastrointestinal tract, parasitizing host intestinal epithelial cells and macrophages as the result of a dysfunction of the NOD2 innate immune surveillance mechanism. As NOD2 provides a surveillance mechanism inside the host infected cells, it would be interesting to determine the fate of AIEC in human intestinal epithelial cells and macrophages, as well as the response of AIEC-infected cells, as determined by the expression of functional NOD2 or CD-associated NOD2 variants. In addition, as bacterial and molecular analysis of the flora associated with the ileal mucosa of patients with CD showed that E. coli, and especially AIEC, are abnormally predominant in both early and chronic ileal lesions, there is the possibility that one or more specific host receptors recognized by bacterial lectin-like surface adhesins in a genetically susceptible host are abnormally expressed on the surface of the intestinal epithelial cells of CD patients. Finally, as the infection cycle of AIEC could depend on the ability of these pathogenic bacteria to colonize the gastrointestinal (GI) tract of genetically predisposed CD patients, antibiotics that can eradicate the bacteria, or probiotics able to replace them in the GI tract, could be of therapeutic value in ileal CD. The efficacy of several probiotics in IBD have been investigated in clinical trials. E. coli Nissle 1917, Saccharomyces boulardii, and the probiotic preparation VSL#3 have been reported as being as effective as standard treatment in preventing relapse of UC, chronic pouchitis, and CD.85–89 It would be of interest to determine whether the efficacy of the probiotics, at least in CD patients, is associated with a decrease in the number of mucosa-associated adherent-invasive E. coli.

REFERENCES

  1. Top of page
  2. Abstract
  3. E. COLI: FROM COMMENSALISM TO PATHOGENICITY
  4. E. COLI AS POTENT INFLAMMATORY RESPONSE INDUCERS
  5. INCREASED NUMBER OF MUCOSA-ASSOCIATED E. COLI IN PATIENTS WITH IBD
  6. HIGH LEVELS OF ANTI-E. COLI OUTER MEMBRANE PROTEIN C ANTIBODIES IN PATIENTS WITH CD
  7. ADHESIVE PROPERTIES OF CD-ASSOCIATED E. COLI STRAINS
  8. INVASIVE PROPERTIES OF CD-ASSOCIATED E. COLI STRAINS
  9. SURVIVAL AND REPLICATION OF CD-ASSOCIATED E. COLI STRAINS WITHIN MACROPHAGES
  10. PREVALENCE OF ADHERENT-INVASIVE E. COLI IN PATIENTS WITH IBD
  11. CD-ASSOCIATED ADHERENT-INVASIVE E. COLI AND FORMATION OF GRANULOMAS
  12. CONCLUSION
  13. REFERENCES