Enterotoxigenic Bacteroides fragilis: a rogue among symbiotes

Abstract

Enterotoxigenic Bacteroides fragilis (ETBF) strains are strains of B. fragilis that secrete a 20-kDa heat-labile zinc-dependent metalloprotease toxin termed the B. fragilis toxin (BFT). BFT is the only recognized virulence factor specific for ETBF. ETBF strains are associated with inflammatory diarrheal disease in children older than 1 year of age and in adults; limited data suggest an association of ETBF colonization with inflammatory bowel disease flare-ups and colorectal cancer. ETBF secretes one of three highly related BFT isoforms. The relationship between BFT isoform and disease expression is unknown. Although the mechanism of action of BFT is incompletely understood, available data suggest that BFT binds to a specific intestinal epithelial cell receptor, stimulating intestinal cell signal transduction pathways that result in cell morphology changes, cleavage of E-cadherin, reduced colonic barrier function, and increased epithelial cell proliferation and cytokine expression (such as the proinflammatory chemokine interleukin-8). Together, the data suggest that in some hosts, ETBF acts via secretion of BFT to induce colitis. However, the full spectrum of clinical disease related to ETBF and the impact of chronic ETBF colonization on the host remain to be defined.

FIG. 1.

ETBF induce murine colitis. Colonization of 4-week-old C57BL/6 mice with ETBF for 2 weeks induces inflammation and hyperplasia throughout the colon. Panels 1 to 3, mice inoculated with phosphate-buffered saline; panels 4 to 6, mice inoculated with ETBF strain 86-5443-2-2. In panels 4 and 5, the rounded and detaching colonic epithelial cells in ETBF-infected mice can be seen. (Reproduced from reference with permission of the publisher. Copyright John Wiley and Sons Ltd.)

FIG. 2.

Effect of BFT on HT29/C1 cells in vitro. HT29/C1 cells (a human colonic carcinoma continuous cell line) exhibit morphological changes, including cell rounding and dissolution of cell clusters, when treated with BFT (5 nM). (Reprinted from reference with permission from Elsevier.)

FIG. 3.

Schematic of the structure of BFT holotoxin. Each of the BFT isotypes (BFT-1, BFT-2, and BFT-3) consists of three protein domains, i.e., the signal peptide, proprotein, and mature toxin. The holotoxin is cleaved by an as yet unidentified B. fragilis protease at amino acids (AAs) arginine (Arg)₂₁₁-alanine (Ala)₂₁₂ prior to release of the mature, ∼20-kDa BFT protein from the bacterial cells into the colon. H, histidine; G, glycine. (Reprinted from references and with permission from Elsevier.)

FIG. 4.

Schematic of the molecular types of B. fragilis. Pattern I B. fragilis strains are ETBF strains possessing at least one 65-kb conjugative transposon (86 CTn), within which is contained the 6-kb BfPAI. The BfPAI contains two genes, one encoding BFT (bft), demonstrated to be important to ETBF pathogenesis (92), and one encoding metalloprotease II (mpII), a putative virulence protein. Pattern II B. fragilis strains lack CTn86 and CTn9343 (or related sequences). Pattern III B. fragilis strains are NTBF strains that possess at least one 65-kb conjugative transposon (9343 CTn). See the text and Fig. 5 for additional details. (Reprinted from reference with permission from Elsevier.)

FIG. 5.

Schematic representations of CTn elements found in ETBF and NTBF strains. Both ETBF and NTBF strains may possess a variety of CTns related to those originally described for ETBF strain 86-5443-2-2 (CTn86) and NTBF strain 9343 (CTn9343) (27). Panels A and B show different patterns of CTns present in a collection of 123 ETBF and 73 NTBF strains, respectively. Gray boxes represent the left end of CTn86, and black boxes represent the left end of CTn9343. (Reprinted from reference .)

FIG. 6.

Model of ETBF colitis pathogenesis. ETBF colonizes the colon, where BFT is released and attaches to a specific colonic epithelial cell (CEC) receptor, triggering complex (and incompletely understood) CEC signal transduction involving β-catenin, tyrosine kinases (TK), mitogen-activated protein kinases (MAPK), and NF-κB. CEC signal transduction results in the cleavage of E-cadherin as well as in new CEC protein synthesis, with increased expression of c-Myc, cyclooxygenase-2 (COX-2), and chemokines/cytokines, including IL-8 and TGF-β. E-cadherin cleavage initiates decreased barrier function of the colonic mucosa, with the potential for increased exposure of the mucosal immune system to antigens of ETBF as well as the colonic flora fostering an inflammatory mucosal response. c-Myc expression stimulates CEC proliferation, at least in part. Release of chemokines/cytokines by CECs into the submucosa enhances mucosal inflammation in response to ETBF colonization. The precise contributions of different mucosal immune cells to the inflammatory response to ETBF colon colonization are unknown, but data suggest that both polymorphonuclear leukocytes and lymphocytes are important (; S. Wu and C. L. Sears, submitted for publication). DC, dendritic cell; MΦ, macrophage; PMNs, polymorphonuclear leukocytes.