Bacteroides spp. and diarrhea

Abstract

Purpose of review: The purpose of this review is to describe recent progress in the understanding of the role of Bacteroides spp. in human diarrheal diseases and newer murine studies implicating certain Bacteroides spp. in colorectal cancer.

Recent findings: Bacteroides fragilis is the only strain of Bacteroides spp. associated with diarrheal disease. Toxin-producing strains of B. fragilis, termed enterotoxigenic Bacteroides fragilis (ETBF), are an established cause of diarrheal disease in people. The clinical syndrome associated with ETBF diarrheal disease encompasses abdominal pain, tenesmus and inflammatory diarrhea. Two new studies conducted in mice have further defined the chronic inflammatory response associated with ETBF infection and observed that in the multiple intestinal neoplasia mouse strain, heterozygotes for the adenomatous polyposis coli gene, ETBF infection enhances development of colonic tumors. Separate murine studies have begun to define the role of nontoxin-producing B. fragilis as a symbiont, serving possibly to protect the host from colonic inflammation.

Summary: B. fragilis remains the leading anaerobe in human disease. ETBF is emerging as an important cause of human diarrheal disease but additional epidemiologic studies are needed to better understand the role of ETBF human disease.

Figure 1. Molecular subtypes of B. fragilis sensu stricto

B. fragilis commonly colonize the colons of humans. Two molecular subtypes of B. fragilis are now identified termed as NTBF and ETBF. ETBF are distinguished by secretion of the BFT. Three distinct isotypes of BFT exist but, to date, a single ETBF strain secretes only one isotype of BFT. Murine data suggest that NTBF promote colon mucosal health (see text) and human data indicate that ETBF induce inflammatory diarrhea but also are frequent clinically silent colon colonizers. BFT, Bacteroides fragilis toxin; ETBF, enterotoxigenic Bacteroides fragilis; NTBF, nonenterotoxigenic Bacteroides fragilis.

Figure 2. Enterotoxigenic Bacteroides fragilis induce colon tumors in multiple intestinal neoplasia mice

The photograph shows the distal colons of Min mice colonized for 2 months with either NTBF or ETBF. Only ETBF-colonized Min mice display excess gross tumors in colons stained with methylene blue. ETBF, enterotoxigenic Bacteroides fragilis; Min, multiple intestinal neoplasia; NTBF, nonenterotoxigenic Bactersoides fragilis. Reproduced with permission from [23•].