Selection of hypotoxigenic Vibrio cholerae population by human intestinal bacteria in gnotobiotic mice

Strains of human intestinal bacteria when cultured with Vibrio cholerae 569B, classical Inaba, exerted either inhibitory or promotive effects on cholera toxin production. An inhibitory strain of Bacteroides fragilis was orally given to germfree mice a week before vibrio administration. In vibrio-monoassociated mice, the toxin production of most vibrios isolated from feces was as high as the inocula, while in Bacteroides-vibrio-diassociated mice toxin production decreased starting on day 3 after vibrio administration and vibrios with 1/250 to 1/1,000 of the toxin production levels of the inocula dominated 1 and 2 wk after administration. The number of vibrios remained basically unchanged in vibrio-monoassociated mice, but was kept low in Bacteroides-vibrio-diassociated mice throughout the experimental period of 4 wk. Fecal concentration of succinic acid was higher in Bacteroides-associated mice than in vibrio-monoassociated mice. An isolate from a vibrio-monoassociated mouse yielded poorer colony formation on a heart infusion agar plate containing 6.25 mM succinic acid than a vibrio from a Bacteroides-vibrio-diassociated mouse. Isolates from agar plates containing succinic acid showed low cholera toxin productivity. Thus, the presence of the Bacteroides in gnotobiotic mice may have resulted in the selection of a less toxigenic vibrio population, and succinic acid produced in abundance in Bacteroides-associated mice may be one of the selecting factors.