The role of pH in determining the species compositionof the human colonic microbiota

摘要

The pH of the colonic lumen varies with anatomicalsite and microbial fermentation of dietary residue.We have investigated the impact of mildly acidic pH,which occurs in the proximal colon, on the growth ofdifferent species of human colonic bacteria in pureculture and in the complete microbial community.Growth was determined for 33 representative humancolonic bacteria at three initial pH values (approxi-mately 5.5, 6.2 and 6.7) in anaerobic YCFA medium,which includes a mixture of short-chain fatty acids(SCFA) with 0.2% glucose as energy source. Repre-sentatives of all eight Bacteroides species testedgrew poorly at pH 5.5, as did Escherichia coli,whereas 19 of the 23 Gram-positive anaerobes testedgave growth rates at pH 5.5 that were at least 50% ofthose at pH 6.7. Growth inhibition of B. thetaiotaomi-cron at pH 5.5 was increased by the presence of theSCFA mix (33 mM acetate, 9 mM propionate and 1 mMeach of iso-valerate, valerate and iso-butyrate). Analy-sis of amplified 16S rRNA sequences demonstrated amajor pH-driven shift within a human faecal bacterialcommunity in a continuous flow fermentor. Bacteroi-des spp. accounted for 27% of 16S rRNA sequencesdetected at pH 5.5, but 86% of sequences at pH 6.7.Conversely, butyrate-producing Gram-positive bacte-ria related to Eubacterium rectale represented 50% ofall 16S rRNA sequences at pH 5.5, but were notdetected at pH 6.7. Inhibition of the growth of a majorgroup of Gram-negative bacteria at mildly acidic pHapparently creates niches that can be exploited bymore low pH-tolerant microorganisms.