Impact of pH on lactate formation and utilization by human fecal microbial communities

Abstract

The human intestine harbors both lactate-producing and lactate-utilizing bacteria. Lactate is normally present at <3 mmol liter(-1) in stool samples from healthy adults, but concentrations up to 100 mmol liter(-1) have been reported in gut disorders such as ulcerative colitis. The effect of different initial pH values (5.2, 5.9, and 6.4) upon lactate metabolism was studied with fecal inocula from healthy volunteers, in incubations performed with the addition of dl-lactate, a mixture of polysaccharides (mainly starch), or both. Propionate and butyrate formation occurred at pH 6.4; both were curtailed at pH 5.2, while propionate but not butyrate formation was inhibited at pH 5.9. With the polysaccharide mix, lactate accumulation occurred only at pH 5.2, but lactate production, estimated using l-[U-(13)C]lactate, occurred at all three pH values. Lactate was completely utilized within 24 h at pH 5.9 and 6.4 but not at pH 5.2. At pH 5.9, more butyrate than propionate was formed from l-[U-(13)C]lactate in the presence of polysaccharides, but propionate, formed mostly by the acrylate pathway, was the predominant product with lactate alone. Fluorescent in situ hybridization demonstrated that populations of Bifidobacterium spp., major lactate producers, increased approximately 10-fold in incubations with polysaccharides. Populations of Eubacterium hallii, a lactate-utilizing butyrate-producing bacterium, increased 100-fold at pH 5.9 and 6.4. These experiments suggest that lactate is rapidly converted to acetate, butyrate, and propionate by the human intestinal microbiota at pH values as low as 5.9, but at pH 5.2 reduced utilization occurs while production is maintained, resulting in lactate accumulation.

FIG. 1.

Effect of initial pH on net production of the major SCFA (acetate [open bars], propionate [vertically striped bars], and butyrate [dotted bars]) and lactate (diagonally striped bars) from 24-h batch culture incubations with a mixture of carbohydrates. Error bars indicate standard errors of the means. ND, not detected.

FIG. 2.

Change in log₁₀ counts per ml in bifidobacteria (Bif) and Eubacterium hallii (Ehal) groups, detected using the probes Bif164 and Ehal1469, in 24-h batch culture containing a mixture of polysaccharides (mix), lactate alone (lact), or the mixture of carbohydrates and lactate together (mix+lact) and incubated at three different pHs: 5.2 (open bars), 5.9 (dotted bars), and 6.4 (striped bars). Data were analyzed as two-way ANOVA with random effect for volunteer and fixed effect for substrate, pH, and their interaction. Error bars indicate standard errors of the means obtained from ANOVA. NS, not significant; different letters indicate statistically significant differences (P < 0.05).

FIG. 3.

Relationship between final abundance of lactate-utilizing bacteria (Eubacterium hallii and Anaerostipes caccae groups, detected using Ehal1469 and Acac194 probes) and lactate utilization over 6 h in batch cultures inoculated with fecal slurries from four different volunteers and incubated for 24 h with lactate at two different pHs (5.9 and 6.4) and in the presence or absence of a mixture of carbohydrates. (Seventy-seven percent of variance was accounted for; P was <0.001 for effect of lactate utilization at 6 h, based on linear regression of bacterial count on lactate utilization.)