Lactate cross-feeding between
Bifidobacterium
species and
Megasphaera indica
contributes to butyrate formation in the human colonic environment
Authors: Zhao Sainan , Lau Raymond , Zhong Yang , Chen Ming-Hsu 
Published: December 2023

Butyrate, a physiologically active molecule, can be synthesized through metabolic interactions among colonic microorganisms. Previously, in a fermenting trial of human fecal microbiota, we observed that the butyrogenic effect positively correlated with the increasing
Bifidobacterium
population and an unidentified
Megasphaera
species. Therefore, we hypothesized that a cross-feeding phenomenon exists between
Bifidobacterium
and
Megasphaera
, where
Megasphaera
is the butyrate producer, and its growth relies on the metabolites generated by
Bifidobacterium
. To validate this hypothesis, three bacterial species (
B. longum
,
B. pseudocatenulatum
, and
M. indica
) were isolated from fecal cultures fermenting hydrolyzed xylan; pairwise cocultures were conducted between the
Bifidobacterium
and
M. indica
isolates; the microbial interactions were determined based on bacterial genome information, cell growth, substrate consumption, metabolite quantification, and metatranscriptomics. The results indicated that two
Bifidobacterium
isolates contained distinct gene clusters for xylan utilization and expressed varying substrate preferences. In contrast,
M. indica
alone scarcely grew on the xylose-based substrates. The growth of
M. indica
was significantly elevated by coculturing it with bifidobacteria, while the two
Bifidobacterium
species responded differently in the kinetics of cell growth and substrate consumption. Coculturing led to the depletion of lactate and increased the formation of butyrate. An RNA-seq analysis further revealed the upregulation of
M. indica
genes involved in the lactate utilization and butyrate formation pathways. We concluded that lactate generated by
Bifidobacterium
through catabolizing xylose fueled the growth of
M. indica
and triggered the synthesis of butyrate. Our findings demonstrated a novel cross-feeding mechanism to generate butyrate in the human colon.

IMPORTANCE

Butyrate is an important short-chain fatty acid that is produced in the human colon through microbial fermentation. Although many butyrate-producing bacteria exhibit a limited capacity to degrade nondigestible food materials, butyrate can be formed through cross-feeding microbial metabolites, such as acetate or lactate. Previously, the literature has explicated the butyrate-forming links between
Bifidobacterium
and
Faecalibacterium prausnitzii
and between
Bifidobacterium
and
Eubacterium rectale
. In this study, we provided an alternative butyrate synthetic pathway through the interaction between
Bifidobacterium
and
Megasphaera indica. M. indica
is a species named in 2014 and is indigenous to the human intestinal tract. Scientific studies explaining the function of
M. indica
in the human colon are still limited. Our results show that
M. indica
proliferated based on the lactate generated by bifidobacteria and produced butyrate as its end metabolic product. The pathways identified here may contribute to understanding butyrate formation in the gut microbiota.