Bifidobacterium pseudolongum has characteristics of a keystone species in bifidobacterial blooms in the ceca of rats fed Hi-Maize starch.

Starches resistant to mammalian digestion are present in foods and pass to the large bowel where they may be degraded and fermented by the microbiota. Increases in relative abundances of bifidobacteria (blooms) have been reported in rats whose diet was supplemented with Hi-Maize resistant starch. We determined that the bifidobacterial species present in the rat cecum under these circumstances mostly belonged to Bifidobacterium animalis However, cultures of B. animalis isolated from the rats failed to degrade Hi-Maize starch to any extent. In contrast, Bifidobacterium pseudolongum also detected in the rat microbiota had high starch-degrading ability. Transcriptional comparisons showed increased expression of a Type 1 pullulanase, alpha amylase, and 'glycogen debranching enzyme' by B. pseudolongum when cultured in medium containing Hi-Maize starch. Maltose was released into the culture medium and B. animalis cultures had shorter doubling times in maltose medium compared to B. pseudolongum Thus B. pseudolongum, which was present at a consistently low abundance in the microbiota, but which has extensive enzymic capacity to degrade resistant starch, showed the attributes of a keystone species associated with the bifidobacterial bloom. IMPORTANCE This study addresses the microbiology and function of a natural ecosystem (the rat gut) using DNA-based observations and in vitro experimentation. The microbial community (microbiota, microbiome) of the large bowel of animals, including humans, has been studied extensively by use of high throughput DNA sequencing methods and advanced bioinformatics analysis. These studies reveal the compositions and genetic capacities of microbiotas, but not the intricacies of how microbial communities function. Our work, combining DNA sequence analysis and laboratory experiments with cultured strains of bacteria, revealed that increased abundance of bifidobacteria in the rat gut, induced by feeding indigestible starch, involved a species that cannot itself degrade the starch ( Bifidobacterium animalis ) but cohabits with a species that can ( Bifidobacterium pseudolongum ). This latter species has the characteristics of a keystone species in the community because it had low abundance but high ability to perform a critical function (hydrolysis of resistant starch).