The Intestinal Life Cycle of <i>Bacillus subtilis</i> and Close Relatives

TLDR

Bacillus subtilis is a soil bacterium whose endospore formation enables long‑term environmental survival. The study aimed to determine what occurs to a spore after ingestion. Researchers used reverse transcription‑PCR to assess expression of vegetative and sporulation genes in the mouse gut following oral spore dosing. In mice, ingested spores germinated, sporulated, and resporulated—eliciting anti‑tetanus toxin fragment C responses—while natural biofilm‑forming isolates persisted longer, grew anaerobically, and produced spores more rapidly, demonstrating that spores complete a full life cycle in the gut and suggesting other Bacillus species may inhabit the intestinal microbiota.

Abstract

ABSTRACT Bacillus subtilis is considered a soil organism for which endospore formation provides a means to ensure long-term survival in the environment. We have addressed here the question of what happens to a spore when ingested. Spores displaying on their surface a heterologous antigen, tetanus toxin fragment C (TTFC), were shown to generate anti-TTFC responses not to the antigen contained in the primary oral inoculum but to those displayed on spores that had germinated and then resporulated. We then used reverse transcription-PCR to determine expression of vegetative genes and sporulation-specific genes in the mouse gut following oral dosing with spores. Significant levels of germination and sporulation were documented. Using natural isolates of B. subtilis that could form biofilms, we showed that these strains could persist in the mouse gut for significantly longer than the laboratory strain. Moreover, these isolates could grow and sporulate anaerobically and exhibited a novel phenomenon of being able to form spores in almost half the time required for the laboratory isolate. This suggests that spores are not transient passengers of the gastrointestinal tract but have adapted to carry out their entire life cycle within this environment. This is the first report showing an intestinal life cycle of B. subtilis and suggests that other Bacillus species could also be members of the gut microflora.