Bacillus colonization of the gastrointestinal tract

Species of the Bacillus genus are gram-positives spore formers, considered to be soil bacteria. Sporulation and germination of Bacillus bacteria are extensively studies in vitro and in various soil models. We propose that as herbivores feed off the ground, they naturally intake bacterial spores together with plant material, and that passage through the GI tract of animals is a common phenomenon that the bacteria has adapted to.

We found a significantly higher load of Bacillus spores in the feces of herbivores compared to the feces of mice or humans. To investigate intestinal colonization of Bacillus, we isolated and characterized a strain of Bacillus clausii derived from feces of mice living in a zoo. We orally inoculated C57BL/6 mice with B. clausii as either spores or as vegetative cells and showed that the mice that were inoculated with spores continued to shed B. clausii for over two weeks after inoculation, whereas the bacteria were quickly eradicated from mice that were inoculated with vegetative cells. Chemically-induced colitis in mice or a pre-treatment with a single dose of antibiotics resulted in a higher B. clausii shedding compared to non-treated mice. Our results suggest that consumption of soil-dwelling Bacillus spores can lead to transient intestinal colonization, affected by the intestinal microbiota and by host physiology.