Bacillus subtilis has been intensively studied and widely used in biotechnology, known as the best characterized of the low GC Gram-positive bacterial species. This rod-shaped bacterium produces endospores that allow it to undergo a series of extreme environmental conditions like heat, gastric hydrochloric acid, pancreatic enzymes, and bile. Bacillus subtilis has demonstrated the ability to produces many enzymes that support metabolism, protect intestinal cells against oxidant-mediated tissue damage and loss of barrier function, and increase the immune reaction of intestinal cells. Additionally, this spore-form bacterium produces several fungicidal compounds, which control fungal pathogens such as Candida.

Bacillus clausii has a unique ability to form spores, allowing it to survive extreme environmental conditions that would otherwise be lethal in active form. This spore formation can inhibit the growth of pathogens in the gastrointestinal tract via three distinct mechanisms: colonization of free ecological niches, which are no longer available for other microorganisms, competition for epithelial cell adhesion, and production of enzymes secreted into the intestinal environment. Bacillus clausii can be taken with the most commonly used antibiotics, guaranteeing the survival of a sufficient number of vegetative form spores, leading to improve antibiotic-associated symptoms and assist high compliance to antibiotic therapy.