Bacillus subtilis has been investigated as a probiotic alternative or adjunct to antibiotics, primarily in the context of animal health and, to a lesser extent, in human applications. The rationale for its use stems from its ability to inhibit pathogenic bacteria through competitive exclusion, production of antimicrobial compounds (such as subtilin), and modulation of the immune system. Several studies, especially in livestock (e.g., poultry and pigs), demonstrate that supplementation with B. subtilis can reduce incidence of gastrointestinal infections and improve growth performance, outcomes commonly targeted by antibiotic growth promoters. These findings have driven its inclusion in animal feed as a strategy to reduce antibiotic use and combat antimicrobial resistance.

In humans, the evidence is more limited and primarily consists of small clinical trials or experimental studies investigating B. subtilis as a probiotic. Some research shows modest benefits in restoring gut flora after antibiotic treatment and in reducing the risk of antibiotic-associated diarrhea, but results are inconsistent. The safety profile is generally favorable, and its spore-forming ability allows survival through the harsh gastric environment.

Overall, while B. subtilis is not a direct substitute for antibiotics in treating infections, the scientific rationale and preliminary evidence support its use as a probiotic adjunct to reduce reliance on antibiotics or to mitigate antibiotic side effects, especially in agriculture. However, high-quality human clinical evidence remains limited, so the evidence rating is moderate to low (2/5).