Bacillus subtilis Intraspecies Interactions Shape Probiotic Activity Against Salmonella Typhimurium

Commercial probiotics are often formulated as multi-strain cocktails, but the effects of social interactions, particularly between strains of a species, are often neglected, despite their potential to contribute to higher-order interactions where these interactions could affect those with a third party. In this study, we investigated the probiotic potential of a collection of Bacillus subtilis strains against Salmonella Typhimurium in single-strain and mixed cultures. The results indicate a promising probiotic potential of B. subtilis as 38 out of 39 strains significantly inhibited the growth of S. Typhimurium. Next, we tested the effect of mixing B. subtilis strains that differ in their inhibitory potency against S. Typhimurium. The results show that strong inhibition by one strain can be significantly reduced by mixing with a less effective strain. Moreover, mixing similarly effective strains mostly resulted in a decreased growth inhibition of the pathogen. Additionally, we found a group of highly aggressive strains, which completely eliminated other B. subtilis strains in the two-strain mixtures. Overall, this work shows that intraspecies interactions between B. subtilis strains can significantly alter the probiotic effect against S. Typhimurium, which is of great importance for future research on the development of multi-strain probiotics.