Disentangling the potential factors defining Bacillus subtilis abundance in natural soils

Bacillus subtilis is ubiquitously and broadly distributed in various environments but mostly isolated from soil. Given that species of B. subtilis are known as key plant growth-promoting rhizobacteria in agriculture, we here aimed to describe the natural distribution of this species and uncover how biotic and abiotic factors affect its distribution. When comparing different soils, we discovered that B. subtilis is most abundant in grasslands, but can rarely be isolated from forest soil, even if the sample sites for the two types of soil are situated in proximity. Differential analysis revealed that spore-forming bacteria exhibited enrichments in the grassland, suggesting niche overlap or synergistic interactions leading to the proliferation of certain Bacillus species in grassland environments. Network analysis further revealed that Bacillus and other Bacillota established a densely interconnected hub module in the grassland soil samples, characterized by positive associations indicating co-occurrence, a pattern not observed in the forest soil. Speculating that this difference was driven by abiotic factors, we next combined amplicon sequencing with physio-chemical analysis of soil samples, and found multiple chemical variables, mainly pH, to affect microbial composition. Our study pinpoints the factors that influence B. subtilis abundance in natural soils and, therefore, offers insights for designing B. subtilis-based biocontrol products in agricultural settings.