Taxonomy of Pseudomonas spp determines interactions with Bacillus subtilis

Bacilli and pseudomonads are among the most well-studied microorganisms commonly found in soil, and frequently co-isolated. Despite this, no systematic approach has been employed to assess the pairwise compatibility of members from these genera. Here, we screened 720 fluorescent soil isolates for their effects on Bacillus subtilis pellicle formation in two types of media and found a predictor for interaction outcome in Pseudomonas taxonomy. Interactions were context-dependent and both medium composition and culture conditions strongly influenced interactions. Negative interactions were associated with Pseudomonas capeferrum, Pseudomonas entomophila and Pseudomonas protegens, and 2,4-diacetylphloroglucinol was confirmed as a strong (but not exclusive) inhibitor of B. subtilis. Non-inhibiting strains were closely related to Pseudomonas trivialis and Pseudomonas lini, but in this case, cocultures with increased B. subtilis pellicle formation were spatially segregated. Our study is the first to propose an overall negative outcome from pairwise interactions between B. subtilis and fluorescent pseudomonads, hence cocultures comprising members from these groups are likely to require additional microorganisms for coexistence.