Resolving the conflict between antibiotic production and rapid growth by recognition of peptidoglycan of susceptible competitors

Abstract

Microbial communities employ a variety of complex strategies to compete successfully against competitors sharing their niche, with antibiotic production being a common strategy of aggression. Here, by systematic evaluation of four non-ribosomal peptides (NRP) produced by B. subtilis clade, we revealed that they acted either synergistically or additively to effectively eliminate phylogenetically distinct competitors. All four major NRP biosynthetic clusters were also imperative for the survival of B. subtilis in a complex community extracted from the rhizosphere. The production of NRP came with a fitness cost manifested in growth inhibition, rendering NRP synthesis uneconomical when growing in proximity to a phylogenetically close species, carrying resistance against the same antibiotics. To resolve this conflict and ease the fitness cost, NRP production was only induced by the presence of peptidoglycan cue from a sensitive competitor. These results experimentally demonstrate a general ecological concept: closely related communities (self) are favoured during competition, due to compatibility in attack and defence mechanisms.

Imaging Flow Cytometry reveals a dual role for exopolysaccharides in biofilms: To promote self-adhesion while repelling non-self-community members

“…B. subtilis was fully resistant to its own conditioned medium, B. atrophaeus was resistant to low concentrations of, but vulnerable to high concentrations of the conditioned medium, (50%v/v., p value<0.0001) and B. mycoides was sensitive to all of the tested concentrations of conditioned medium (p=0.0006) (Figure 1). These results are consistent with our previous work where we reported that the conditioned rich medium of B. subtilis pellicles was inert to self, slightly toxic to the related B. atrophaeus, and significantly toxic to B. mycoides [38].…”

“…Here we report that in addition to their structural role, manifested by their necessity for B. subtilis to generate pellicles (floating biofilms) with itself [57] and with a related bacterium, B. atrophaeus (Figure 1), the exopolysaccharides act to induce the production of bacillaene, a polyketide antibiotic [33,[58][59][60] which was recently shown to be essential for the elimination of phylogenetically distinct Bacillus species [38,61]. Our results indicate that this non-structural role of the exopolysaccharides is manifested by a capacity of B. subtilis to repel the phylogenetically distinct competitor, B. mycoides from the pellicle and its surrounding media.…”

“…Interestingly, the protein matrix component TasA induced Bacillaene while repressing the production of the surfactant and antibiotic surfactin [62,63], also involved in horizontal gene transfer [64]. This result may explain the heterogeneity in gene expression of antibiotics within the biofilm [38], and predicts that their local expression may also reflect the local concentration of the extracellular matrix components.…”

“…Recently, we found that two antibiotics: the NRP (non-ribosomal peptide) surfactin and the polyketide bacillaene play an important role in the elimination of phylogenetically distinct competitors over semi-solid surfaces and in shaking culture [38]. As we found that the role of exopolysaccharides (∆eps) during competition is largely indirect, and was pronounced in the free-living cells present in suspension, we tested whether the exopolysaccharides are capable to regulate surfactin and bacillaene production.…”

“…As we found that the role of exopolysaccharides (∆eps) during competition is largely indirect, and was pronounced in the free-living cells present in suspension, we tested whether the exopolysaccharides are capable to regulate surfactin and bacillaene production. The expression of operons encoding surfactin and bacillaene biosynthetic clusters tagged to a luciferase reporter [38] were compared for ECM mutants. Both exopolysaccharides and TasA were required for optimal bacillaene expression (Figure 4A); however, a deletion of TasA induced surfactin production (Figure S3).…”