A toxin-mediated policing system in Bacillus improves population fitness via penalizing non-cooperating phenotypic cheaters

Microbial cooperation is vulnerable to exploitation by social cheaters. Although the strategies for controlling genotypic cheaters have been well investigated, the mechanism and significance of preventing phenotypic cheating remain largely unknown. Here, we revealed the molecular mechanism and ecological significance of a policing system for punishing phenotypic cheaters in the community of a plant beneficial strain Bacillus velezensis SQR9. Coordinated activation of extracellular matrix (ECM) production and autotoxin bacillunoic acids (BAs) biosynthesis/self-immunity, punished public goods-nonproducing cheaters in strain SQR9’s community. Spo0A was identified to be the co-regulator for triggering both ECM production and BAs synthesis/immunity, which activates acetyl-CoA carboxylase (ACC) to produce malonyl-CoA, an essential precursor for BAs biosynthesis, thereby stimulating BAs production and self-immunity. Elimination of phenotypic cheaters by this policing system, significantly enhanced population fitness under different stress conditions and in plant rhizosphere. This study provides insights into our understanding of maintenance and evolution of microbial cooperation.