Cereulide synthesis in emetic Bacillus cereus is controlled by the transition state regulator AbrB, but not by the virulence regulator PlcR

Cereulide, a depsipeptide structurally related to the antibiotic valinomycin, is responsible for the emetic type of gastrointestinal disease caused by Bacillus cereus. Recently, it has been shown that cereulide is produced non-ribosomally by the plasmid-encoded peptide synthetase Ces. Using deletion mutants of the emetic reference strain B. cereus F4810/72, the influence of the well-known transcription factors PlcR, Spo0A and AbrB on cereulide production and on the transcription of the cereulide synthetase gene cluster was investigated. Our data demonstrate that cereulide synthesis is independent of the B. cereus specific virulence regulator PlcR but belongs to the Spo0A-AbrB regulon. Although cereulide production turned out to be independent of sporulation, it required the activity of the sporulation factor Spo0A. The σ^A-promoted transcription of spo0A was found to be crucial for cereulide production, while the σ^H-driven transcription of spo0A did not affect cereulide synthesis. Overexpression of the transition state factor AbrB in B. cereus F4810/72 resulted in a non-toxic phenotype. Moreover, AbrB was shown to bind efficiently to the main promoter region of the ces operon, indicating that AbrB acts as a repressor of cereulide production by negatively affecting ces transcription.