SrrAB modulates Staphylococcus aureus cell death through regulation of cidABC transcription

The death and lysis of a subpopulation in Staphylococcus aureus biofilm cells are thought to benefit the surviving population by releasing extracellular DNA, a critical component of the biofilm extracellular matrix. Although the means by which S. aureus controls cell death and lysis is not understood, studies implicate the role of the cidABC and lrgAB operons in this process. Recently, disruption of the srrAB regulatory locus was found to cause increased cell death during biofilm development, likely as a result of the sensitivity of this mutant to hypoxic growth. In the current study, we extended these findings by demonstrating that cell death in the ΔsrrAB mutant is dependent on expression of the cidABC operon. The effect of cidABC expression resulted in the generation of increased reactive oxygen species (ROS) accumulation and was independent of acetate production. Interestingly, consistently with previous studies, cidC-encoded pyruvate oxidase was found to be important for the generation of acetic acid, which initiates the cell death process. However, these studies also revealed for the first time an important role of the cidB gene in cell death, as disruption of cidB in the ΔsrrAB mutant background decreased ROS generation and cell death in a cidCindependent manner. The cidB mutation also caused decreased sensitivity to hydrogen peroxide, which suggests a complex role for this system in ROS metabolism. Overall, the results of this study provide further insight into the function of the cidABC operon in cell death and reveal its contribution to the oxidative stress response.