Molecular analysis of the Staphylococcus aureus CidR regulon and its effect on cell death and lysis

The Staphylococcus aureus cid and lrg operons encode homologous proteins that regulate murein hydrolase activity, antibiotic tolerance, and stationary phase survival. It has been shown that expression of the IrgAB operon is positively regulated by a two-component regulatory system encoded by the lytSR operon located immediately upstream to IrgAB. In contrast, the cidABC operon lies downstream from the cidR gene, encoding a potential LysR-type transcriptional regulator. The experiments described in this thesis tested the hypothesis that the cidR gene product affects expression of cid and lrg regulatory system and, consequently, viability and lysis of S. aureus . In the first part of this study, it was demonstrated that the cidR gene product positively regulates cidABC expression by increasing transcription in the presence of acetic acid. As a result of this increase, extracellular murein hydrolase activity was also increased. Also, it was demonstrated that the cidR gene affects antibiotic tolerance and stationary phase survival of S. aureus. The second part of this study elucidated a role for cidR in the regulation of IrgAB expression and the dicistronic operon, alsSD. It was shown that CidR positively regulates expression of IrgAB in the presence of acetic acid in a proton motive force (PMF)-independent manner. Surprisingly, Northern blot analyses revealed that transcription of cidABC and IrgAB is uninducible in an alsSD knock-out mutant, suggesting that the CidR-mediated upregulation of cidABC and IrgAB expression is dependent on the presence of intact alsSD genes. In addition, the alsSD mutation caused a significant decrease in murein hydrolase activity and had a dramatic impact on cell viability and lysis in stationary phase. Overall, the results presented in this thesis define the CidR regulon and provide more details on the molecular mechanisms controlling cell death and lysis in S. aureus.