The Staphylococcus aureus cidC gene encodes a pyruvate oxidase that affects acetate metabolism and cell death in stationary phase

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Abstract
The Staphylococcus aureus cid and lrg operons have previously been shown to affect murein hydrolase activity and antibiotic tolerance. Based on their similarities to the holin family of proteins it was proposed that the functions of the cidA and lrgA gene products are analogous to bacteriophage-encoded holin and antiholin proteins respectively. The cid operon expresses two overlapping transcripts, one that spans the cidA, cidB and cidC genes and whose expression is induced by the acetic acid generated by aerobic growth in the presence of excess glucose, and the other that spans the cidB and cidC genes only and is expressed in a sigma B-dependent manner. In the study presented here, we have focused primarily on the third gene of this operon, cidC. A sequence analysis of the cidC gene product suggested that it encodes a pyruvate oxidase that catalyses the oxidative decarboxylation of pyruvate yielding acetate and CO2. Indeed, a ferricyanide-based spectrophotometric assay revealed that the cidC mutant produced decreased pyruvate oxidase activity relative to the parental and complemented strains. In the presence of excess glucose the cidC mutant accumulated normal levels of acetic acid in the growth medium, likely because of the activity of the pyruvate dehydrogenase complex. However, in contrast to the wild type and complemented strains, the pH of the cidC mutant culture began to increase gradually until it was able to utilize the acetate for a secondary round of growth. Finally, a mutation in cidA caused reduced cell lysis in stationary phase but only minimally affected cell death. These results indicate that the cidC gene product is involved in the generation of acetic acid that contributes to the cell death and lysis that occurs in high-glucose stationary phase cultures, while the cidA gene product, a putative holin, controls lysis of the dying cells.