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Nitric oxide (NO) is emerging as an important regulator of bacterial stress resistance, biofilm development, and virulence. One potential source of endogenous NO production in the pathogen  Staphylococcus aureus  is its NO-synthase (saNOS) enzyme, encoded by the  nos  gene. Although a role for saNOS in oxidative stress resistance, antibiotic resistance, and virulence has been recently-described, insights into the regulation of  nos  expression and saNOS enzyme activity remain elusive. To this end, transcriptional analysis of the  nos  gene in  S. aureus  strain UAMS-1 was performed, which revealed that  nos  expression increases during low-oxygen growth and is growth-phase dependent. Furthermore,  nos  is co-transcribed with a downstream gene, designated  pdt , which encodes a prephenate dehydratase (PDT) enzyme involved in phenylalanine biosynthesis. Deletion of  pdt  significantly impaired the ability of UAMS-1 to grow in chemically-defined media lacking phenylalanine, confirming the function of this enzyme. Bioinformatics analysis revealed that the operon organization of  nos-pdt  appears to be unique to the staphylococci. As described for other  S. aureus nos  mutants, inactivation of  nos  in UAMS-1 conferred sensitivity to oxidative stress, while deletion of  pdt  did not affect this phenotype. The  nos  mutant also displayed reduced virulence in a murine sepsis infection model, and increased carotenoid pigmentation when cultured on agar plates, both previously-undescribed  nos  mutant phenotypes. Utilizing the fluorescent stain 4-Amino-5-Methylamino-2',7'-Difluorofluorescein (DAF-FM) diacetate, decreased levels of intracellular NO/reactive nitrogen species (RNS) were detected in the  nos  mutant on agar plates. These results reinforce the important role of saNOS in  S. aureus  physiology and virulence, and have identified an  in vitro  growth condition under which saNOS activity appears to be upregulated. However, the significance of the operon organization of  nos-pdt  and potential relationship between these two enzymes remains to be elucidated.

Contribution of the nos-pdt Operon to Virulence Phenotypes in Methicillin-Sensitive Staphylococcus aureus