Nitric oxide and its derivative peroxynitrites are generated by host defense system to control bacterial infection. However certain Gram positive bacteria including Staphylococcus aureus possess a gene encoding nitric oxide synthase (SaNOS) in their chromosome. In this study it was determined that under normal growth conditions, expression of SaNOS was highest during early exponential phase of the bacterial growth. In oxidative stress studies, deletion of SaNOS led to increased susceptibility of the mutant cells compared to wild-type S. aureus. While inhibition of SaNOS activity by the addition of L-NAME increased sensitivity of the wild-type S. aureus to oxidative stress, the addition of a nitric oxide donor, sodium nitroprusside, restored oxidative stress tolerance of the SaNOS mutant. The SaNOS mutant also showed reduced survival after phagocytosis by PMN cells with respect to wild-type S. aureus. 1. Introduction Staphylococcus aureus is a Gram-positive bacterial pathogen that colonizes anterior nares and mucosal surfaces in humans and is responsible for causing a wide array of diseases from mild skin infections to life-threatening conditions such as bacteremia, pneumonia, and endocarditis [1–4]. The emerging resistant strains of S. aureus exacerbate efforts to control or properly treat staphylococcal infections . The host immune system responds to bacterial infections in a concerted manner to eliminate this pathogen. This involves recruitment of polymorphonuclear leukocytes and macrophages to the site of infection and ingestion of invading bacteria. Uptake of bacteria triggers oxygen-dependent and oxygen-independent microbicidal pathways in the phagocytic cells. The oxygen-dependent pathway generates superoxide anion ( ) that serves as a precursor for additional reactive oxygen species (ROS) such as hydrogen peroxide (H2O2), hydroxyl radical, singlet oxygen, hypochlorous acid (HOCl), and peroxynitrite [6–9]. S. aureus utilizes various strategies to defend itself against host immune attack. It produces antioxidant enzymes such as superoxide dismutase that converts superoxide anion to H2O2, catalase that converts H2O2 to water and oxygen, and alkyl hydroperoxide reductases that detoxify H2O2, peroxynitrites and hydroperoxides [10, 11]. In addition to their ability to protect from host’s oxidants, S. aureus infections impose oxidative stress in a host . During infection with a methicillin resistant S. aureus strain, host neutrophils respond by an increase in nitric oxide production . Nitric oxide (NO) is a free radical synthesized by nitric
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