Oxidation of bacillithiol during killing of Staphylococcus aureusUSA300 inside neutrophil phagosomes

Targeting immune evasion tactics of pathogenic bacteria may hold the key to treating recalcitrant bacterial infections. Staphylococcus aureusproduces bacillithiol (BSH), its major low‐molecular‐weight thiol, which is thought to protect this opportunistic human pathogen against the bombardment of oxidants inside neutrophil phagosomes. Here, we show that BSH was oxidized when human neutrophils phagocytosed S. aureus, but provided limited protection to the bacteria. We used mass spectrometry to measure the oxidation of BSH upon exposure of S. aureusUSA300 to either a bolus of hypochlorous acid (HOCl) or a flux generated by the neutrophil enzyme myeloperoxidase. Oxidation of BSH and loss of bacterial viability were strongly correlated (r= 0.99, p< 0.001). BSH was fully oxidized after exposure of S. aureusto lethal doses of HOCl. However, there was no relationship between the initial BSH levels and the dose of HOCl required for bacterial killing. In contrast to the HOCl systems, only 50% of total BSH was oxidized when neutrophils killed the majority of phagocytosed bacteria. Oxidation of BSH was decreased upon inhibition of myeloperoxidase, implicating HOCl in phagosomal BSH oxidation. A BSH‐deficient S. aureusUSA300 mutant was slightly more susceptible to treatment with either HOCl or ammonia chloramine, or to killing within neutrophil phagosomes. Collectively, our data show that myeloperoxidase‐derived oxidants react with S. aureusinside neutrophil phagosomes, leading to partial BSH oxidation, and contribute to bacterial killing. However, BSH offers only limited protection against the neutrophil's multifaceted killing mechanisms. Bacterial bacillithiol was oxidized during phagocytosis but provided minimal protection against the neutrophil's killing mechanisms.