Mycobacterium smegmatis Resists the Bactericidal Activity of Hypochlorous Acid Produced in Neutrophil Phagosomes

Neutrophils are often the major leukocyte at sites of mycobacterial infection, yet little is known about their ability to kill mycobacteria. In this study we have investigated whether the potent antibacterial oxidant hypochlorous acid (HOCl) contributes to killing of when this bacterium is phagocytosed by human neutrophils. We found that were ingested by neutrophils into intracellular phagosomes but were killed slowly. We measured a of 30 min for the survival of inside neutrophils, which is 5 times longer than that reported for and 15 times longer than Live-cell imaging indicated that neutrophils generated HOCl in phagosomes containing ; however, inhibition of HOCl production did not alter the rate of bacterial killing. Also, the doses of HOCl that are likely to be produced inside phagosomes failed to kill isolated bacteria. Lethal doses of reagent HOCl caused oxidation of mycothiol, the main low-m.w. thiol in this bacterium. In contrast, phagocytosed maintained their original level of reduced mycothiol. Collectively, these findings suggest that can cope with the HOCl that is produced inside neutrophil phagosomes. A mycothiol-deficient mutant was killed by neutrophils at the same rate as wild-type bacteria, indicating that mycothiol itself is not the main driver of resistance. Understanding how avoids killing by phagosomal HOCl could provide new opportunities to sensitize pathogenic mycobacteria to destruction by the innate immune system.