Dual Role of Hydrogen Peroxide as an Oxidant in Pneumococcal Pneumonia

Significance: Streptococcus pneumoniae ( Spn ), a facultative anaerobic Gram-positive human pathogen with increasing rates of penicillin and macrolide resistance, is a major cause of lower respiratory tract infections worldwide. Pneumococci are a primary agent of severe pneumonia in children younger than 5 years and of community-acquired pneumonia in adults. A major defense mechanism toward Spn is the generation of reactive oxygen species, including hydrogen peroxide (H_{2}O_{2}), during the oxidative burst of neutrophils and macrophages. Paradoxically, Spn produces high endogenous levels of H_{2}O_{2} as a strategy to promote colonization. Recent Advances: Pneumococci, which express neither catalase nor common regulators of peroxide stress resistance, have developed unique mechanisms to protect themselves from H_{2}O_{2}. Spn generates high levels of H_{2}O_{2} as a strategy to promote colonization. Production of H_{2}O_{2} moreover constitutes an important virulence phenotype and its cellular activities overlap and complement those of other virulence factors, such as pneumolysin, in modulating host immune responses and promoting organ injury. Critical Issues: This review examines the dual role of H_{2}O_{2} in pneumococcal pneumonia, from the viewpoint of both the pathogen (defense mechanisms, lytic activity toward competing pathogens, and virulence) and the resulting host-response (inflammasome activation, endoplasmic reticulum stress, and damage to the alveolar-capillary barrier in the lungs). Future Directions: An understanding of the complexity of H_{2}O_{2}-mediated host-pathogen interactions is necessary to develop novel strategies that target these processes to enhance lung function during severe pneumonia.