Lactoperoxidase-generated hypothiocyanite efficiently kills Streptococcus pneumoniae

Abstract

Streptococcus pneumoniae (pneumococcus) infections affect millions of people worldwide, cause serious mortality and represent a major economic burden. Airway epithelial cells, the primary responders to pneumococcal infection, orchestrate an extracellular antimicrobial system consisting of lactoperoxidase, thiocyanate anion and hydrogen peroxide (H2O2). Lactoperoxidase oxidizes thiocyanate using H2O2 into the final product hypothiocyanite. While hypothiocyanite has known antimicrobial effects against a wide range of microorganisms, its effect on pneumococcus has never been studied. To test this, pneumococci were exposed to hypothiocyanite generated enzymatically in a cell-free in vitro system. Bacterial killing was determined by colony forming units on agar plates while bacteriostatic effects were measured by changes in optical density of liquid cultures. We demonstrate hypothiocyanite exerted robust killing of several pneumococcal strains. Nonencapsulated pneumococcal mutants were killed to the same extent as their parental strains, indicating no role of the capsule in protection against hypothiocyanite. Catalase, an H2O2 scavenger, inhibited pneumococcal killing by hypothiocyanite under all circumstances. Interestingly, a pneumococcal mutant deficient in pyruvate oxidase, the main bacterial H2O2 source, had enhanced susceptibility to hypothiocyanite indicating the role of this gene in partial protection of the bacterium against this oxidizing agent. Overall, numerous pneumococcal strains were found to be susceptible to lactoperoxidase-generated hypothiocyanite in vitro, which presents an opportunity to explore hypothiocyanite as a potential novel anti-pneumococcal therapy.