Inhibitory and bactericidal effects of hydrogen peroxide production by Streptococcus pneumoniae on other inhabitants of the upper respiratory tract

An inverse correlation between colonization of the human nasopharynx by Str eptococcus pneumoniae and Haemophilus influenzae, both common upper respira tory pathogens, has been reported. Studies were undertaken to determine if either of these organisms produces substances which inhibit growth of the o ther. Culture supernatants from S, pneumoniae inhibited growth of H. influe nzae, whereas culture supernatants from H. influenzae had no effect on the growth of S. pneumoniae. Moreover, coculture of S. pneumoniae and H. influe nzae led to a rapid decrease in viable counts of H. influenzae. The additio n of purified catalase prevented killing of H. influenzae in coculture expe riments, suggesting that hydrogen peroxide may be responsible for this bact ericidal activity, H. influenzae was killed by concentrations of hydrogen p eroxide similar to that produced by S pneumoniae. Hydrogen peroxide is prod uced by the pneumococcus through the action of pyruvate oxidase (SpxB) unde r conditions of aerobic growth. Both an spxB mutant and a naturally occurri ng variant of S. pneumoniae, which is downregulated in SpxB expression, wer e unable to kill H. influenzae. A catalase-reversible inhibitory effect of S. pneumoniae on the growth of the respiratory tract pathogens Moraxella ca tarrhalis and Neisseria meningitidis was also observed. Elevated hydrogen p eroxide production, therefore, may be a means by which S. pneumoniae is abl e to inhibit a variety of competing organisms in the aerobic environment of the upper respiratory tract.