The role of hydrogen peroxide production in the biology of Streptococcus pneumoniae

An inverse correlation between colonization of the human nasopharynx by Streptococcus pneumoniae and Haemophilus influenzae , both common upper respiratory pathogens, has been reported. Coculture of S. pneumoniae and H. influenzae led to rapid killing of H. influenzae. Exogenous catalase prevented killing of H. influenzae, suggesting that hydrogen peroxide may be responsible for this bactericidal activity. S. pneumoniae which did not produce hydrogen peroxide were unable to kill H. influenzae . H2O2 produced by S. pneumoniae also inhibited the growth of the respiratory tract pathogens Moraxella catarrhalis and Neisseria meningitidis. Loss of function mutations were detected in seven genes, pspA, spxB, xba, licD2, lytA, nanA and atpC. Factors associated with these mutations included: (1) frameshifts caused by reversible gain and loss of single bases within homopolymeric repeats as short as six bases, (2) deletions caused by recombinational events between non-tandem direct repeats as short as eight bases, and (3) substitutions of guanine residues caused at an increased frequency by endogenous hydrogen peroxide production. The latter accounted for a spontaneous mutation rate as high as 2.8 × 10−6 for resistance to optochin but was 10 to 200-fold lower in the absence of detectable H2O2. Hydrogen peroxide is produced in S. pneumoniae by pyruvate oxidase (SpxB), with concentrations in the media often exceeding 1mM. SpxB mutants were found to exhibit 102–10 3-fold decreased survival in 20mM H2O2. SpxB activity was required, rather than a peroxide-inducible pathway. SpxB mutants had decreased levels of acetyl-phosphate, a potential source of ATP. During exposure to H2O2, ATP levels decreased more rapidly in SpxB mutants than in wild-type cells, suggesting their increased killing was due to more rapid ATP depletion. These data support the hypothesis that pyruvate oxidase maintains a reserve energy source that helps to maintain viability during oxidative stress. Although more H 2O2-resistant than E. coli, S. pneumoniae contained similar amounts of free iron, suggesting that S. pneumoniae prevents iron from participating with H2O2 in the production of hydroxy radicals or somehow avoids the effects of these radicals. The ability of S. pneumoniae to produce and survive high concentrations of H2O2 may be important in colonization or disease