Short-Sequence Tandem and Nontandem DNA Repeats and Endogenous Hydrogen Peroxide Production Contribute to Genetic Instability of Streptococcus pneumoniae

Loss-of-function mutations in the following seven pneumococcal genes were detected and analyzed: pspA, spxB, xba, licD2, lytA, nanA, and atpC. Factors associated with these mutations included (i) frameshifts caused by reversible gain and loss of single bases within homopolymeric repeats as short as 6 bases, (ii) deletions caused by recombinational events between nontandem direct repeats as short as 8 bases, and (iii) substitutions of guanine residues caused at an increased frequency by the high levels of hydrogen peroxide (>2 mM) typically generated by this species under aerobic growth conditions. The latter accounted for a frequency as high as 2.8 × 10(−6) for spontaneous mutation to resistance to optochin and was 10- to 200-fold lower in the absence of detectable levels of H(2)O(2). Some of these mutations appear to have been selected for in vivo during pneumococcal infection, perhaps as a consequence of immune pressure or oxidative stress.