Identifying Mutator Phenotypes among Fluoroquinolone-Resistant Strains of Streptococcus pneumoniae Using Fluctuation Analysis

The occurrence of mutator phenotypes among laboratory-generated and clinical levofloxacin-resistant strains of Streptococcus pneumoniae was determined using fluctuation analysis. The in vitro selection for levofloxacin-resistant mutants of strain D39, each with point mutations in both gyrA and parC or parE, was not associated with a significant change in the mutation rate. Two of eight clinical isolates resistant to levofloxacin (MIC, >8 μg/ml) had estimated mutation rates of 1.2 × 10−7 and 9.4 × 10−8 mutations per cell division, indicating potential mutator phenotypes, compared to strain D39, which had an estimated mutation rate of 1.4 × 10−8 mutations per cell division. The levofloxacin-resistant isolates with the highest mutation rates showed evidence of dysfunctional mismatch repair and contained missense mutations in mut genes at otherwise highly conserved sites. The association of hypermutability in levofloxacin-resistant S. pneumoniae clinical isolates with mutations in DNA mismatch repair genes provides further evidence that mismatch repair mutants may have a selective advantage in the setting of antibiotic pressure, facilitating the development of further antibiotic resistance.