Frequency of 1st- and 2nd-step topoisomerase mutations in Streptococcus pneumoniae following levofloxacin and moxifloxacin exposure

Seven Streptococcus pneumoniae isolates were exposed to inhibitory concentrations of levofloxacin and moxifloxacin in antibiotic-containing agar dilution plates. Colony counts were used to calculate the frequency of mutation. DNA was sequenced to detect mutations in the quinolone resistance-determining regions of the gyrA, gyrB, parC, and parE genes. The wild-type S. pneumoniae isolate developed a parC mutation after exposure to levofloxacin more frequently than it developed a gyrA mutation after exposure to moxifloxacin. The 1st-step gyrA mutant developed a 2nd-step gyrA–parC mutation more frequently after exposure to levofloxacin. Conversely, the transformation from a 1st-step parC mutant to a 2nd-step parC–gyrA mutant occurred more frequently following exposure to moxifloxacin. Our data suggest that the occurrence of a 2nd mutation will be contingent on the location of the 1st mutation and the preferential binding site of the fluoroquinolone that drives the transformation from 1st- to 2nd-step mutant.