Abstract
It is assumed that bacteria always pay a significant physiological price for the acquisition of resistance to antibiotics. To test whether this was the case for a strain of
Streptococcus pneumoniae
that develops resistance to fluoroquinolone antibiotics, we selected resistance to these agents in a wild-type strain and measured their fitness in comparative growth experiments. The relative growth rate
of a mutant strain selected on ciprofloxacin (
parC
Serine 79 to Tyrosine) was compared with its susceptible isogenic parent and no significant deficit was found (relative fitness 1.15 95% C.I. ±
0.2.). A double mutant, however, had a relative fitness of 0.81 (
parC
Serine 79 to Tyrosine
gyrA
Serine 81 to Tyrosine). Mutant strains selected on gemifloxacin had only a modest increase
in minimum inhibitory concentration; thus, second-round mutants were competed with a first-round
gyrA
Serine 81 to Tyrosine or the susceptible isogenic parent. The growth rate of three double-mutant
strains
parC
Serine 79 to Tyrosine
gyrA
Serine 81 to Phenylanine,
parC
Serine 79 to Tyrosine, and Asparagine 83 to Phenylalanine were similar to the isogenic susceptible parent 1.16
(95% C.I. ± 0.17), 0.99 (95% C.I. ± 0.05), and 0.95 (95% C.I. ± 0.05), respectively. These data suggest that mutation in the
parC
and
gyrA
genes may, on some occasions,
not be associated with a physiological deficit.