[In vitro activity of six fluoroquinolones and penicillin against 101 viridans group streptococci characterized by their susceptibility to erythromycin]

Abstract

The increase in the use of the fluoroquinolones, including for the treatment of lower respiratory tract infections, seems to be associated with a parallel increase in bacterial resistance. We studied the activity of penicillin and six fluoroquinolones against 101 viridans group streptococci isolated from blood (58 erythromycin-susceptible and 43 erythromycin-resistant). The percentage of strains not susceptible to penicillin was 35% and there were statistically significant differences in the percentages of penicillin resistance between erythromycin-susceptible and -resistant strains. The fluoroquinolones studied showed good activity against our viridans group streptococci independent of their susceptibility to erythromycin. The norfloxacin MIC(50) and MIC(90) were 8 and 16 mg/l, respectively. Ofloxacin and ciprofloxacin showed the same activity (MIC(50) 1 mg/l and MIC(90) 2 mg/l); levofloxacin was similar with MIC(50) and MIC(90) of 1 mg/l. The fluoroquinolones with enhanced activity were moxifloxacin and trovafloxacin (MIC(50) 0.12 mg/l and MIC(90) 0.25 mg/l). However, we found two strains resistant to trovafloxacin (MIC = 4 mg/l) that showed reduced susceptibility to all the fluoroquinolones tested, including moxifloxacin (MIC = 2 mg/l). Mutations in the topoisomerase genes parC and gyrA have been shown to occur in strains with reduced susceptibility; therefore, ongoing surveillance for the development of fluoroquinolone resistance in viridans streptococci is necessary.