The response of Streptococcus faecalis to ciprofloxacin, norfloxacin and enoxacin

Abstract

The response to three quinolones of a clinical isolate of Streptococcus faecalis was studied by continuous turbidimctric monitoring, by viable counting, and in an in-vitro model of the treatment of bacterial cystitis. Dense populations (c. 8 ×10⁶ cfu/ml) responded to ciprofloxacin, norfloxacin and enoxadn at concentrations below the conventionally determined MIC, but variants, shown to exhibit decreased susceptibility, emerged during overnight incubation. The variants were cross-resistant to each of the three quinolones, but the resistance was lost on subculture in drug-free broth. At a concentration of ten-fold MIC, ciprofloxacin was more bactericidal than norfloxacin or enoxadn. In experiments in the bladder model, doses of ciprofloxacin achieving a peak concentration of 10, SO or 250 mg/l suppressed bacterial growth for 20 h or more. The doses of norfloxacin or enoxadn required to achieve a comparable effect were higher than those of ciprofloxacin. A reduction in susceptibility, as judged by comparative disc testing, occurred after two cycles of exposure to norfloxacin or enoxacin in experiments in which the peak concentration achieved was SO mg/1, but not when the peak was 250 mg/1. Reduced susceptibility to ciprofloxacin developed only in experiments in which a lower peak concentration of 10 mg/1 was achieved.