Prevalence of a Putative Efflux Mechanism among Fluoroquinolone-Resistant Clinical Isolates of Streptococcus pneumoniae

Twenty-three norfloxacin-selected first-step mutants ofStreptococcus pneumoniae showed low-level fluoroquinolone resistance. Their susceptibility to norfloxacin in the presence or absence of reserpine and known efflux pump substrates was determined by an agar dilution method. Five mutants showed four- to eightfold increases in their susceptibility to norfloxacin in the presence of reserpine and four- to eightfold decreases in their susceptibility to acriflavine and ethidium bromide. This phenotype is suggestive of an efflux mechanism of resistance. A representative of these mutants, 1N27, accumulated significantly less ethidium bromide than the parent strain; reserpine abolished these differences. No changes in the quinolone resistance-determining regions ofparC ,parE ,gyrA , orgyrB were found in this mutant. By our validated agar dilution method, the efflux phenotype was sought in clinical isolates ofS. pneumoniae . Of 1,037 clinical isolates examined from the United Kingdom, 273 showed reduced susceptibility to norfloxacin or ciprofloxacin. Of these, 45.4% showed the efflux phenotype. Our findings suggest that an efflux mechanism may be a frequent cause of clinically significant fluoroquinolone resistance in pneumococci.