Fluoroquinolone resistance in staphylococci: new challenges.

Staphylococci show only marginal susceptibility to the newer fluoroquinolones. Minimum inhibitory concentrations (MICs) for susceptible strains usually range from 0.25 to 2.0 mg/l. As a single mutational event involving the gyrase target or permeability diminishes fluoroquinolone susceptibility fourfold on average, such a mutation in staphylococci would lead to a clinical level of resistance. Therefore, it is not surprising that in some institutions, the use of fluoroquinolones has been quickly followed by greatly increased prevalence of fluoroquinolone-resistant staphylococci. The greatest increase in resistance has been seen among methicillin-resistant staphylococci although increased prevalence of resistance among Staphylococcus saprophyticus and other methicillin-susceptible staphylococci has also been reported. Clinical isolates of staphylococci recovered since the introduction of the fluoroquinolones fall into three fluoroquinolone susceptibility groups: susceptible (MIC < 0.5 mg/l), moderately resistant (MIC 0.5 to 4 mg/l) and highly resistant (MIC > 4 mg/l). The first group represents wild type strains while the second and third groups represent single- and multiple-step mutants, respectively. To prevent increasing prevalence of isolates in the second and third groups, it would be prudent to avoid use of quinolones whenever possible. However, when it is necessary, a fluoroquinolone, which achieves serum/tissue levels eight times the MIC of the infecting strain, should be used.