In vitro activity of fluoroquinolones against gram-positive bacteria

This paper reviews the in vitro activities of several newer fluoroquinolone antimicrobials that exhibit enhanced potency against Gram-positive bacteria. Several of these agents demonstrate 10-fold greater activity than older members of this class against Staphylococcus aureus and inhibit [minimum inhibitory concentration (MIC90) values < or = 2 mg/L] many isolates resistant to ciprofloxacin or ofloxacin. Markedly enhanced activity is also noted against Streptococcus pneumoniae, 90% of isolates being inhibited at concentrations 10- to 100-fold lower than those of the older agents. Enterococci also exhibit greater susceptibility to several of the newer fluoroquinolones, although relative cross-resistance with the earlier drugs is noted. As determined by dilution techniques, the new fluoroquinolones generally demonstrate bactericidal activity at concentrations at or near their MIC values. The activities of the new compounds described here are decreased at low pH, but are not affected by the addition of up to 50% human serum to the test medium. Resistance is rarely detected (frequency < 10(-9)) when high density bacterial suspensions are plated in the presence of 4 times the MIC of these compounds. However, colonies displaying relative resistance to the new agents can be selected by serial passage in incremental antimicrobial concentrations.