Comparative In Vitro Activities of Linezolid, Quinupristin-Dalfopristin, Moxifloxacin, and Trovafloxacin against Erythromycin-Susceptible and -Resistant Streptococci

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Abstract
The in vitro activities of the new agents linezolid, quinupristin-dalfopristin, moxifloxacin, and trovafloxacin were determined and compared with those of penicillin, clindamycin, and four macrolides against 53 erythromycin-resistant Streptococcus pneumoniae , 117 S. pyogenes (64 erythromycin-susceptible and 53 -resistant), and 101 S. agalactiae (53 erythromycin-susceptible and 48 -resistant) isolates. Differentiation of macrolide resistance phenotypes was performed by the double-disk method. The genetic basis for macrolide resistance in 52 strains was also determined. The M phenotype was found in 84.9, 6.3, and 1.9% of S. pyogenes , S. agalactiae , and S. pneumoniae isolates, respectively. These strains were susceptible to miocamycin and clindamycin. Strains with the inducible phenotype accounted for 27.1% of S. agalactiae isolates and 9.4% each of S. pyogenes and S. pneumoniae isolates. All erythromycin-resistant isolates were also resistant to the 14- and 15-membered macrolides tested. Strains with all three phenotypes were susceptible to ≤2 μg of linezolid per ml. Quinupristin-dalfopristin exhibited good in vitro activity against all strains, irrespective of their resistance to erythromycin (MICs at which 90% of the isolates tested were inhibited [MIC 90 s], 0.2 to 1 μg/ml). Against the erythromycin-resistant S. pyogenes and S. agalactiae strains, moxifloxacin and trovafloxacin were the most active agents (MIC 90 s, 0.1 μg/ml). The new antimicrobials evaluated may be alternative agents to treat infections caused by macrolide-resistant as well as macrolide-susceptible streptococci.