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Two unrelated strains of Staphylococcus aureus, one with a single mutation in grlA, the other with multiple mutations in gyrA, gyrB, grlA, grlB, norA and the norA promoter region, encoding low-level and high-level ciprofloxacin resistance, respectively, were studied. The characterized mutations in these genes were conserved when both strains were passaged for at least 500 generations in an antibiotic-free environment. New, rapidly stabilized mutations and higher MICs were detected for strains passaged in sub-MIC ciprofloxacin concentrations. The seeming irreversibility of quinolone resistance may affect the long-term success of this drug class.

Macrolide, lincosamide and streptogramin (MLS) antibiotics are chemically distinct inhibitors of bacterial protein synthesis. Resistance to MLS antibiotics may be constitutive or inducible. The purpose of this study is to update our understanding of the prevalence of different forms of MLS resistance in Europe. The analysis of 3653 clinical pneumococcal, staphylococcal and enterococcal isolates exhibited an average percentage of 21.3% and 6.2% intermediate and high-level penicillin-resistant Streptococcus pneumoniae, 21.8% methicillin-resistant Staphylococcus aureus and 11% vancomycin-resistant Enterococcus faecium. Geographical differences in erythromycin and clindamycin resistance in isolates of S. pneumoniae and S. aureus strongly reflect geographical variations in susceptibility to penicillin and methicillin, respectively. A very narrow range of MICs was obtained with quinupristin/dalfopristin, with no S. pneumoniae, S. aureus and E. faecium isolate having an MIC of > 4 mg/L, indicating a possible role of quinupristin/dalfopristin in the treatment of infections by multi-resistant Gram-positive bacteria.