Limoncu M.H.Ermertcan S.Çetin C.B.Cosar G.Dinç G.2025-04-102025-04-102003http://hdl.handle.net/20.500.14701/53072The emergence of phenotypic resistance to ciprofloxacin and levofloxacin in methicillin-sensitive and methicillin-resistant Staphylococcus aureus (MRSA) strains was studied. Twenty MRSA and 77 methicillin-sensitive S.aureus (MSSA) strains susceptible to both quinolones were investigated for resistance after single step or serial passages. No growth of 20 MRSA strains was observed at 4×MIC of levofloxacin after 48 h incubation, but 4 of 77 (5%) MSSA strains grew at the same concentration. At 4×MIC concentration of ciprofloxacin, 10 MSSA (13%) and five MRSA (25%) strains were grown. In the serial passages of MRSA strains, resistance to ciprofloxacin was 75 and 5% for levofloxacin by the third passage. In the seventh passage this resistance was 100 and 15%, respectively. In MSSA strains, resistance to ciprofloxacin was 75 and 19% to levofloxacin at the third passage and at the seventh passage, 100 and 61%, respectively. Emergence of ciprofloxacin resistance was more common and developed more rapidly than resistance to levofloxacin in both MRSA and MSSA strains. © 2003 Elsevier Science B.V. and the International Society of Chemotherapy. All rights reserved.Article10.1016/S0924-8579(03)00006-2