Methicillin-resistant Staphylococcus aureus with the MIC of linezolid

35 Methicillin-resistant Staphylococcus aureus with the MIC of linezolid 4 μg/ml, isolated from an 36 unsuccessful linezolid therapy patient, yielded linezolid-resistant mutants in blood agar at 48 h of 37 incubation. The resistant clones showed a MIC of linezolid ranging from 8 to 64 μg/ml and 38 accumulated the T2500A mutation(s) of the ribosomal RNA genes. Emergence of these resistant clones 39 appears to be facilitated by a cryptic mutation or mutations associated with chloramphenicol resistance. 40 41 42 Infection by methicillin-resistant Staphylococcus aureus (MRSA) is a serious problem in hospitals 43 and communities because this bacterium shows resistance to major chemotherapeutic agents. To 44 combat MRSA infections, powerful antibiotics, such as vancomycin and linezolid, have been 45 developed. However, vancomycin-resistant MRSA has emerged world-wide. Recently developed 46 linezolid is probably one of a few choices for the treatment of vancomycin-resistant MRSA. Most S. 47 aureus including MRSA is found to be linezolid-susceptible with a breakpoint of 4 μg/ml (3, 4). 48 However, the emergence of linezolid-resistant MRSAs with an MIC of over 4 μg/ml has been 49 reported recently (1, 5, 6, 8, 11, 12). The resistant cells were found to have either the G2576T (5, 11, 50 on July 5, 2017 by gest httpaac.asm .rg/ D ow nladed fom