cases. Compared with the MIC90 among the high-level resistant strains, rifampin and vancomycin showed high antimicrobial activity and the MICs were ≤0.5μg/ml and 0.5μg/ml, respectively. The MICs of ciprofloxacin and Table 1. Rates of mupirocin resistant MRSA isolates in four Korean hospitals Hospital A B C D Total No. of MRSA isolates tested 103 30 30 30 193 Mupirocin resistance No. 11* 2 13 1 27 % 10.7 6.7 43.3 3.3 14.1 MRSA: methicillin-resistant Staphylococcus aureus. *High level mupirocin resistant MRSA isolates. High- and Low-level Mupirocin Resistant MRSA tetracycline were 64μg/ml, respectively and those for erythromycin and fusidic acid were ≥128μg/ml, respec- tively. For the low-level mupirocin resistant MRSA strains, vancomycin showed the highest antimicrobial activity (MIC=1μg/ml) and this was followed by rifampin (2μg/ ml), tetracycline (32μg/ml) and ciprofloxacin (64μg/ml). The MICs of clindamycin, erythromycin and fusidic acid were ≥128μg/ml, respectively. The high-level mupirocin resistant MRSA strains were susceptible to rifampin and vancomycin, but 90% were resistant to fusidic acid, and 100% were resistant to ciprofloxacin, clindamycin, tetracycline and erythromycin. While the low-level mupirocin resistant MRSA strains were susceptible to vancomycin, 6% were resistant to rifampin, 24% were resistant to erythromycin, 77% were resistant to fusidic acid and 100% were resistant to Table 2. Antimicrobial susceptibilities of the 11 high-level and 16 low-level mupirocin resistant MRSA isolates Mupirocin resistant isolates Antimicrobial agents Resistant rate (%) High level (n=11) Low level (n=16) Mupirocin Ciprofloxacin Clindamycin Tetracycline Erythromycin Fusidic acid Rifampin Vancomycin Mupirocin Ciprofloxacin Clindamycin Tetracycline Erythromycin Fusidic acid Rifampin Vancomycin Range 512∼≥1,024 32∼≥128 128 64 ≥128 ≤0.25∼≥128 ≤0.5 0.5∼1 16∼32 16∼≥128 ≥128 16∼64 ≤0.5∼≥128 ≤0.25∼≥128 ≤0.5∼≥128 0.5∼1 MIC (μg/ml) 50% ≥1,024 32 128 64 ≥128 ≥128 ≤0.5 0.5 16 32 ≥128 16 ≥128 ≥128 ≤0.5 1 90% ≥1,024 64 128 64 ≥128 ≥128 ≤0.5 0.5 16 64 ≥128 32 ≥128 ≥128 2 1 100 100 100 100 100 90 0 0 100 100 100 100 24 77 6 0 MRSA: methicillin-resistant Staphylococcus aureus, MIC: minimum inhibitory concentration. Fig. 5. A dendrogram generated with the Gelcompar II program showing the five PFGE types (1-5) of SmaI- restricted chromosome DNA of the 16 mupirocin-resistant MRSA isolates. H1-H10: isolates with high-level mu- pirocin resistance, L16-21: isolates with low-level mupirocin resistance. PFGE: pulsed-field gel electrophore- sis, MRSA: methicillin-resistant Sta- phylococcus aureus. Vol. 24, No. 1, 2012 35