SY Park, et al ciprofloxacin, clindamycin and tetracycline. Genotyping the mupirocin resistant MRSA using the PFGE patterns The PFGE patterns of the genomic DNA of the 16 strains of mupirocin resistant MRSA were classified into five clusters (Fig. 5). Of the 10 high-level resistant strains from (A) hospital, 9 (H1-H7, H9-H10) were classified in cluster 1 with 92% similarity. Among these, H6 and H10, H1-4 and H9, and H5 and H7 were the same clones. The H8 strain was in cluster 4 with 72% similarity. Of the 6 low-level resistant strains, L16-L18 from (B) hospital were classified in cluster 2 and they were the same clone with 100% similarity. The L19 and L20 from (A) hospital were in cluster 4 with 93% similarity, and L21 from (D) hospital was in cluster 5 with 79% similarity. DISCUSSION Mupirocin is a topical antibiotic that shows a high level of activity against streptococci and staphylococci, and certain gram-negative bacteria, including Haemophilus influenzae and Neisseria gonorrhoae, but it is much less active again- st most gram-negative bacilli and anaerobes1. Therefore, it has been used to treat skin infection and to eradicate MRSA. Mupirocin was introduced into clinical practice in 1985 and the use of mupirocin ointment has been pro- gressively increasing worldwide. Clinical isolates resistant to mupirocin were first reported in 1987, and the resistance rate has been increasing14. Prolonged use and frequent use of mupirocin were thought to be the most important factors associated with the in- crease of mupirocin resistance3,15. A Canadian study by Simor et al.2 reported an increase in mupirocin resistance among MRSA over time. From 1995 to 1999, the propor- tions of MRSA strains with high- and low-level mupirocin resistance were 1.6% and 6.4%, respectively, whereas from 2000 to 2004, the resistant rates were 7.0% and 10% respectively. Pérez-Fontán et al.16 reported the emergence of mupirocin-resistant S. aureus in peritoneal dialysis pati- ents who applied mupirocin over 10 years. From 1990 to 1996, there were no mupirocin resistant strains, but they increased to 8.3% from 1997 to 1998, and to 12.4% from 1999 to 2000. According to Wise and Johnson17, the mupirocin resistance rate was 8.3% in a dermatological hospital, while it was 0.2% in the nearby general hospitals over the same period. In Korea, topical mupirocin has been used since 1994 to eradicate staphylococcal infection in hospitals and the use of mupirocin has been dramatically increasing. However, there has been very little awareness and research about 36 Ann Dermatol mupirocin resistance. The study conducted in Korea up to 1999 failed to detect mupirocin resistant strains18. Yet Yun et al.4 first identified 16 high-level mupirocin resistant isolates in 2003 and the prevalence of mupirocin resis- tance was 5%. In a study from long-term-care facilities in Korea, the rate of mupirocin resistance was 11.3% (6.1% high-level mupirocin resistance and 5.2% low-level mupi- rocin resistance)5. Another study carried out at intensive care units reported that the mupirocin resistance rate was 25.3% and all the isolates showed low-level resistance. In our study, the prevalence of mupirocin resistance was 14.1%. The 5.7% of the isolates were high-level resistant strains (11/193) and all of them were from (A) hospital. The low-level resistant strains were from the other three hospitals. Also, the prevalence of mupirocin resistance of (C) hospital was 43.3%, which is much higher than that of the other hospitals, and the reason for this was thought to be there were more dermatological samples from (C) hospital. Mupirocin resistance is divided into two groups: low-level resistance (MIC=8∼256μg/ml) and high-level resistance (MIC＞256μg/ml). Low-level mupirocin resistance is due to point mutation in the chromosomally encoded native IleS gene, whereas high-level resistance is related to the acquisition of a plasmid containing a modified additional IleS-2 gene mupA3,4. However, the mupA gene was detec- ted in the low-level mupirocin resistant strains by western blot analysis in a small case study10. It is also possible that high- and low-level mupirocin resistance are mediated by different, but closely related genes that hybridize with the mupA probe, so further studies are needed to resolve this issue. Low-level mupirocin resistant strains are not considered to have clinical significance because the concentration of mupirocin in the 2% ointment (20,000μg/ml) exceeds the MICs of the low-level mupirocin resistant strains, so low- level mupirocin resistant strains can be treated by topical mupirocin19. On the contrary, high-level mupirocin resis- tant strains that can not be treated by mupirocin are more clinically important. Fortunately, low-level strains have so far been more frequently isolated than high-level strains. In our study, the prevalence of low-level mupirocin resis- tance (8.3%) was higher than the prevalence of high-level resistance (5.7%)2,10,20. However, high-level mupirocin resistant strains have been increasing in prevalence recen- tly. Also, the mupirocin resistance rates were higher for the methicillin-resistant isolates than for the methicillin- susceptible isolates4,21. Chaves et al.21 detected a much higher percentage of mupirocin resistance among the isolates of MRSA (14.8%) than among the isolates of methicillin-susceptible S. aureus (0.6%). Therefore, the