evaluation of mupirocin resistance, and especially high- level resistance, is very important in MRSA isolates. High-level mupirocin resistant strains have an additional mupA gene in a plasmid that can be transferred to other strains by plasmid conjugation22,23. Bastos et al.23 showed that mupirocin resistance is transferred from mupirocin resistant strains of S. aureus to mupirocin susceptible S. aureus via a conjugative plasmid. Furthermore, this plas- mid was transferred between S. aureus and Staphylococ- cus epidermidis. This result indicates the possibility of horizontal transfer of the conjugative plasmid among Staphylococcus species and this suggests that Staphylo- coccus epidermidis could be a reservoir of this plasmid. This enables the wide spread of mupirocin resistance. Further, the mupA gene may co-transfer with other antibac- terial resistance, such as that against triclosan, tetracycline and trimethoprim3. PFGE is the most widely used genotyping or genetic fingerprinting tool, although it requires technical skill, a long processing time and expensive instruments. It is commonly considered the gold standard method for epidemiologic typing and the determination of genetic relatedness11,24. Yoo et al.5 showed 20 of 25 high-level and 20 of 21 low-level mupirocin resistant S. aureus from eight long-term-care facilities belonged to the same PFGE groups. A study by Chaves et al.21 also identified that 13 of 14 high-level mupirocin resistant strains showed the same PFGE band patterns. These results indicate that clonal transmission of mupirocin-resistant S. aureus occur- red in hospitals. In our study, 9 of 10 high-level mupirocin resistant MRSA isolates from (A) hospital and 5 of 6 low- level resistant isolates (2 from (A) hospital, 3 from (B) hos- pital) belonged to the same PFGE group. The antimicrobial susceptibility of the mupirocin resistant strains can vary depending on different studies. A study conducted on 4980 MRSA strains showed mupirocin- resistant strains were more likely to be resistant to fusidic acid and to be susceptible to tetracycline, trimethoprim- sulfamethoxazole and ciprofloxacin as compared to that of the mupirocin-susceptible strains2. Yoo et al.5 showed all the mupirocin resistant S. aureus were resistant to oxacil- lin, penicillin and erythromycin and they were susceptible to ampicillin and vancomycin. In our study, the mupirocin resistant MRSA isolates were susceptible to vancomycin and rifampin, and they were resistant to ciprofloxacin, clindamycin and tetracycline. Compared to the low-level mupirocin resistant strains, the high-level mupirocin resis- tant strains were more resistant to erythromycin and fusi- dic acid. Several agents have been shown to have activity against mupirocin resistant S. aureus3. These include azelaic acid, High- and Low-level Mupirocin Resistant MRSA nitrofurazone and silver sulphadiazine. Maple et al.25 sho- wed that the concentrations of azelaic acid, nitrofurazone and silver sulphadiazine close to the MIC were bacteri- cidal, but mupirocin was only bactericidal at concentra- tions substantially greater than the MIC. Prolonged use and frequent usage of mupirocin appear to be the most important factors for increasing mupirocin resistance. Riley et al.26 recommended less than 10 days treatment and at least 1 month between treatment. The increasing number of reports of high-level mupirocin resistance could mean the potential loss of one of the major treatment methods for controlling MRSA. Therefore, mupirocin treatment should be used cautiously and judi- ciously. Careful monitoring of mupirocin usage and test- ing of S. aureus, including MRSA, for mupirocin resistance seems to be indicated. REFERENCES 1. Sutherland R, Boon RJ, Griffin KE, Masters PJ, Slocombe B, White AR. Antibacterial activity of mupirocin (pseudomonic acid), a new antibiotic for topical use. Antimicrob Agents Chemother 1985;27:495-498. 2. Simor AE, Stuart TL, Louie L, Watt C, Ofner-Agostini M, Gravel D, et al; Canadian Nosocomial Infection Surveillance Program. Mupirocin-resistant, methicillin-resistant Staphylo- coccus aureus strains in Canadian hospitals. Antimicrob Agents Chemother 2007;51:3880-3886. 3. Cookson BD. The emergence of mupirocin resistance: a challenge to infection control and antibiotic prescribing practice. J Antimicrob Chemother 1998;41:11-18. 4. Yun HJ, Lee SW, Yoon GM, Kim SY, Choi S, Lee YS, et al. Prevalence and mechanisms of low- and high-level mupiro- cin resistance in staphylococci isolated from a Korean hospi- tal. J Antimicrob Chemother 2003;51:619-623. 5. Yoo JI, Shin ES, Cha JO, Lee JK, Jung YH, Lee KM, et al. Clonal dissemination and mupA gene polymorphism of mupirocin-resistant Staphylococcus aureus isolates from long-term-care facilities in South Korea. Antimicrob Agents Chemother 2006;50:365-367. 6. Yang JA, Park DW, Sohn JW, Yang IS, Kim KH, Kim MJ. Molecular analysis of isoleucyl-tRNA synthetase mutations in clinical isolates of methicillin-resistant Staphylococcus aureus with low-level mupirocin resistance. J Korean Med Sci 2006; 21:827-832. 7. Clinical and Laboratory Standards Institute. Methods for dilu- tion antimicrobial susceptibility tests for bacteria that grow aerobically; approved standard - eighth edition. Wayne: Clinical and Laboratory Standards Institute, 2009. 8. Lee HK, Lee EJ, Pahk YJ, Kim BK, Kang MW, Shim SI. Relationship between the level of methicillin resistance and mecA, mecI, femA genes genes in Staphylococci. Korean J Infect Dis 1998;30:36-44. 9. Yokoyama T. Study on mec gene in methicillin-resistant staphylococci. Kansenshogaku Zasshi 1993;67:1203-1210. 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