Background: Methicillin-resistant Staphylococcus aureus (MRSA) accounts for more than 70% of S. aureus isolates from tertiary-care hospitals in Korea. Recently, a multilocus sequence typing (MLST) scheme has been used to study the local and global epidemiologies of MRSA. The aim of this study is to compare the genetic background of MRSA strains isolated in the same ward during two different periods. 

Methods: To investigate clonal changes of endemic MRSA isolates between 1996 and 2004, we studied a total of 33 MRSA strains (16 from 1996 and 17 from 2004) isolated in the intensive care units of a tertiary-care hospital in Korea. The isolates were analyzed for their sequence types by MLST and for their antimicrobial susceptibilities by the disk diffusion method. 

Results: ST5 was the most frequent type (n=11, 68.7%) in 1996, followed by ST254 (n=3, 18.8%) and ST1 (n=2, 12.5%). In 2004, ST239 was the most frequent type (n=10, 58.8%), followed by ST5 (n=6, 35.3%). 

Conclusion: The major clone type of MRSA isolates from intensive care unit patients changed from ST5 in 1996 to ST239 in 2004. (Korean J Clin Microbiol 2006;9(2):84-89)