Objective      To explore the mechanisms underlying the replacement of methicillin-resistant Staphylococcus aureus (MRSA) straim ST239-MRSA-Ⅲ-t037 by ST239-MRSA-Ⅲ-t030 in Chongqing

China.       Methods      Eight ST239-MRSA-Ⅲ-t030 and ST239-MRSA-Ⅲ-t037 isolates collected from Chongqing were divided into four t030/t037 pairs. The independent growth rates of all isolates were then determined. The competitive growth rates of the bacterial pairs were detected in vitro and in vivo

and cross inhibition assays were also performed.       Results      The ST239-MRSA-Ⅲ-t030 isolates were rifampicin-resistant

whereas the ST239-MRSA-Ⅲ-t037 strains were all rifampicin-sensitive and trimethoprim/sulfamethoxazole-resistant. The ST239-MRSA-Ⅲ-t030 strains had shorter lag phases than the ST239-MRSA-Ⅲ-t037 isolates. The ST239-MRSA-Ⅲ-t030 strains could out-compete their rivals in a competition assay in vitro as well as in a mouse model. No significant cross-inhibition effect was observed between ST239-MRSA-Ⅲ-t030 and ST239-MRSA-Ⅲ-t037. The growth of ST239-MRSA-Ⅲ-t037 strains on the BHI nonselective plates had smaller colonies as compared to ST239-MRSA-Ⅲ-t030 strains.       Conclusion      ST239-MRSA-Ⅲ-t030 replacing ST239-MRSA-Ⅲ-t037 as the first predominant clone may result from its higher growth rate and the specific drug resistance profiles.