Piggery-associated methicillin-resistant Staphylococcus aureus (MRSA) is a potential zoonotic pathogen. We constructed the population structure and dynamics of staphylococcal chromosome cassette mec (SCCmec) in MRSA ST9 isolates from different geographical areas of Taiwan. A total of 140 MRSA (135 piggery and 5 human clinical) isolates from three populations located in western Taiwan (n=96 including the 5 clinical isolates), central eastern Taiwan (n=22), and Penghu Island (n=22) were collected and characterized by analysis of the mec-associated direct repeat unit (dru). Twenty-eight dru types (with 24 novel) and 15 dru-Clonal Complexes (CCs) were identified. The predominant novel dt12w type (48.6%) was widespread in all populations and may have a superior ability to transmit among populations. The minimum spanning network showed that at least two ancestral dru types (dt11a and dt12w) were identified, and the genetics between different populations could be differentiated. Temporal distributions of clone population dynamics estimated through the Bayesian skyline plot indicated a stable population with a long evolutionary history for MRSA ST9 in Taiwan. Findings indicating that some dru types are shared between piggery-associated and human-clinical MRSA ST9 suggest the occurrence of cross-species horizontal transmission of SCCmec.
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