Purpose: The current study was designed to investigate the frequency of MRSA isolations from raw milk in Korea, and characterize the patterns of antimicrobial resistance, virulence and genotype of the MRSA.
Methods: A total of 1,222 raw milk samples were collected from 47 dairy farms in Gyeonggi province during 2011 to 2012, and 643 milk samples were determines as mastitic milk based on the somatic cell counts. MRSA were isolated from mastitic milk and characterized for the profile of antimicrobial susceptibility. The presence of staphylococcal enterotoxins (SE) were screened by target specific PCR and the genetic profiles were determined by pulsed-field gel electrophoresis (PFGE), Staphylococcus protein A (spa) typing, and the SCCmectyping.
Results: Of 165 S. aureus isolated from mastitic milk samples, 23 S. aureus (13.9%) were determined as MRSA based on the detection of mecA gene, and all the MRSA isolates showed multi-drug resistance to more than 3 different antimicrobials tested. The most frequently detected SE gene profile was seg, sel, sek, sem, sen and seo genes (20 isolates, 87%). PFGE analysis showed the 23 MRSA isolated were grouped to 4 different PFGE types, and molecular typing revealed there were 3 kinds of SCCmec-spatypes; IVa-t148 (n=11, 47.8%), IVa-t324 (n=9, 39.1%), II-t002 (n=3, 13.0%).
Significance: Previous studies and the current results indicate that the isolation rate of MRSA from raw milk has consistently increased in Korea. Since MRSA in raw milk is a serious threat to the public health, an effective monitoring and control program should be implemented to reduce the contamination.