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Sir,

Methicillin-resistant Staphylococcus aureus (MRSA) can cause a wide variety of infections in animals with bovine mastitis being the predominant staphylococcal infection in dairy cattle. MRSA isolates—mainly of multilocus sequence type (ST) 398—have recently been shown to be also involved in bovine mastitis.1 Although a variety of antimicrobial resistance genes have been identified in MRSA isolates from cases of bovine mastitis,1 the multidrug resistance gene cfr has not yet been identified in bovine CC398 isolates,1 although it has been found in a single porcine MRSA ST398 isolate and a single porcine methicillin-susceptible S. aureus ST9 isolate.2 Although pilot studies have indicated that MRSA ST9 isolates are commonly found among pigs in China,3,4cfr-positive MRSA isolates of animal origin have not yet been reported in China.

In 2010, during a routine surveillance study on antimicrobial resistance on dairy farms in China, an MRSA strain (designated SA16) was isolated from fresh raw milk of a cow suffering from mastitis. The MRSA strain was further characterized by multilocus sequence typing (MLST; http://saureus.mlst.net/), spa typing (http://spaserver.ridom.de) and SCCmec typing, as previously described.1 This strain belonged to ST9 (allelic profile 3-3-1-1-1-1-10), belonged to spa type t899 (allelic profile 07-16-23-02-34), and harboured an SCCmec element of type III. The strain displayed resistance to oxacillin (MIC = 128 mg/L), erythromycin (MIC ≥64 mg/L) and clindamycin (MIC ≥64 mg/L), had a linezolid MIC of 4 mg/L and showed high florfenicol and tiamulin MICs of ≥128 mg/L and 256 mg/L, respectively. The genes mecA (oxacillin resistance), cfr and fexA (phenicol resistance) were detected by PCR1,5 and confirmed by sequencing of the respective amplicons.

Issue Section:
Research Letters
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