High incidence of oxacillin-susceptible mecA-positive Staphylococcus aureus (OS-MRSA) associated with bovine mastitis in China

Abstract

Staphylococcus aureus is a main cause of bovine mastitis and a major pathogen affecting human health. The emergence and spread of methicillin-resistant Staphylococcus aureus (MRSA) has become a significant concern for both animal health and public health. This study investigated the incidence of MRSA in milk samples collected from dairy cows with clinical mastitis and characterized the MRSA isolates using antimicrobial susceptibility tests and genetic typing methods. In total, 103 S. aureus isolates were obtained from dairy farms in 4 different provinces in China, including Gansu, Shanghai, Sichuan, and Guizhou. Antimicrobial susceptibility testing of these isolates revealed that the resistance rates to penicillin and sulfamethoxazole were high, while the resistance rates to ciprofloxacin and vancomycin were low. Among the 103 isolates, 49 (47.6%) were found to be mecA-positive, indicating the high incidence of MRSA. However, 37 of the 49 mecA-positive isolates were susceptible to oxacillin as determined by antimicrobial susceptibility assays and were thus classified as oxacillin-susceptible mecA-positive S. aureus (OS-MRSA). These isolates could be misclassified as methicillin susceptible Staphylococcus aureus (MSSA) if genetic detection of mecA was not performed. Molecular characterization of selected mecA-positive isolates showed that they were all negative with Panton-Valentine leukocidin (PVL), but belonged to different spa types and SCCmec types. These results indicate that OS-MRSA is common in bovine mastitis in China and underscore the need for genetic methods (in addition to phenotypic tests) to accurately identify MRSA.

Figure 1. PCR detection of mecA-positive S. aureus isolates.

The results were analyzed by Agarose gel electrophoresis. (A) mecA-positive isolates from Gansu province. Lane M: DNA size Marker. Lane 1: positive control strain ATCC43300. Lane 2: negative control strain ATCC25923. Lanes 3–10: isolate QY4, QY6, QY8, QY10, HG2, HG3, HG4 and HG5, respectively. (B) mecA-positive isolates from Shanghai. M: DNA size Marker. Lane 1: positive control strain ATCC43300. Lane 2: negative control strain ATCC25923. Lane 3–8: isolate SX5, SX6, SX10, SX11, SX13 and SX15, respectively; and lanes 9–22: isolates SH1, SH2, SH3, SH4, SH7, SH8, SH9, SH10, SH13, SH14, SH16, SH17, SH18 and SH20, respectively. (C) mecA-positive isolates from Guizhou. M: DNA size Marker. Lane 1positive control strain ATCC43300. Lane 2: negative control strain ATCC25923. Lanes 3–12: isolates zy1, zy2, zy4, zy5, zy6, zy8, zy11, zy12, zy14, and zy15, respectively. (D). mecA-positive isolates from Sichuan. M: DNA size Marker. Lane 1positive control strain ATCC43300. Lane 2: negative control strain ATCC25923. Lanes 3–14: isolates cx1, cx2, cx5, cx6, cx8, cx9, cx10, cx13, cx14, cx17, cx18, and cx19, respectively.