Antimicrobial resistance, virulence gene profiles and molecular typing of Staphylococcus aureus isolates from dairy cows in Xinjiang, Northwest of China.

BACKGROUND: Staphylococcus aureus (SA) is one of the major pathogens causing dairy cow mastitis and endometritis. Recently, animal-derived SA strains have posed a serious threat to public health. However, little is known about antimicrobial resistance and virulence factors of SA strains isolated from dairy cows in Xinjiang China. Here, we characterized antimicrobial resistance, virulence gene profiles and genotypes of SA isolates from clinical mastitis and endometritis in dairy cows.

METHODS: A total of 337 clinical samples (including 186 milk samples from cases of clinical mastitis and 151 swab samples of endometritis) were collected from 15 large-scale dairy farms in Xinjiang China and screened for SA. All SA isolates were subjected to antimicrobial susceptibility testing, detection of virulence genes and molecular typing.

RESULTS: A total of 155 strains of SA were isolated from 337 samples, in which 22 strains were methicillin-resistant Staphylococcus aureus (MRSA) with a detection rate of 14.2%. The resistance of MRSA isolates was significantly higher than that of methicillin-sensitive SA (MSSA). Nevertheless, the percentage of different virulence genes varied between MSSA and MRSA. Staphylococcal Cassette Chromosome mec (SCCmec) typing showed that 155 strains could be divided into two genotypes: SCCmec I and SCCmec IVa. Agr typing identified three genotypes: Agr I, Agr II and Agr III. Spa typing identified four genotypes: t779, t2883, t13751 and t1939. MLST typing identified fourteen genotypes: ST188, ST294, ST9, ST805, ST2139, ST1, ST2700, ST903, ST2454, ST2990, ST63, ST968, ST2373 and STX, among which ST1 and ST9 had relatively high detection rates.

CONCLUSIONS: The above findings revealed that ST9-t1939-Agr I was the main genotype of MSSA isolates, while ST1-SCCmec I-t1939-Agr I was the main genotype of MRSA isolates from dairy cows. More significantly, STX was a novel genotype that was identified for the first time in this study. The fact that the majority of SA strains from dairy cows were multidrug resistant and carried multiple virulence genes may pose a potential risk to public health.