Antimicrobial resistance and molecular typing of Staphylococcus aureus isolates from raw milk in Hunan Province.

BACKGROUND: Staphylococcus aureus is one of the most important foodborne pathogens in the world and the main cause of dairy cow mastitis. Few studies have investigated the epidemic pedigree of S. aureus of bovine origin in Hunan, China. Therefore, we aimed to analyze the capsular polysaccharides (CP), molecular typing, and antibiotic resistance characteristics of S. aureus isolated from raw milk of dairy farms in Hunan Province.

METHODS: Between 2018 and 2022, 681 raw milk samples were collected from dairy cows from farms in Changsha, Changde, Shaoyang, Yongzhou, and Chenzhou in Hunan Province. S. aureus was isolated from these samples, and the isolates were subjected to molecular typing, CP typing, and determination of antibiotic resistance through broth dilution and polymerase chain reaction (PCR).

RESULTS: From 681 raw milk samples, 76 strains of S. aureus were isolated. The pathogenicity of 76 isolates was determined preliminarily by detecting cp5 and cp8 CP genes. Eighteen types of antimicrobial resistance phenotypes of 76 S. aureus strains were detected by the broth dilution method, and 11 kinds of related resistance genes were amplified by PCR. The S. aureus isolates had CP5 (42.10%) and CP8 (57.89%). S. aureus had a multiple antimicrobial resistance rate of 26.75%. The isolated strains had the highest resistance rate to penicillin (82.89%) and showed varying degrees of resistance to other drugs, but no isolate showed resistance to doxycycline. The 76 isolates all carried two or more antibiotic resistance genes, with a maximum of eight antibiotics resistance genes. FemB was detected in all isolates, but none of isolates carried vanA , ermA , or glrA . The 76 isolates were divided into 22 sequence types (ST) and 20 spa types by MLST and spa typing, and the number of t796-ST7 ( n  = 15) isolates was the highest, which may be the major epidemic strain of multidrug-resistant S. aureus .

CONCLUSION: The present findings indicate the need to increase production of the CP8 S. aureus vaccine in Hunan Province and strengthen resistance monitoring of t796-ST7 isolates with the prevalent molecular type of multi-drug resistant strains. The use of β -lactam, macrolides, and lincosamides should be reduced; doxycycline, sulfonamides, and glycopeptides could be appropriately added to veterinary antibiotics to treat infectious diseases in dairy cows.