Antimicrobial resistance profiles of common mastitis pathogens on large Chinese dairy farms

The primary objective of this study was to determine the antimicrobial resistance (AMR) profile of common mastitis pathogens on large Chinese dairy farms. A total of 673 isolates, including Staphylococcus aureus (S. aureus) (14.41%, 97/673), coagulase-negative staphylococci (CoNS, 52.30%, 352/673), Streptococcus agalactiae (S. agalactiae, 5.64%, 38/673), non-agalactiae streptococci (7.42%, 50/673), Acinetobacter spp. (7.72%, 52/673), Escherichia spp. (6.39%, 43/673) and Klebsiella spp. (6.09%, 41/673), were collected from 15 large Chinese dairy farms in 12 provinces. The AMR profiles of all isolates were measured using a microdilution method for 8 antimicrobials [penicillin (PEN), ampicillin (AMP), amoxicillin/clavulanate (AMC), oxacillin (OX), cephalexin (LEX), ceftiofur (EFT), lincomycin (LIN) and rifaximin (RIX)]; these are commonly used in practice for the treatment of bovine mastitis. Our results showed that more than 75% of S. aureus (87/97) and CoNS (291/352) were resistant to PEN. More than 30% of Escherichia spp. (15/43) showed resistance to AMP. However, less than 10% CoNS and non-agalactiae streptococci showed resistance to AMC (1/352; 0/50)/LEX (1/352; 0/50)/EFT (10/352; 0/50)/RIX (21/352; 2/50), less than 10% S. aureus showed resistance to AMC (1/97)/OX (3/97)/LEX (1/97)/EFT (2/97)/RIX (2/97), less than 10% S. agalactiae showed resistance to PEN (3/38)/AMC (0/38)/LEX (0/38)/EFT (0/38)/RIX (0/38), and less than 10% Escherichia spp. showed resistance to AMC (1/43)/EFT (4/43). These results suggested that most mastitis pathogens were susceptible to most antimicrobials with exceptions of S. aureus tested against penicillin or ampicillin and CoNS against penicillin or oxacillin. To control the AMR threat in Chinese dairy farms, a nationwide surveillance program for AMR of bovine mastitis pathogens is needed.