For the study, the researchers investigated the antimicrobial susceptibility of mastitis-causing bacteria Staphylococcus epidermidis, as well as the isolates’ ability to spread AMR genes. The possibility of S. epidermidis spreading resistance genes to bacteria that are significant to human health, like methicillin-resistant Staphylococcus aureus (MRSA), is a concern.

The researchers collected 310 milk samples—approximately half from cattle and half from ewes—and tested them for subclinical mastitis. S. epidermidis isolates from the milk were subjected to antimicrobial susceptibility testing and polymerase chain reaction (PCR) screening for AMR genes.

Nearly 13 percent of the samples were contaminated with S. epidermis, and a striking 95 percent of the S. epidermis isolates were highly resistant to both penicillin and erythromycin. Notably, half (50 percent) of the isolates were multi-drug resistant (MDR), meaning they showed resistance to three or more drug classes.

Regarding AMR genes, ermC was most prevalent (87.5 percent), followed by tetK (80 percent) and mecA (45 percent). The identification of these key resistance genes, especially ermC and tetK, amplify concerns about potential AMR transmission to human pathogens through the food chain.  

The authors say that the use of antibiotics to treat mastitis and other diseases are likely the driver of high levels of AMR among S. epidermis isolates in cows and sheep. Their findings indicate that S. epidermidis may be an important pathogen impacting food safety as well as animal health, and call for responsible antibiotic use and improved hygiene practices by the Pakistani dairy sector to reduce the risk of foodborne AMR transmission.