Study warns raw milk may be carrying antibiotic-resistant superbugs

By Stephen Beech

High rates of antibiotic-resistant bacteria have been found in raw milk.

Raw cow and sheep milk is frequently contaminated with antibiotic-resistant bacteria that could pose a threat to human and animal health, according to an international study.

Half of the strains of a common milk bacterium, Staphylococcus epidermidis, were found to be multi-drug resistant in Pakistan, where over 95% of milk is consumed in its "raw" form, which has not been pasteurised to kill off harmful bacteria.

Study leader Dr. Tahir Usman said: "Milk can become contaminated by bacteria through improper handling or from infections in the teat, called subclinical mastitis.

"The overuse of antibiotics to treat subclinical sumastitis has led to the emergence of multidrug-resistant bacterial strains, which could then be transmitted to humans through raw milk."

For the new study, researchers investigated the risk posed by Staphylococcus epidermidis, a subclinical mastitis-causing bacterium that often does not lead to visible symptoms in the cow, but results in contaminated, lower-quality milk.

The team collected 310 milk samples, about half from cattle and half from ewes, and tested them for subclinical mastitis.

They also isolated strains of Staphylococcus epidermidis from the milk samples and screened them for antibiotic resistance.

Around one in four of the samples showed evidence of subclinical mastitis and one in eight were contaminated with Staphylococcus epidermidis, according to the findings published in the journal PLOS One.

Dr. Usman, of Abdul Wali Khan University, Pakistan, said: "Strikingly, 95% of Staphylococcus epidermidis bacteria isolated from the milk were resistant to penicillin and erythromycin, and half were resistant to three or more antibiotics."

In humans, Staphylococcus epidermidis is a common, generally harmless inhabitant of the skin.

But the researchers say that multi-drug resistant Staphylococcus epidermis bacteria in raw milk could spread antimicrobial resistance to more harmful pathogens, such as Staphylococcus aureus, the MRSA pathogen.

Dr. Usman said: "The study’s findings underscore the high rates of subclinical mastitis in cattle and ewes, and indicate that Staphylococcus epidermidis might be an important pathogen impacting both animal health and food safety."

He added: “The presence of multidrug-resistant Staphylococcus epidermidis in raw milk highlights how on-farm antibiotic use directly shapes public health risks.

"These findings emphasise the urgent need for responsible antibiotic use and improved hygiene practices in the dairy sector to reduce the risk of antimicrobial resistance transmission through the food chain.”