The study, published in the journal Microbiome, says that raw or unpasteurized cows’ milk from U.S. retail outlets can contain a large number of antimicrobial-resistant genes if left at room temperature.

The researchers also discovered that bacteria with antimicrobial-resistant genes can pass them on to other bacteria, potentially spreading resistance if consumed.

Unpasteurized, or raw, milk, which has not been heated to kill pathogens and improve shelf-life, is consumed by an estimated 3 percent of the US population. When compared to pasteurised milk, raw milk is often marketed to customers as containing an abundance of probiotics, or good microorganisms. UC Davis researchers discovered that this was not the case.

If bacteria with antimicrobial-resistant genes are passed on to a pathogen, they have the potential to become “superbugs,” making medicines used to treat infection or disease ineffective. According to the Centers for Disease Control, almost 3 million individuals suffer antibiotic-resistant infections each year, and over 35,000 die as a result.

Researchers from UC Davis examined over 2,000 retail milk samples from five states, including raw milk and milk pasteurized in different ways. When raw milk was left at room temperature, it had the highest prevalence of antibiotic-resistant microorganisms, according to the study.

Some people purposefully leave raw milk out of the refrigerator at room temperature to ferment in order to make clabber. According to David Mills, co-author and Peter J. Shields Chair of Dairy Food Science, if consumers ingest raw milk clabber, they are likely contributing a large number of antimicrobial-resistant genes to their gut.

“You could just be flooding your gastrointestinal tract with these genes,” said the study author.

“We don’t live in an antibiotic-free world anymore. These genes are everywhere, and we need to do everything we can to stop that flow into our bodies.”

While additional research is needed to determine whether antibiotic-resistant genes in raw milk pose health problems to people, the study authors recommend that if customers wish to ferment raw milk, they should use a starter culture, which has certain strains of bacteria to inoculate the milk.

Image Credit: GEtty