University of California-Davis food scientists found raw milk purchased at retail stores can hold a large amount of antimicrobial-resistant genes if left at room temperature. 

Their findings are published in the journal Microbiome.

The UC Davis researchers collected 2,034 samples of unpasteurized milk from retail stores in California, Idaho, Arizona, South Carolina and Maine.  

The study was open to four kinds of milk processing techniques including completely unpasteurized, vat-pasteurized, high temperature short time (HTST), and ultra-pasteurized (UHT) products.

The UC Davis food scientists found raw milk holds large volumes of antimicrobial-resistant genes which can very swiftly spawn dangerous bacteria when left at room temperature.

They also found bacteria that harbored antimicrobial-resistant genes can transfer them to other bacteria, potentially spreading resistance if consumed. 

“We don’t want to scare people, we want to educate them. If you want to keep drinking raw milk, keep it in your refrigerator to minimize the risk of it developing bacteria with antibiotic-resistant genes,” said lead author Jinxin Liu, a postdoctoral researcher in the Department of Food Science and Technology at UC Davis.

only a very small percentage of U.S. consumers — perhaps 3 percent — are raw milk drinkers.  Raw milk is not heated to kill dangerous pathogens such as E. coli, Salmonella and various other bacteria and viruses.  Raw milk is often promoted for benefits on the basis that it has extra probiotics or healthy bacteria compared with pasteurized milk

“Two things surprised us,” said Liu. “We didn’t find large quantities of beneficial bacteria in the raw milk samples, and if you leave raw milk at room temperature, it creates dramatically more antimicrobial-resistant genes than pasteurized milk.”

Bacteria with antimicrobial-resistant genes, if passed to a pathogen, have the potential to become “superbugs,” so that pharmaceuticals to treat infection or disease no longer work. Each year, almost 3 million people get an antibiotic-resistant infection, and more than 35,000 people die, according to the Centers for Disease Control and Prevention.

“Our study shows that with any temperature abuse in raw milk, whether intentional or not, it can grow these bacteria with antimicrobial resistant genes,” said co-author Michele Jay-Russell, research microbiologist and manager with the UC Davis Western Center for Food Safety. “It’s not just going to spoil. It’s really high risk if not handled correctly.”

Some consumers are intentionally letting raw milk sit outside of the refrigerator at room temperature to ferment, in order to make what’s known as clabber. Co-author and Peter J. Shields Chair of Dairy Food Science David Mills said if consumers eat raw milk clabber, they are likely adding a high number of antimicrobial-resistant genes to their gut.

“You could just be flooding your gastrointestinal tract with these genes,” said Mills. “We don’t live in an antibiotic-free world anymore. These genes are everywher, and we need to do everything we can to stop that flow into our bodies.”

While more work is needed to fully understand whether antibiotic-resistant genes in raw milk translate into health risks for humans, Mills suggests that consumers instead use a starter culture if they want to ferment raw milk, which carries specific strains of bacteria to inoculate the milk.