Raw or unpasteurized cows' milk can hold a huge amount of antimicrobial-resistant genes if left at room temperature, according to a new study published in the journal Microbiome from researchers at the University of California-Davis (UC-Davis). The study also found that bacteria that harbor antimicrobial-resistant genes can transfer those genes 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 UC-Davis department of food science and technology.
Lacking in probiotics
UC-Davis said an estimated 3% of the U.S. population consumes unpasteurized/raw milk, which has not been heated to kill pathogens and extend shelf life. Raw milk is often touted to consumers as having an abundant supply of probiotics, or healthy bacteria, compared with pasteurized milk.
In the new study, the UC-Davis researchers did not find that to be the case.
"Two things surprised us," Liu said. "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.
The UC-Davis researchers analyzed more than 2,000 retail milk samples from five states, including raw milk and milk pasteurized in different ways. The study found raw milk had the highest prevalence of antibiotic-resistant microbes when left at room temperature, UC-Davis said.
"Our study shows that with any temperature abuse in raw milk, whether intentional or not, it can grow these bacteria with antimicrobial resistance 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 David Mills, the Peter J. Shields chair of dairy food science at UC-Davis, 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," Mills said. "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 more work is needed to fully understand whether antibiotic-resistant genes in raw milk translate into health risks for people, Mills suggested that consumers instead use a starter culture if they want to ferment raw milk, which carries specific strains of bacteria to inoculate the milk.
Other authors include Yuanting Zhu with UC-Davis and Danielle Lemay with the Western Human Nutrition Research Center of the U.S. Department of Agriculture's Agricultural Research Service. This study was funded with support from the National Institutes of Health and the Peter J. Shields endowed chair in dairy food science.