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Raw or unpasteurised cow’s milk can hold a huge amount of antimicrobial-resistant genes if left at room temperature, warn researchers. The study, published in the journal Microbiome, also found that bacteria that harboured antimicrobial-resistant genes can transfer them to other bacteria, potentially spreading resistance if consumed.
“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 study lead author Jinxin Liu from the University of California, Davis in the US. Raw milk is often touted to consumers as having an abundant supply of probiotics, or healthy bacteria, compared with pasteurized milk. The researchers did not find that to be the case.
“Two things surprised us. 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 pasteurised milk,” Liu added.
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 three million people get an antibiotic-resistant infection, and more than 35,000 people die, according to the US Centers for Disease Control.
For the findings, the research team analysed more than 2,000 retail milk samples from five states, including raw milk and milk pasteurised in different ways. The study found raw milk had the highest prevalence of antibiotic-resistant microbes when left at room temperature. “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.
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. If consumers eat raw milk clabber, they are likely adding a high number of antimicrobial-resistant genes to their gut, the researchers said.
“You could just be flooding your gastrointestinal tract with these genes. 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,” the study authors wrote.