Raw milk linked to antimicrobial-resistant genes

Unpasteurized, or raw, milk can serve as a reservoir for antimicrobial-resistant genes, according to a study published in Microbiome.

In addition, researchers found that raw milk does not have the beneficial properties it is commonly believed to have, and contains limited lactic acid bacteria.

The sale of raw milk for human consumption in retail stores or local farms is legal in 30 states across the U.S., according to the study authors.

When consumed, antibiotic-resistant genes can transfer resistances to pathogens in the body, resulting in an antibiotic-resistant pathogen, David A. Mills, PhD, co-author of the study and a professor in the departments of food science & technology and viticulture & enology at the University of California, Davis, told Healio Primary Care.

“Many antibiotic-resistant genes are on genetic elements that are readily transferred to other bacteria,” Mills said. “This is a common activity between bacteria. Moreover, the gut has trillions of bacteria all crowded together and genetic exchange of this sort has been repeatedly proven to occur routinely in the gastrointestinal tract.”

The issue of antibiotic resistance “is an ever growing, worldwide problem,” he added.

Study findings

Mills and colleagues tested 2,304 pasteurized and unpasteurized retail milk samples from five states.

As raw milk is often consumed as clabber — the product created when raw milk is left at room temperature for 1 day to 5 days — and to account for unintentional incubation at room temperature during delivery, the researchers evaluated samples that were kept in a cold environment or at room temperature for up to 24 hours.

They determined that raw milk samples had the highest prevalence of viable bacteria — including aerobic bacteria, coliform and Escherichia coli — and had a microbiota that was distinct from pasteurized milk.

Using gene sequencing, they determined that Pseudomonadaceae was dominant in raw milk samples; a limited level of lactic acid bacteria was also found in the samples.

“Raw milk proponents have argued that pasteurization denatures the enzymes and other structures in milk that provide benefit to the neonate,” Mills said. “Some also argue that the microbes that milk delivers, or the microbes that get enriched in production of raw milk clabber, are beneficial, kind of like probiotics.”

Mills noted that these reported benefits of raw milk are countered by the “indisputable and dramatically higher incidence” of food-borne illness with raw milk consumption.

In addition, he said that his research team “did not witness the beneficial class of microbes (i.e. ‘probiotics’) commonly referred to as enriched in raw milk.”

Discussing raw milk consumption in practice

“Raw milk myths have been around for quite some time,” Kristi L. King, MPH, RDN, LD, CNSC, senior pediatric dietitian at Texas Children's Hospital in Houston and a clinical instructor at Baylor College of Medicine, told Healio Primary Care. “The main beliefs about raw milk have been [that] it has more nutrients, protects against allergies, is better for lactose intolerance, and offers probiotics.”

Despite these myths, “none of the peer-reviewed studies have found any of those claims to be true when looking at commercially — homogenized and pasteurized — prepared milk vs. raw milk,” King said.

“These findings reiterate what we already know: raw milk contains a significant amount of bacteria that could be harmful to one’s health,” she said. “And with the uptick in interest in farmers markets and buying local, I think it is extremely important in timing — just because some of the states allow the purchase of raw milk does not mean we need to purchase and consume raw milk.”

Due to their elevated risk of being unable to fight off food-borne illnesses, King stressed that avoiding raw milk consumption is especially important in people with compromised immune systems, infants, young children, older adults and pregnant women.

Because of the risks associated with raw milk consumption, “we should be focusing on food safety and consumer safety, and look at tighter regulations of raw milk,” she said.

“Primary care physicians should not be recommending raw milk under any circumstances,” she added.