A new study by US researchers suggests bacteria found in meat could be a significant source of human urinary tract infections (UTIs).
The study, published last month in the journal One Health, applied comparative genomic analysis and a novel modeling method to more than 3,000 Escherichia coli isolates from human clinical infections and raw turkey, chicken, and pork products in a small US city. Their analysis found that 8% of the clinical E coli isolates, which were mostly from UTIs, originated in the meat.
If extrapolated to the entire US population, that would mean foodborne E coli could account for as many as 480,000 to 640,000 of the 6 to 8 million UTIs recorded in the United States each year. E coli is the leading cause of UTIs.
The authors say the findings of the study provide compelling evidence that potentially dangerous strains of E coli are making their way from animals to people through the food system.
Looking for the link between foodborne E coli and UTIs
For the study, a team led by researchers with the Antibiotic Resistance Action Center at George Washington University's Milken Institute of Public Health analyzed 3,111 E coli isolates collected from a hospital (1,188 isolates) and from meat samples in several retail grocery stores (1,923 isolates) in Flagstaff, Arizona, in 2012. They used whole-genome sequencing to identify E coli sequence types (STs) and pieces of DNA known as mobile genetic elements (MGEs) that were associated with human isolates and meat isolates.
Foodborne E coli is generally associated with gastrointestinal illness and certain diarrhea-causing strains are tracked by US health officials to make sure they are not contaminating the food supply. But the idea that the enteric bacteria could also be a cause of UTIs was proposed more than 60 years ago and has subsequently been supported by sporadic outbreak investigations, the researchers note. They added further evidence in a study published in mBio in 2018.
That study, which used the same collection of E coli isolates, found that ST131-H22, a lineage of a multidrug-resistant E coli strain that causes complicated UTIs, was prevalent in clinical samples and chicken and turkey meat. One of the discoveries that confirmed the findings from that study was that both the human- and poultry-associated isolates of that E coli strain shared an MGE that likely originated in poultry.
With the current study, the research team—which also included scientists from Northern Arizona University, Translational Genomics Research Institute, and the University of Michigan, and the University of Minnesota—wanted to identify the total number of MGEs in the entire collection of E coli isolates and figure out whether those MGEs originated in humans or meat samples.
"Whether the isolates were from the meat samples or people, we wanted to know what is the most likely source," corresponding author and Antibiotic Resistance Action Center Director Lance Price, PhD, told CIDRAP News. "Did it come from meat, or from people?"
Among the isolates, Price and his colleagues identified 443 STs—247 that only included, meat isolates, 120 that only included human isolates, and 76 that included both. Core-genome phylogenetic analysis suggested host transitions, but they needed more to identify recent zoonotic transmission.
Analysis of E coli accessory genes from the clinical samples and the meat samples identified 17 MGEs, six of which were associated with humans and 11 with meat. The statistical model used by Price and his colleagues then used that information to predict the likely origin of each isolate. Of the 1,162 clinical E coli isolates, the model identified 98 (8.4%) as originating in meat.
Further analysis of these foodborne zoonotic E coli (FZEC) isolates found that while they were as likely to cause symptomatic UTIs and sepsis as human-origin E coli. Two particular sequence types, ST131 and ST58, had the highest virulence potential.
"I think this is an indication that these strains can infect us, and can cause serious infections," Price said. "UTIs sometimes get dismissed as a painful annoyance, but they can kill you if they ascend from the bladder and get into the kidneys and the blood."
A One Health problem
CIDRAP research associate and public health veterinarian Jamie Umber, DVM, MPH, said the study is an important contribution to One Health research, which has become a critical tool in efforts to address antimicrobial resistance (AMR).
"Given the complex relationship between humans, animals, and the environment and the sharing of AMR genes, studies like this one can help fill knowledge gaps and attempt to quantify risks related to AMR spread between these sectors," she said.
Although there was not a significant difference in resistance between the FZEC and the human-origin E coli isolates, antibiotic stewardship advocates and officials from the World Health Organization (WHO) and the United Nations Food and Agriculture Organization (FAO) have long been concerned that widespread antibiotic use in livestock is helping create a reservoir of resistant bacteria and AMR genes that can spread through the environment and infect people.
There's a chance for us to work together to improve food, animal production, and public health...and I think that's exciting.
Price said he hopes that the model can be further refined to differentiate which animal or meat product a strain of E coli originally came from, which could help with outbreak investigations. He and his colleagues also want to conduct similar studies in other parts of the world to identify the highest risk FZEC strains, determine their sources, and investigate whether antibiotic use in livestock is impacting clinical resistance levels.
And that could lead to new strategies that could help improve human and animal health and reduce antibiotic use in both populations. Price theorized, for example, that one day food-producing animals might be vaccinated against virulent FZEC strains that cause illness in animals and people, which could prevent those strains from entering the food supply and potentially reduce UTI incidence in people.
"There's a chance for us to work together to improve food, animal production, and public health," he said. "And I think that's exciting."