Despite it has usually been associated with undercooked chicken or other food, antibiotic-resistant E. coli is actually more likely to be spread through poor toilet hygiene, according to new research.

Millions of bacteria naturally populate the guts of humans and animals alike, with different species coexisting in a fine balance that ensures a state of health. Some strands of E. coli form part of the natural gut microbiome and are usually harmless.

However, sometimes, a person may come into contact with strains of this bacterium that have developed antibiotic resistance. When this happens, E. coli may cause food poisoning, urinary tract infections, or intestinal infections.

Two possible sources of E. coli infections are contaminated food items and poor personal hygiene. But it remains unclear which one of these sources is most likely to lead to infection, and that is what researchers set out to learn.

In their study, published in The Lancet, the researchers collected antibiotic-resistant E. coli strains from meat (chicken, pork, and beef), animal slurry, salad, and fruit, on the one hand, and human bloodstream infections, feces, and sewage, on the other. The samples came from the National Health Service (NHS) laboratories.

Typically, antibiotic-resistant strains of this bacterium feature extended-spectrum beta-lactamases (ESBLs), enzymes that neutralize the action of antibiotics that people use to fight E. coli, such as penicillin and cephalosporin. Scientists refer to such strains of E. coli as “ESBLs-E. coli.“

The researchers’ analysis revealed that resistant E. coli strains present in the samples of human blood, feces, and sewage had lots of similarities. The dominant strain present in samples of human origin was ST131. In samples of food, however, the researchers found barely any traces of ST131. Instead, they noticed the presence of other ESBL-E. coli strains.

The almost complete lack of a crossover of E. coli strains between samples of human origin and those taken from contaminated foods suggested to the study authors that most infections with antibiotic-resistant E. coli are, most likely, transmitted from human to human as a result of poor hygiene practices.

“Critically — there’s a little crossover between strains from humans, chickens, and cattle. The great majority of strains of ESBL-E. coli causing human infections aren’t coming from eating chicken, or anything else in the food chain,” notes Prof. Livermore. “Rather — and unpalatably — the likeliest route of transmission for ESBL-E. coli is directly from human to human.”

Still, researchers noted that the findings do not mean people should stop being careful about how they handle foods, as food remains a source of infection.

“We need to carry on cooking chicken well and never to alternately handle raw meat and salad,” the lead author says. “There are plenty of important food-poisoning bacteria, including other strains of E. coli, that do go down the food chain.”