The bacterium Escherichia coli is normally present in both human and animal guts. However, some strains of E. coli have developed resistance to antibiotics and can cause dangerous infections. But how do these harmful bacteria end up infecting people?
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 the bacterium 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. In the worst case scenario, an E. coli infection may leak into the bloodstream and lead to bacteremia, which, in turn, can trigger septic shock.
Two possible sources of E. coli infections are contaminated food items — such as raw chicken or raw cookie dough — and poor personal hygiene, for example, not washing one’s hands after using the toilet.
But it remains unclear which one of these sources is most likely to lead to infection, and that is what researchers from the University of East Anglia in Norwich, United Kingdom, set out to learn.
“E. coli bacteria normally live in the intestines of healthy people and animals. Most varieties are harmless or cause brief diarrhea. But E. coli is also the most common cause of blood poisoning […],” notes lead author Prof. David Livermore.
Infections from the highly resistant ESBL-E. coli bacteria are difficult to treat, the researcher explains, adding, “And they are becoming more common in both the community and hospitals.”
“Mortality rates among people infected with these superbug strains are double those of people infected with strains that [a]re susceptible to treatment,” Prof. Livermore continues.
For this reason, the researcher and his colleagues “wanted to find out how these superbugs are spread — and whether there is a crossover from the food chain to humans.”
In their study — the findings of which appear in The Lancet Infectious Diseases — 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 National Health Service (NHS) laboratories in five different regions in the U.K., namely London, East Anglia, Northwest England, Scotland, and Wales.
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, particularly ST23, ST117, and ST602.
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.
“[C]ritically — there’s 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, with fecal particles from one person reaching the mouth of another,” he goes on.
Still, he also notes 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.”
“But here — in the case of ESBL-E. coli — it’s much more important to wash your hands after going to the toilet. And it’s particularly important to have good hygiene in care homes, as [the] most of the severe E. coli infections occur among the elderly.”
Prof. David Livermore