Why E. coli O157 only infects the gut

There are thousands of strains of E. coli and most are harmless, with many being a normal part of the gut flora in healthy people; however, some strains can cause illness in humans.

The bacterium can cause a wide range of infections including those of the gut, bladder, bloodstream and brain. These can be very common, for example, over half of all women suffer from E. coli associated bladder infections at some point in their lives.

The genetic variety between strains of E. coli is huge, with around 2000 ‘core’ genes and 18,000 genes that vary between strains. Different strains are able to attach themselves to different tissues, causing a range of different infections.

Among the most well known is E. coli O157, typically acquired via contaminated food, which causes severe diarrhoea and can lead to kidney damage.

The O157 strain only infects the gut so scientists at the University of Glasgow wanted to know what stopped it from spreading to other parts of the body.

The team led by Dr Andrew Roe, and PhD student James Connolly, of the Institute of Infection, Immunity & Inflammation, analysed the genome sequence of 1500 strains of E.coli.

They wanted to see how the genes of the bug, which enable it to attach to and infect a host, responded to varying concentrations of D-Serine, an amino acid produced in the brain where it plays a role in nerve signalling.

They found that E. coli O157 is unable to attach itself to host tissue in high concentrations of D-Serine. Other strains, such as those that cause meningitis, thrive in the presence of the amino acid.

The discovery, published in the ISME journal, opens up the possibility of altering the diet to increase levels of D-Serine to prevent E. coli O157 infection or perhaps treat it.

Dr Roe said: “This work provides new insights into the infection process with the aim of developing compounds that block such bugs from attaching to the host.

“With many strains of E. coli developing resistance to traditional antibiotics, such approaches are urgently needed.

“If we can disarm such bacteria rather than killing them, it puts less pressure on the bacteria to evolve into something that is resistant to treatment.”

E. coli O157 doesn’t normally live in humans, instead residing in the gut of cattle. Eating contaminated food is the most common cause of infection but it can also be picked up in the environment, through contact with the bacteria in fields, for example.