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The Other E. coli: Studying non-O157 Strains

Move over, E. coli O157:H7. It’s time to share the spotlight, or at least the microscope, with other types of foodborne illness-causing E. coli bacteria.

While the most serious and widely publicized E. coli outbreaks are usually linked to one particular strain of the bacteria — O157:H7 — other pathogenic species are becoming more widely recognized by food scientists as a threat to human health, particularly E. coli O26, O45, O103, O111, O121 and O145.

These six serogroups, known as the “Big Six” in the food science business, are the focus of new research being conducted by scientists at the Agricultural Research Service (ARS), the research arm of the USDA.

The Big Six have been identified as responsible for 70 percent of non-O157:H7 E. coli-related illnesses in the United States, but little is known about the genetic differences between them, or what makes some types within each group particularly more virulent than others.

“For many years, we knew about E. coli O157:H7,” says Pina Fratamico, microbiologist at ARS and lead researcher of the project in an interview with Food Safety News. “That was kind of an easy one, because we knew it caused very serious illness and there were ways we could detect it in food because it had some characteristics we could use,” she explains.

Non-O157:H7 Bacteria: Genetically Camouflaged

Non-O157 E. coli, on the other hand, don’t have as many distinguishing features. Within the Big Six, “strains are fairly homogeneous in terms of the genes they possess,” says Fratamico. This means that not only are the six groups themselves harder to separate (for example, O26 from O45), but serotypes within each group also look more alike.

So far, Fratamico and her team have developed a method to test for the presence of each of the six leading non-O157 groups in a sample of meat. The challenge that remains, she says, is figuring out what genetic characteristics make some strains within a group dangerous, and others harmless.

This is an important distinction to make. While a piece of beef might test positive for O26 in general, the particular strain of O26 present might not be a dangerous one, Fratamico explains. Instead, another bacteria in the sample, such as an O45, might be the one producing shiga toxins, the poisonous element carried by pathogenic E. coli that causes diarrhea and can lead to other more harmful diseases.

“All these non-O157 Shiga toxin-producing E. coli (STECs) look like non-pathogenic E. coli. So how do we know what it is that makes them different?” asks Fratamico. “That’s been the real struggle. People all over the world are struggling with this,” she says.

For now, the research team sorts through each strain of Big Six bacteria individually, testing a sample for the dangerous duo in an E. coli bacteria: shiga-toxin and eae, a protein that helps it attach to cells, and then attempting to isolate the harmful microbes.

They are currently working on developing a more universal way to isolate STECs from non-STECs.

Ultimately, the group’s research will provide inspectors from USDA’s Food Safety Inspection Service (FSIS) with a method for testing meat for non-O157 E. coli. One guidance paper has already been published.

Policy: Will non-O157 E. coli Tests Be Required?

However, whether or not this research is put into play will be a matter of policy. Currently, non-O157 E. coli, with the exception of E. coli O26, are not considered adulterants by FSIS; therefore meat is not randomly tested for their presence as it is for O157:H7.

Many food safety advocates have begun to push for non-0157 strains of E. coli to be classified as adulterants as concerns about the risks of these bacteria increase.

According to the Centers for Disease Control (CDC), approximately 170,000 people in the United States are sickened each year by non-O157 E. coli. That’s about two thirds of E. coli illnesses, with the “Big Six” accounting for the largest share.

E. coli O157:H7 still appears more virulent than its relatives, however, as it accounts for a far greater share of hospitalizations and deaths. This does not mean that non-O157s are harmless though.

“We’re finding that a lot of these top six serogroups can cause very serious illness, so we should do something about them,” says Fratamico.

For example, in 2008, an outbreak of E. Coli 0111 traced back to a restaurant in Oklahoma sickened at least 341 people and resulted in 70 hospitalizations and one death. The outbreak source was never determined.

For now, whether or not FSIS will deem this and similar non-O157 E. coli strains adulterants remains up in the air.

“It’s all unclear now as to what’s going to happen. Nobody knows,” says Fratamico. “We’re just preparing. If we do need to start screening for these non-0157 STECs in beef, we’ll have a method ready for that,” she says.

Fratamico says the group’s testing method, while developed on beef, could eventually be used on other foods as well.

This research is being conducted as a joint initiative between ASA and FSIS. So far, researchers have published a paper in Foodborne Pathogens and Disease, and expect to publish more results in the near future.

© Food Safety News
  • Toni Allardyce

    Does this include the generic e.coli that some plants screen for using Petrifilm plates? Can we determine from the plates if they are just gram (-) bacteria or are they another strain of e.coli that gives a non- gas result/

  • http://twitter.com/gi Rajat Chander

    thanks, gretchen. this was a prescient article. now O104 in germany & europe, so that makes “big seven”.