a head of lettuce and a tomato on a cutting board

With the Public Health Agency of Canada advising Canadians to avoid all lettuce grown in Salinas, Calif. because of an outbreak of E. coli 0157, the University of Guelph has experts who can comment on why this leafy green is so often pulled from stores.

This is the fifth E. coli outbreak linked to romaine lettuce in the last two years, with this latest recall coming less than a year after the last one.

There have been at least 40 illnesses in the U.S.  and one illness in Canada with a similar “genetic fingerprint” to illnesses reported in the U.S. in this outbreak.

Prof. Lawrence Goodridge

Prof. Lawrence Goodridge holds the Leung Family Professorship in Food Safety in U of G’s Department of Food Science.

He notes that genetic analysis of the E. coli bacteria in the current outbreak shows it is similar to the E. coli that caused some of the earlier outbreaks — indicating there may be a common link.

Romaine is typically grown in open, irrigated fields. Goodridge says several changes have been proposed to avoid further outbreaks, such as treating irrigation water and investing in better traceability methods for romaine. But such changes are voluntary for the lettuce industry and have not been implemented by all.

He believes there needs to be better E. coli testing methods of irrigation water and romaine itself.

a photo of Prof. Keith Warriner
Prof. Keith Warriner

Prof. Keith Warriner is a food microbiologist in the Department of Food Science and researches food-borne pathogens, food-borne hazards detection technologies and intervention technologies.

He notes it’s striking that the E coli strain associated with this recall is the same as the 2017 and 2018 outbreaks, because it would be expected the strain would die off in the environment. That might suggest there is a contamination source higher up the waterway that is continuing to spread the bacteria, he said.

E. coli is typically found in the feces of animals such as cows and the outbreak in spring, 2018, was traced to tainted irrigation water near a cattle lot. Five people died in that outbreak.

In the last recall, in the fall of 2018, no common supplier, distributor or retailer of romaine was ever identified as the definitive source of the outbreak.

Prof. Jeffrey Farber is also a food microbiologist with the Department of Food Science and the director of the Canadian Research Institute for Food Safety.

Prof. Jeffrey Farber

Farber feels that the use of whole genome sequencing technology is playing a part in detecting these repeated outbreaks. Whole genome sequencing is more sensitive and specific than previous food-borne illness investigation techniques, and allows for the quick and sensitive matching of bacteria recovered from the patients to the organisms isolated from the lettuce.

He wonders whether similar contaminations have been occurring for longer than most realized, and this new technology is simply finding and linking more cases than would have been detected in the past.

At the same time, he says, public health authorities are more attuned to outbreaks involving fresh produce, which may help explain why lettuce contaminations are being identified more often.


All three experts are available for interviews and can be contacted directly.


Prof. Lawrence Goodridge

Prof. Keith Warriner

Prof. Jeffrey Farber