Lettuce More Susceptible to E. coli Than Kale and Other Brassicas

Lettuce has a weakness for Escherichia coli when compared with its Brassica cousins like kale and spinach.

After exposing the vegetables to the bacteria and various temperatures, the researchers from the University of Illinois observed that lettuce was the most vulnerable to E. coli at room temperature but coped well when refrigerated at 4 °C (39 °F).

The findings were published in Food Microbiology.

Leaves that are green turn to brown

Leafy greens account for the majority (68.9%) of vegetable illness outbreaks in the US. Between 2014 and 2021, leafy green outbreaks resulted in 2,028 illnesses, 477 hospitalizations and 18 deaths, according to the Centers for Disease Control.

To better understand why these vegetables are so susceptible to bacterial growth, researchers from the University of Illinois inoculated several of them with E. coli and studied the outcomes when the greens were refrigerated at 4 °C (39 °F), 20 °C (68 °F) and 37 °C (98.6 °F).

At the end of the experiment, they found that susceptibility was determined by a combination of temperature and leaf surface properties such as roughness and the natural wax coating.

“At room temperature or higher, E. coli grows very fast on lettuce, but if lettuce is refrigerated at 4 °C, we see a sharp decline in the E. coli population,” said lead author Mengyi Dong, who conducted the research as a doctoral student at the university’s Department of Food Science and Nutrition.

“However, for waxy greens like kale and collard, we get the opposite results. On these vegetables, E. coli grows slower under warmer temperatures, but if it is already present, it can survive longer under refrigeration.”

Overall, though, kale and collard were less susceptible to E. coli contamination than lettuce. Moreover, kale and chard are usually cooked – which kills or inactivates E. coli – while lettuce is consumed raw.

Washing lettuce does help, says Dong, but doesn’t remove all the bacteria because of their tight attachment to the leaf.  

The researchers also inoculated cut leaves with E. coli to observe any differences between intact surfaces and cut surfaces.

“Whole leaves and freshly cut leaves present different situations. When the leaf is cut, it releases vegetable juice, which contains nutrients that stimulate bacterial growth,” Dong explained.

In their experiment, however, this juice actually exhibited antimicrobial properties that protect against E. coli.

Dong and her colleagues say the juice has the potential to be used in antimicrobial sprays or coatings to control foodborne pathogen contaminations at both pre-harvest and post-harvest stages.

“We can’t completely avoid pathogens in food. Vegetables are grown in soil, not in a sterile environment, and they will be exposed to bacteria,” said Pratik Banerjee, an associate professor at the University of Illinois Department of Food Science and Nutrition.

“It’s a complex problem to solve, but we can embrace best practices in the food industry and food supply chain. There's a lot of interest from the research community and federal agencies to address these issues, and the [United States Department of Agriculture] imposes high standards for food production, so overall the US food supply is quite safe.”

Reference: Dong M, Holle MJ, Miller MJ, Banerjee P, Feng H. Fates of attached E. coli o157:h7 on intact leaf surfaces revealed leafy green susceptibility. Food Microbiology. 2024;119:104432. doi: 10.1016/j.fm.2023.104432

This article is a rework of a press release issued by the University of Illinois. Material has been edited for length and content.