Listeria on lettuce — you can wash but you can't eliminate

You can wash all you like but you can’t get rid of all of the foodborne pathogens on your lettuce leaves because some hide within the plant tissue.

Conventional post-harvest sanitisation practices might not be sufficient to kill the potentially lethal pathogen because, according to a Purdue University study, Listeria monocytogenes can live inside the tissue of romaine lettuce.

Amanda Deering, clinical assistant professor in the Department of Food Science, and her team have established that the bacteria can live within lettuce in every stage of the plant growth process.

When ingested, L. monocytogenes can be deadly to those with vulnerable immune systems, including pregnant women, the elderly, infants or those with HIV. L. monocytogenes can also cross the placental barrier in pregnant women, triggering a miscarriage.

L. monocytogenes is killed by heat but this kill-step is not possible with minimally processed produce that is consumed raw. In 2011 31 people in the US died from listeriosis after the outside of raw rockmelons was contaminated with L. monocytogenes.

The source of listeriosis can be difficult to establish as people may only develop the disease two months after they consumed the contaminated product. Getting people to remember what they ate last week is problematic — getting them to reliably remember what they ate two months ago is extremely difficult.

In the US, romaine lettuce is the fastest growing crop in terms of production, export and consumption, and Deering, Haley Oliver, associate professor of food science, and Archana Shenoy, a graduate research assistant, began to investigate the persistence and internalisation of L. monocytogenes in the lettuce. The researchers found that exposing lettuce to the bacteria could lead to infection of plant tissue with the bacterium able to gain entry into the plant through cracked seed coats, small tears in root tissue during germination and damaged plant tissue. Only 30 minutes’ exposure was needed for the bacteria to enter the plant.

Their research showed L. monocytogenes can persist up to 60 days or until the time of harvest in romaine lettuce. They are now working on detection technologies as they shift their focus to what can happen to the seed and seedlings before planting. They aim to find pre-harvest control strategies to prevent produce contamination, particularly as sanitisers can only treat produce externally.

Amanda Deering, Purdue professor in the Department of Food Sciences, researches the ability of foodborne pathogens to grow and remain on or in plant tissue such as romaine lettuce. (Photo provided.)

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