Bacteria Listeria monocytogenes is a lethal pathogen and can live inside the tissue of romaine
lettuce, suggesting that conventional post-harvest sanitization practices might not be sufficient to kill it.
Research led by Amanda Deering, clinical assistant professor in the Department of Food Science, showed that the bacteria could live within lettuce in every stage of the plant growth process, residing inside the plant tissue.
‘Sanitizers that are used in the fresh produce industry only contact the external
portions of the fruit or vegetable. L. monocytogenes can gain entry into the plant through
cracked seed coats, small tears in root. If bacteria are inside the plant, they are not
exposed to the sanitizer and therefore not killed.’
L. monocytogenes can gain entry into the plant through cracked seed coats, small tears in root
tissue during germination and damaged plant tissue. The researchers found that exposing
lettuce to the bacteria could lead to infection of plant tissue in as little as 30 minutes.
"Knowing this can happen, we need to keep it on our radar as we continue to follow good
agricultural practices," Deering said.
When ingested, the bacteria 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, which can trigger a miscarriage.
"For immune-compromised consumers, it's important to remember, that canned or cooked produce
is better," Deering said.
While commonly associated with ready-to-eat deli meat and hot dogs, outbreaks of listeriosis -
the disease associated with the bacteria - have also been caused by contaminated celery,
cantaloupe, sprouts and apples.
A 2011 outbreak from cantaloupe was the second most deadly foodborne bacterial outbreak in
U.S. history. The bacteria can only be killed by heat and the contaminated fruit that was
minimally processed and consumed raw.
Symptoms of listeriosis may take as long as two months to appear, and by that time, most
people don't connect the illness to something they ate, Deering said.
After the 2016 recall of contaminated packaged salads, Deering, Haley Oliver, associate
professor of food science, and Archana Shenoy, a graduate research assistant, began to
investigate the persistence and internalization of L. monocytogenes in romaine lettuce, the
fastest growing crop in the U.S. in terms of production, export and consumption.
Their research showed L. monocytogenes in romaine lettuce can persist up to 60 days or until
the time of harvest. The bacteria could be found throughout the plant tissue, indicating yet
another way foodborne pathogens can reach consumers, especially in ready-to-eat foods.
At the Purdue Center for Food Safety Engineering, researchers are 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 sanitizers can only treat produce externally.
"Continued education, training and research to minimize exposure of human foodborne pathogens
in our soil, water, seeds, plants and produce have become my priority in research," said