Few words strike horror into the hearts of food producers like “Listeria,” and a new study out of Purdue University’s Department of Food Science carries the unwelcome news that the bacteria can live within the tissue of romaine lettuce.
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Research led by Professor Amanda Deering, PhD, reveals that Listeria monocytogenes bacteria is able to gain entry to romaine lettuce through cracked seed coats, tears in root tissue, and other damage to the plant. It can subsequently survive within the tissue of lettuce throughout each stage of the plant growth process.
“We know the Listeria is naturally present on the soil so the assumption would be that it could contaminate the surface of the plant during production or harvest,” Dr. Deering explains. “I have published research, as well as others, that has shown the ability of human pathogenic to internalize into plant tissue. This is the first study, however, that has demonstrated that Listeria can internalize within lettuce tissue. Once the bacteria are inside, postharvest sanitization treatments that are typically done in the industry to wash/clean the lettuce will not likely be effective.”
Listeria bacteria can easily be killed by heat, meaning many foods exposed to it during the growing process may be made safe, but the problem Dr. Deering notes is that lettuce is consumed raw. Careful not to overstate risk, she underlines the principal concern is for consumers who are immunocompromised.
“From a production standpoint I don’t think there can be any changes made to inhibit the potential of Listeria contamination,” she says. “Plants grow in soil, and Listeria is naturally present in the soil, so there’s not really a way around it. I think my research points to the fact that if Listeria is able to internalize in the plant tissue then what we typically do for postharvest sanitization methods won’t do much, however, it will help reduce the bacteria that are present on the outside of the lettuce leaves. Growers need to continue to follow Good Agricultural Practices and consumers need to keep in mind there is a risk to eating certain food.”
In a strongly worded response, Dr. Jennifer McEntire, PhD, VP Food Safety and Technology for the United Fresh Produce Association in Washington, D.C., argued this study actually contains some good news. Dr. McEntire begins by underlines Dr. Deering’s point that Listeria is so much a part of soil it is considered ubiquitous.
“The fact that when a seed is doused in Listeria some of it winds up in the plant (before eventually dying) is no surprise,” Dr. McEntire says. “If this was a disaster waiting to happen, it would have happened already. The study actually shows that while internalization can occur, the pathogen dies off in romaine lettuce. The authors cite literature claiming that 17.5 percent of produce fields are contaminated with L. monocytogenes, and yet there has never been an outbreak traced to the field level. Why? Presumably because, as suggested by research and risk assessments, it takes a certain dose of pathogenic Listeria to make someone sick.”
Dr. McEntire cautions against sensationalizing the research, arguing the study isolated Listeria pathogens from romaine lettuce under “unrealistic conditions.”
“The research on romaine lettuce in fact shows that, despite using ridiculously high inoculum levels, they could not detect the pathogen in romaine lettuce after 18 to 45 days (depending on soil type),” she counters. “To focus media attention on such a limited portion of this study is misleading and not reflective of the key message, which is that romaine lettuce does not support the growth of Listeria, and it dies off over time. The industry would be well served to focus on true areas of risk which are the processing environments, not the field level.”