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Listeria in Lettuce: How much protection is enough?

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Listeria monocytogenes: How much protection is enough?

By: Ryan Robinson, PhD

Recent studies involving Listeria monocytogenes found in lettuce is causing concern for food safety. In foods such as lettuce, which are not heated prior to eating, there is a concern as to how safe ready-to-eat foods can be, even with precautionary action. Is washing the lettuce enough? How much do we need to do to ensure we are doing what we can to prevent ourselves, and those we care about, from becoming sick? When is enough, well, enough?

Listeria monocytogenes, most frequently referred to as simply “Listeria”, is a pathogenic, rod-shaped bacterial organism. It is commonly associated with foodborne illnesses originating from consumption of tainted deli meat, hot dogs, and unpasteurized dairy, but may also be associated with contaminated produce.

While outbreaks of Listeria are less common than other foodborne illnesses (e.g. Salmonella or E. coli infections) they are incredibly dangerous. Several studies focusing on listeriosis (the medical condition resulting from a Listeria monocytogenes infection) indicate that the mortality rate from this infection may be as high as 30% (CDC MMWR report, June 7, 2013) (Ramaswamy et al, 2007). Listeria is far more likely to claim the life of an individual infected with it than other, more widely-known foodborne-illnesses, including those caused by Salmonella (<1% mortality), STEC (<1% mortality), and even Clostridium botulinum (botulism) (5-10% mortality).

Listeria owes its exceptional lethality to a handful of intrinsic traits. First, unlike most bacterial organisms that result in foodborne-illness Listeria monocytogenes is invasive, meaning it can readily penetrate the human body’s natural barriers and take root in locations where one would not normally find bacteria, for example the bloodstream or the nervous system (Gray and Killinger, 1966). As if this were not bad enough, Listeria has developed a rather insidious method of avoiding the host immune system; Listeria bacilli invade your own body’s cells, living and replicating inside them! By doing so, the Listeria pathogen disguises itself, and avoids detection and destruction by immune cells and antibodies circulating in the bloodstream.

Are normal food-safety processes enough to protect us from Listeria?

Unfortunately, they are not. In addition to being incredibly lethal, Listeria is also notoriously hard to defend against. Refrigeration is one of the key tools for food-safety, and low temperatures will typically halt bacterial replication. Listeria, however, will continue to reproduce in cold environments, unhindered by temperatures consistent with those normal food refrigeration (Azizoglu et al, 2009). Worse yet, at room temperature Listeria bacilli will sprout “flagella”, whip-like tail structures that allow them to move about freely on solid surfaces, contaminating surrounding food products.

Some of the standard battery of food-safety methods are effective for preventing Listeria infection. For example, Listeria monocytogenes is killed by heat. Cooking meat to an appropriate temperature and pasteurizing dairy products is generally sufficient to eliminate the pathogen in these sources (foodsafety.gov). However, removing Listeria from uncooked produce appears to not be as simple as previously thought.

As with other foodborne-illnesses, general guidelines for prevention include washing produce thoroughly on all external surfaces. However, recent evidence published in the Journal of Food Protection indicates that Listeria bacilli can attach to, and invade internal edible portions of plant structures (Shenoy et al, 2017). This means that simply washing the exterior of your produce may not be sufficient to protect your family.

According to the research, conducted in the laboratory of Dr. Amanda Deering at Purdue University, if lettuce is grown in soil that is contaminated with Listeria monocytogenes bacilli the bacteria do not simply remain on the exterior of the lettuce, but innervate internal tissues that are unaffected by normal wash-and-rinse steps. Even more concerning, her research showed that exposure to Listeria bacteria for as little as 30 minutes can contaminate seeds, and produce lettuce that contains a large quantity of the infectious pathogen. Deering’s paper cites particular cause for concern, as previous research has indicated that as many more than 1 in 8 produce generating farms in the United States are contaminated with Listeria monocytogenes bacteria (Strawn et al, 2013)

It is worth noting Dr. Deering and her colleagues are some of the foremost experts on Listeria and other bacterial forms of foodborne illness. Previous work in the food-science department at Purdue identified a stark difference between high standards for Listeria protection in meatpacking/manufacturing facilities, and relatively lax standards in deli-counters and retail establishments across the US (Simmons et al, 2014).

How do we protect ourselves?

Thankfully Listeria infections are fairly rare, due in no-small part to intense scrutiny and advanced monitoring at food-processing and packaging facilities around the United States. When contamination sources are identified, the FDA mandates a total stop on food-processing, a recall of any potentially affected products, and thorough cleaning and replacement of food processing equipment until the processing plant is deemed safe.

Unfortunately, the research previously mentioned above indicates that the same standards are often not held by delicatessens and other retail eateries. Consumers need to be particularly cautious when frequenting these establishments to ensure that proper cleaning and sanitation standards are maintained. They should pay particularly close attention to warnings issued by state and local health authorities.

Protection from Listeria on produce is a particularly difficult challenge, as new food-safety guidelines have not yet been established in the wake of the most recent scientific evidence regarding the Listeria pathogens ability evade sanitation. It is possible that modern, advanced monitoring and intervention technology may be need to be developed and implemented in produce fields in order help safeguard food sources from this deadly pathogen.

Some hope arrives in the form of new biotechnology. Researchers have identified a series of bacteriophages (special viruses that selectively attack bacteria) that target and kill the Listeria pathogen with superb efficiency (Carlton et al, 2005). Thus far, several phage treatments have been approved by the FDA for use on food products (Agency letter GRN 000198). While they are promising, whether they are effective at wide-scale at prevention of Listeria contamination in produce remains to be seen.

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