Listeria monocytogenes: Why it is challenge for Food Industry?

Listeria monocytogenes (L. monocytogenes) is a Gram-positive facultatively anaerobic microorganism, motile at the temperature range of 22–28°C but non-motile above 30°C, characterized by the growth at temperature range from −0.4°C to 45°C, with an optimum temperature of 37°C. It is able to survive at a relatively low water activity (aW < 0.90) and a broad pH range between 4.6 and 9.5 as well as to tolerate salt conditions up to 20%. 

It is a species of pathogenic (disease-causing) bacteria that can be found in moist environments, soil, water, decaying vegetation and animals, and can survive and even grow under refrigeration and other food preservation measures. When people eat food contaminated with L. monocytogenes, they may develop a disease called listeriosis. The disease is especially dangerous for the elderly, pregnant women, unborn babies and immunocompromised people.

According to the recent European Food Safety Authority (EFSA) and European Center for Disease Prevention and Control (ECDC) common report for year 2020, a total of 1,876 confirmed cases of invasive listeriosis in humans were noted in the European Union member states, with the notification rate of 0.42 cases per 100,000 population and 97.1% hospitalizations. In the United States, the Centers for Disease Control and Prevention (CDC) estimate that each year about 1,600 persons are infected with L. monocytogenes, with the hospitalization rate of ca. 94%. L. monocytogenes is generally transmitted when food is harvested, processed, prepared, packed, transported or stored in environments contaminated with L. monocytogenes. Environments can be contaminated by raw materials, water, soil, and incoming air. Pets can also spread the bacteria in the home environment if they eat food contaminated with L. monocytogenes.

L. monocytogenes can adapt, survive and even grow over a wide range of food production environmental stress conditions such as temperatures, low and high pH, high salt concentration, ultraviolet lights, presence of biocides and heavy metals. Furthermore, this bacterium is also able to form biofilm structures on a variety of surfaces in food production environments which makes it difficult to remove and allows it to persist for a long time. This increases the risk of contamination of food production facilities and finally foods. The present review focuses on the key issues related to the molecular mechanisms of the pathogen survival and adaptation to adverse environmental conditions. L. monocytogenes’ virulence factors include but are not limited to intracellular mobility via actin polymerization and the ability to replicate at refrigerator temperatures. This makes it difficult for food industries to control. 

Key facts

• Listeriosis is a serious, but preventable and treatable disease.

• Pregnant women, the elderly or individuals with a weakened immune system, such as people with immuno-compromised status due to HIV, leukaemia, cancer, kidney transplant and steroid therapy, are at greatest risk of severe listeriosis and should avoid high risk foods.

• High risk foods include deli meat and ready-to-eat meat products (such as cooked, cured and/or fermented meats and sausages), soft cheeses and cold smoked fishery products.

• Listeria monocytogenes are widely distributed in nature. They can be found in soil, water, vegetation and the faeces of some animals and can contaminate foods.

• Listeriosis is an infectious disease caused by the bacterium Listeria monocytogenes.

  
  

  In what foods is listeria found?

• L. monocytogenes is ubiquitous as it can be found in soil, water, and decaying vegetation. The bacterium can also be found in the human digestive tract. 

  Foods that have the highest rates of L. monocytogenes related infections include:

  • Raw sprouts

  • Unpasteurized milk

  • Soft cheeses

  • Cold deli meats

  • Cold hot dogs

  • Smoked seafood

  
  

  These foods have the highest risk of L. monocytogenes contamination. However, listeriosis can affect almost any improperly handled or prepared food.

  The bacteria L. monocytogenes is unique from many other foodborne illnesses. It survives and even multiplies in low temperatures, including refrigerators and freezers. The bacteria can multiply to dangerous levels during storage.

  
  

  Food that has spoiled breaks down and develops an unpleasant smell, taste or change in texture. When L. monocytogenes bacteria contaminate food, the smell, taste and texture don’t change.

History and Physical

Always consider treating for L. monocytogenes in the following patient populations:

• Neonates (Usually younger than 29 days old)

• Elderly

• Immunocompromised

• Pregnant women

Listeria meningitis presents with fever, neck stiffness, headache, altered mental status, neurological deficits, and other classic signs of meningitis. The patient may demonstrate a positive Brudzinski's or Kernig's sign on physical exam. The patient may have an altered mental status and may not be alert and oriented to their name, location, and date. Inquire about the use of stomach acid suppressors as they increase the risk of L. monocytogenes infection.

Pregnant women or otherwise healthy adults may present with general "flu-like" illness including fever, diarrhea, headache, chills, nausea, vomiting, myalgias. Although, some patients may be completely asymptomatic.

Control methods

The control of L. monocytogenes is required at all stages in the food chain and an integrated approach is needed to prevent the multiplication of this bacteria in the final food product. The challenges for controlling L. monocytogenes are considerable given its ubiquitous nature, high resistance to common preservative methods, such as the use of salt, smoke or acidic condition in the food, and its ability to survive and grow at refrigeration temperatures (around 5 °C). All sectors of the food chain should Implement Good Hygienic Practices (GHP) and Good Manufacturing Practices (GMP) as well as implement a food safety management system based on the principles of Hazard Analysis Critical Control Points (HACCP).

Food manufacturers should also test against microbiological criteria, as appropriate, when validating and verifying the correct functioning of their HACCP based procedures and other hygiene control measures. In addition, producers manufacturing food associated with risks of Listeria must conduct environmental monitoring to identify and eliminate niche environments, including areas that favor the establishment and proliferation of L. monocytogenes.

Modern technologies using genetic fingerprint – Whole Genome Sequencing (WGS) – allow for more rapid identification of the food source of listeriosis outbreaks by linking L. monocytogens isolated from patients with those isolated from foods.

Prevention

L. monocytogenes in food are killed by pasteurization and cooking.

In general, guidance on the prevention of listeriosis is similar to guidance used to help prevent other foodborne illnesses. 

This includes practicing safe food handling and following the WHO Five Keys to Safer Food:

• Keep clean

• Separate raw and cooked

• Cook thoroughly

• Keep food at safe temperatures

• Use safe water and raw materials.

  Persons in high risk groups should:

  • Avoid consuming dairy products made of unpasteurized milk; deli meats and ready-to-eat meat products such as sausages, hams, patés and meat spreads, as well as cold-smoked seafood (such as smoked salmon);

  • Read and carefully follow the shelf life period and storage temperatures indicated on the product label.

  It is important to respect the shelf-life and storage temperature written on labels of ready-to-eat foods to ensure that bacteria potentially present in these foods does not multiply to dangerously high numbers. Cooking before eating is another very effective way to kill the bacteria

Listeria Outbreaks

Active Outbreak:

• Listeria Outbreak Linked to Meats Sliced at Delis-

  
  
  
  

  

  
  

Epidemiologic, laboratory, and traceback data showed that meats sliced at delis, including Boar's Head brand liverwurst, were contaminated with Listeria and made people sick.

Products sold at the deli, especially those sliced or prepared at the deli, can be contaminated with Listeria. Listeria spreads easily among deli equipment, surfaces, hands, and food. Refrigeration does not kill Listeria, but reheating to a high enough temperature before eating will kill any germs that may be on these meats.

Previous Outbreaks:

• Listeria Outbreak Linked to Queso Fresco and Cotija Cheese - February 2024-

Data showed that queso fresco and cotija cheese made by Rizo-López Foods made people sick in this outbreak. Many foods including cheeses, crema, and yogurts were recalled. Recalled foods are past their shelf life.

• Listeria Outbreak Linked to Peaches, Nectarines, and Plums - November 2023-

  
  
  
  

Data showed that whole peaches, nectarines, and plums distributed by HMC Farms made people in this outbreak sick.

• Of the seven people interviewed, all (100%) reported eating peaches, nectarines or plums.

• The outbreak strain of Listeria was found in a sample of HMC Farms peaches. WGS showed that the Listeria in the peaches were closely related to bacteria from sick people.

• Listeria Outbreak Linked to Ice Cream - August 2023

Data showed that ice cream made by The Ice Cream House of Brooklyn, NY, made people in this outbreak sick.

• One sick person ate "Soft Serve on The Go" ice cream cups, and the other person likely ate the same ice cream cup at a long-term care facility they lived.

• The outbreak strain of Listeria was found in "Soft Serve On The Go" ice cream cups, "Ice Cream House" mini dessert cigar novelty ice cream, and several environmental samples taken from two of The Ice Cream House stores.

• Listeria Outbreak Linked to Leafy Greens - February 2023

When this investigation began, there was not enough information to identify the source of the outbreak.

State and local health officials collected as much information as they could by asking people or their caregivers about foods they ate before getting sick. Epidemiologic data showed that leafy greens were a likely source of this outbreak, but there was not enough other data to identify a specific type or producer of leafy greens.

• Of 14 people who answered questions about leafy greens, 13 (93%) people ate leafy greens.

• Analysis showed that people in this outbreak were 8 times more likely to eat iceberg lettuce and 5 times more likely to eat romaine lettuce before getting sick compared to people who got sick with Listeria but were not part of an outbreak.

• Listeria Outbreak Linked to Enoki Mushrooms - November 2022-

  
  

  

  
  

This outbreak is over. Data showed that enoki mushrooms made people in this outbreak sick.

• Two sick people reported eating enoki mushrooms or eating at restaurants with menu items containing enoki mushrooms. Three people did not report eating enoki mushrooms, but two of them reported shopping at grocery stores or eating at restaurants that sell Asian foods.

• The outbreak strains were found in several samples of enoki mushrooms.

• Listeria Outbreak Linked to Brie and Camembert Cheese - September 2022-

Data showed that cheese made by Old Europe Cheese, Inc. made people in this outbreak sick.

• Of the five people interviewed, four (80%) reported eating brie or camembert cheese.

• Most people did not remember the brand of the cheese they ate, but one person reported eating Lidl Premium Brand Brie. Old Europe Cheese was the only manufacturer of Lidl Premium Brand Brie.

• The outbreak strain of Listeria was found in the Old Europe Cheese facility.