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Tom Pula
Listeria Bacteria

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Listeria Bacteria

I happened to hear on the radio recently about a food recall and that Listeria might be the source of illness resulting in the recall. Beginning on the 1st of January, 2017 the FDA began the establishment of new and more stringent standards for controlling Listeria in food production. Cleanliness during the production process is an area of particular concern.

Listeria monocytogenes is the species of bacteria that is often referred to when illness results due to ingestion of some foods. It is generally called an opportunistic pathogen meaning that it primarily affects individuals with impaired immune systems. It is anaerobic, but can also survive in the presence of oxygen. It is a virulent foodborne pathogen. Reportedly Listeriosis infections of those with compromised immune systems between 20 and 30 percent are fatal. Reportedly, infection and death rate are increasing.


Its ability to survive and grow at very low temperatures makes it more difficult to control in foods even when refrigerated at low temperatures. This bacterium was first referred to as Bacterium monocytogenes when its description was first published in 1927, but the genus name was changed to Listeria in 1940. This was in honor of Joseph Lister who spearheaded the use of antiseptic medicine.

The onset of gastrointestinal symptoms after exposure to Listeria probably exceeds 12 hours. These can include fever, chills, headache, nausea, vomiting, diarrhea, etc. These can be the only symptoms or could precede more serious ones which could appear anywhere from a few days to a few weeks. More severe symptoms seldom affect individuals with normally operating immune systems. These symptoms can include septic shock, encephalitis and meningitis, which can manifest as stiff neck, severe headache with nausea and vomiting. Those with impaired immune systems can include people taking immunosuppressant medications or procedures for such things as cancer and organ transplants.

Listeria is reported to be the third most common cause of meningitis in newborn infants. Infections during pregnancy usually occur during the third trimester when the mother’s immunity is lowest. Fever, headache, backache and muscle pain are the common symptoms. Infections of the mother can result in infection of the newborn or even premature labor, miscarriage or stillbirth. Mothers usually survive infections contracted during pregnancy, but mortality is particularly high with infant infections, either prenatal or neonatal. Treating with antibiotics has been shown to be effective.

Listeria has been associated with raw milk products, soft cheeses, ice cream, a wide variety of meats including deli meats, and especially raw meats and fish as well as raw vegetables. Refrigerated foods can be of concern due to this bacteria’s ability to survive and multiply well at relatively low temperatures.

Listeria infections were not recognized as a major cause of foodborne illness until 1981. Numerous cases of Listeriosis in some Canadian locations caused a number of deaths. Coleslaw with cabbage that was contaminated with Listeria from sheep manure was determined to be the source of the outbreak.

While E-Coli and Salmonella often dominate the news due to massive recalls of certain types of food, Listeria infections often fly under the radar due to its status as an opportunistic pathogen. However, its importance and impact should not be underestimated. According to a number of studies Listeria monocytogenes may be present in up to 10% of human gastrointestinal tracts. Ingestion of Listeria due to its very significant presence in the food supply has been described as an “exceedingly common occurrence.” According to Food Quality and Safety online magazine, “Without meticulous and proper cleaning, plant equipment, including any equipment used for refrigeration and HVAC, can become breeding grounds for microorganisms, intermittent defrost systems, which are subject to frequent wash-downs, encourage microbial growth. Because they often contain moisture from standing water or condensation, special attention should be paid to these systems to prevent their contamination.”

Due to Listeria bacteria being common in soil and water, animals can carry the bacteria and infect meat and dairy products. Thus ensuring surfaces used for preparing or cutting meat are kept very clean is quite important. This is as important in the home as it is in the food industry. Storing Listeria-contaminated food in your refrigerator can contaminate other food stored there or can contaminate the entire refrigerator. So amplification of Listeria in the home is entirely possible so precautions should be taken such as ensuring meats are enclosed in plastic bags or containers that are clean both on the interior and exterior. Thoroughly washing fruits and vegetables before preparing or serving should be a standard practice.