Focus on Listeria

Listeria Gram Stain

Although Listeria monocytogenes is the only member of the Listeria family which causes human illness, the presence of any member of the Listeria family in a food processing environment indicates that conditions are favorable for L. mono.  The other species of the Listeria family include L. innocua, L. seeligeri, L. welshimeri, L. ivanovii, and L. grayi.  Unlike most other bacteria that can cause food-borne illness, which grow only at room temperature and slightly above, L. monocytogenes can also grow in cold, refrigerated environments.  

L. monocytogenes are small Gram-positive bacteria.  They are facultatively anaerobic, meaning they can grow with little or no oxygen.  What makes this bacterium such a dangerous food-borne pathogen is the ability to grow at a wide range of temperatures, 0 to 45 Celsius, and a wide pH range, from 4.5 to 9.6.  They are able to survive for long periods under adverse conditions including very saline environments (up to a 10% NaCl solution).  

Listeriosis is the general name given to a variety of illnesses caused by L. monocytogenes.  While most healthy people do not develop severe forms of listeriosis, certain populations are at special risk for serious illness.  At-risk groups include fetuses, newborns, the elderly, and people with weakened immune systems due to chronic illness or medical treatments.  The less severe cases of listeriosis may result in flu-like symptoms, fever, muscle aches, vomiting, and diarrhea.  More severe forms of listeriosis result when the organism infects the blood and eventually the nervous tissue or brain.  Ensuing symptoms include headaches, stiff neck, confusion, loss of balance or convulsions and possibly death in high-risk individuals.  While listeriosis during pregnancy may only cause flu-like symptoms in the mother, it can lead to severe infection of the newborn, premature delivery, spontaneous abortion or stillbirth.  

Where is Listeria found?

L. monocytogenes is widespread in the environment and has been isolated from water, soil, dust, plants, animal feed, feces, and sewage.  While some studies suggest that 1-10% of humans may be intestinal carriers of L. monocytogenes, many animals, including dairy cows, can carry the bacterium in their intestinal tract without becoming sick.  L. mono has been isolated from raw milk and foods made from raw milk.  Various studies have found that 2-5% of raw milk samples test positive for L. monocytogenes.  Even though Listeria is killed in the pasteurization process, post-pasteurization contamination can occur within the processing plant.  Because these bacteria are capable of growing at refrigeration temperatures, even low numbers of L. monocytogenes in processed products can multiply to dangerous levels.  In the dairy plant, Listeria has been isolated from a variety of sites, though it is most often found in moist environments or areas with condensed or standing water or milk, including drains, floors, conveyors, coolers, and case washing areas.  

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Controlling Listeria in the Plant

Control of Listeria in the plant requires both reducing the number of bacteria on equipment surfaces using physical means, and preventing growth and proliferation of the bacterium by managing the environment.  Cleaning and sanitizing treatments given to the equipment, walls, and drains should be adequate to remove the bacterium.  

Training all employees within the plant on GMPs and HACCP is recommended to reduce the risk of contamination of any product produced by the plant.  

Control the entry of Listeria into the processing areas.  As much as possible, sanitize all receiving and processing areas.  Ensure that operations where food is directly exposed are protected from contamination.  Plan and implement some type of traffic control to ensure critical areas are isolated from external sources as much as possible.  Make sure that product is handled as least as possible. It is important to remember that Listeria can grow at room temperatures and in moist environments.  Floors, walls, and ceilings in cold rooms and refrigerators should be kept clean and dry.  The presence of mold on refrigerator and cold room door gaskets, surfaces, cooling motors, and ice machines indicate that there is inadequate cleaning and sanitizing of those areas and makes them a prime environment for Listeria.   

Controlling possible cross-contamination is a critical step in controlling Listeria in the plant.  Attention should be given to the following types of cross-contamination:

• Pathways between raw materials and finished product are key target areas for control.  Keeping food contact surfaces, utensils, containers, hands, sinks, faucet handles, towels, and cleaning cloths clean will help prevent cross-contamination as well as controlling the presence of insects, flies, and rodents. 

• Avoid using high pressure hosing of drains and sanitize all drains daily.

• Control the flow of air from areas where raw materials are stored to areas where finished product is stored.  Also control flow of air through air conditioning units with improper or dirty filters.  

As up to 10% of humans carry Listeria, personnel cleanliness is important and the following practices should be observed: 

• Wash hands often.  Hands should be washed after using the restroom, before entering the work area, immediately after handing raw materials, after using nasal tissues, or whenever hands may have been contaminated.  

• Use disposable gloves.  Do not reuse gloves.  They can be just as unclean as hands are if used inappropriately.

• Avoid eating and drinking in food handling areas.

• Restrict jewelry wear on hands and wrists.  Jewelry can impair effective hand washing.

Finally, keep floors clean.  Pools of water or other wastes such as found in irregular floor surfaces, grout, cracks, and holes should be minimized. 

Sanitizing suggestions:

• It may be beneficial to spray 200 ppm quaternary aerosol into a room as a final sanitation step either weekly or monthly.

• A 2 to 3% solution of citric acid can be poured directly into all drains.  The acid will make the area difficult for Listeria to survive.

• Chemical sanitizers such as iodophor and quaternary ammonia are most effective on equipment and surfaces.  Chlorine may be used, but it quickly becomes deactivated.

• Remember to rotate sanitizers at least quarterly.  Not doing so may result in bacteria building a resistance to a particular sanitizer.  

• Clean drains, floors, waste containers and storage daily. 

• Every week clean walls, condense drip pans, HVAC, and coolers.  

It is important that each plant develops and implements Standard Operating Procedures (SOP) for cleaning all pieces of equipment within the plant.  When cleaning, be sure to clean "Top-to-Bottom".  Always remember to clean and sanitize mops, buckets, and other cleaning materials. 

Visual inspection and routine microbiological testing are necessary to develop an idea of what you are dealing with in the plant.  A generic Listeria monitoring system is also recommended. 

The most important point in limiting the risk for L. monocytogenes contamination may be in ensuring that personnel are aware of the severity of contamination with the bacteria, and what practices increase this risk.  Many of the seemingly insignificant practices such as setting equipment on the floor to clean it, not wearing clean gloves, or handling dirty equipment and then touching food contact surfaces may seem insignificant to the uninformed employee, but can be catastrophic for a processing system.  Make sure that every employee feels a sense of personal responsibility towards maintaining the sanitation and safety of the plant. 

Additional Information 

Reducing the risk of Listeria monocytogenes - Regulation & Food Safety

Listeria monocytogenes - FDA Bad Bug Book

Food Safety Information Website contains many links for information on Listeria

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