Ohio State University Extension Bulletin

Research and Reviews: Meat

Special Circular 172-99


A Review of Listeria monocytogenes –A Pathogen That Likes Refrigerated Temperatures

H. W. Ockerman1 and L. Basu
The Ohio State University Department of Animal Sciences

Abstract

The fact that Listeria monocytogenes can grow at refrigerated temperatures, which is often the technique that the meat industry uses to control pathogens, makes this microorganism a special problem for meat processors. Since this pathogen can be deadly and can cause abortions, it is receiving a lot of attention from a food-safety standpoint. Fortunately, non-pregnant healthy adults are fairly resistant. Also, pasteurization and chemical additives are fairly effective at protection, but a rapid analytical technique for identification would be extremely helpful in combating this troublemaker for meat products.

Review

There are several types of Listeria spp., but the most pathogenic one is Listeria monocytogenes. Others include Listeria innocua, Listeria seeligeri, Listeria welshimeri, Listeria ivanovii, Listeria grayi, and Listeria murrayi. Listeria monocytogenes is responsible for 98% of the human listerosis cases identified. This organism was first described in 1923, and the first human listeriosis was reported in 1929. This pathogen has been shown to be a problem in more than 50 mammal types, and the overall mortality rate can be from 10 to 70%. Listeria may be found in decaying vegetation, soil, animal feces, sewage, silage, water, and contaminated food, including meat. It is obvious that this contamination can occur in most any food item and is particularly a problem with perishable items. Listeria spp. is fairly common in the household and can be found in 47% of household supplies. Of this 47%, Listeria monocytogenes makes up about 41% of this population. The highest percentage is found on dishcloths and wash-up brushes, and so these areas need special attention in the cleanup operation. Listeria is also moderately common in meat products, with about 0.5% beef jerky containing this microfile, about 1.3% large-diameter cooked sausage may contain Listeria, 2.3% of cooked poultry, 2.8% of corned beef and cooked roast beef, 4.2% of small-diameter cooked sausage, and 6% of ham slices and luncheon meat. It is pertinent that we be aware that meat products can be a problem and that special precautions should be taken.

Listeria monocytogenes is especially harmful to people with AIDS, alcoholism, diabetes, cardiovascular disease, renal transplants, and corticosteroid therapy. A major area of concern is abortion as fetuses are highly susceptible. Fortunately, non-pregnant healthy adults are highly resistant. This translates into a variable dose rate to cause infection. Epidemiology data indicates that consumption of fewer than a thousand total organisms in milk by susceptible people may cause the disease. However, a study with healthy primates indicates that consumption of as high as 100 million cells was required to cause the disease. USDA product recalls due to Listeria were one in 1997, five in 1998, and nine in 1999, suggesting that this problem is not going away and, in fact, may be increasing. The Center For Disease Control has suggested that in 1996 for every 100,000 individuals, Listeria was responsible for 0.4 cases; in 1997, 0.5 cases; and in 1998, 0.5 cases. In 1996—97 this resulted in 77 cases in the United States, with 15 deaths. Therefore, the problem can be serious, and special precautions should be taken.

Listeria monocytogenes likes warm temperatures of about 50°F but can also survive and grow at refrigerated temperatures, which makes it a special problem. For example, at 40°F, the germination period (time required to double in number) is approximately three days. At 32°F, the germination time would be 62 to 131 hours, and at 40°F, it has been estimated at 18 to 30 hours, all of which is a problem for products that are protected by refrigeration.

Government tolerance levels permitted in food are as follows: For the United States, it is zero cells for sampling unit, and for Canada, the European community, and Australia, it is less than 100 units per gram. So in all cases, the permissible legal level is very, very low. How can the product be protected – by paying attention to raw materials, educating employees, cleaning floors and equipment properly, and handling cleanup equipment and material properly.

Cooking is always a critical control point in the production of this type of product and particularly where Listeria is involved, but time and temperature will be different for each individual product. Smokehouse and cook chambers should also receive special attention to make sure they are not a contaminating source. Cooking temperature to kill Listeria is 158°F for two minutes, and this will destroy this pathogen. Frankfurters should be cooked to 160°F. This will result in a three-log reduction. Initial contamination as well as cooking temperature is important.

Continuous training and monitoring is essential for all microbial problems. This should be concentrated particularly in the area of hygiene and product-handling procedures. Areas that need special attention are personnel entrances, lift-truck entrances, and work areas around equipment. Equipment should also be designed so it is easy to maintain, clean, and sanitize and even non-food contact surfaces should receive special attention particularly if they are in the proximity of food contact surfaces which, are of course, absolutely critical from a sanitation perspective. Following appropriate pasteurization techniques along with using chemical additives are needed to help the food processor fight against Listeria monocytogenes. A more closed-system environment, with more automation and improved sanitation techniques, is very critical as is a rapid analytical tool for detecting this microorganism in a short period of time. The meat industry has recognized the problem and is taking steps to make sure your meat supply is safe. Handling after it leaves the meat plant, however, is still an area that needs added emphasis.


1 For more information, contact at: The Ohio State University 15 Animal Science, 2029 Fyffe Road, Columbus, OH 43210, (614) 292-4317, Fax (614) 292-2929; email:ockerman.2@osu.edu


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