Listeria : Old, Tough Germ

by Food and Drug Administration (FDA)

(Page 3 of 3)

L. monocytogenes is not "new." What is relatively new is the recognition that L. monocytogenes bacteria may be spread in food. It has been only in the past decade that researchers have recognized L. monocytogenes as an agent of food-borne illness.

Since 1911, scientists have known it infects animals, and in 1929 the first case of human infection was detected. Researchers have isolated L. monocytogenes from soil, leaf litter, sewage, silage, dust, and water. The organism often moves through the animal and human intestinal tract without causing illness, and has been found in many domestic and wild animals, including birds and fish. (See also, "Listeria: Battling Back Against One Tough Bug," in the July-August 1988 FDA Consumer.)

In those earlier times, many believed farm animals transmitted L. monocytogenes to farm workers. But, when listeriosis appeared in city dwellers, public health authorities realized that animal contact was not always the method of disease transmission.

For example, farm animals may pick it up from consuming improperly fermented silage, and vegetables may then become contaminated when animal manure carrying the organism is used for fertilizer. Animals in a herd also may pick up L. monocytogenes from other animals or manure containing the organism.

Because L. monocytogenes is widely present in the environment, it would be impossible to prevent animals from coming in contact with the bacteria. However, farmers, animal producers, food processors, and food handlers can all take steps to reduce contamination and keep food safe from L. monocytogenes.

L. monocytogenes is remarkably tough. It resists heat, salt, nitrite, and acidity much better than many organisms. The bacteria survive on cold surfaces and also can multiply slowly at temperatures as low as 34 degrees Fahrenheit, defeating one traditional food safety defense — refrigeration. (Refrigeration at 40 F or below stops the multiplication of — but does not kill — many other food-borne bacteria.) Freezer temperatures of zero degrees Fahrenheit, however, will stop L. monocytogenes from multiplying.

FDA researchers and FDA-funded research have confirmed that commercial pasteurization will eliminate the organism in dairy products.

And studies funded by FSIS confirmed that current regulatory cooking standards will eliminate L. monocytogenes in FSIS-regulated products, such as meat and poultry.

When L. monocytogenes is found in processed products, it is likely contamination occurred after processing, rather than because of failure of heating or pasteurizing.

FSIS scientists developed a more precise method of detecting L. monocytogenes in meat and poultry products. FDA scientists developed a similar method appropriate for dairy products, seafood and vegetables. These methods have steadily been improved over the years.

As more is learned about the bacteria and their control, new and even better tests are being developed. An example is FDA's gene-probe method. This method, developed by agency scientists, can detect L. monocytogenes in a food sample in two days, a significantly shorter time than older methods. n Proper Food Handling Helps

Listeria bacteria do not change the taste or smell of a food. CDC, FDA, FSIS, and the National Advisory Committee on Microbiological Criteria for Foods (which includes food scientists from federal health agencies, universities, and private industry) have developed food handler advice for preventing listeriosis.

Although most people are at very low risk for listeriosis, the risk of listeriosis and other food-borne illnesses can be reduced by following these tips:

Avoid raw/unpasteurized milk.

Keep raw and cooked foods separate when shopping and preparing, cooking and storing foods. Otherwise, bacteria in juices from raw meat, poultry or fish might contaminate a cooked food. For instance, transfer cooked meat, poultry or fish to a clean platter — never to the dish that held the raw food.

Wash hands, knives, and cutting boards after handling uncooked foods.

Wash raw vegetables thoroughly before eating.

Thoroughly cook all food of animal origin, including eggs. Cook raw meat to an internal temperature of 160 degrees Fahrenheit, raw poultry to 180 F, and raw fish to 160 F, or until it is white and flaky. Reheat leftovers thoroughly.

Read and follow label instructions to "keep refrigerated" and "use by" a certain date.

Keep hot foods hot (above 140 F). Do not keep them out for longer than two hours at room temperature (at which L. monocytogenes can thrive) before eating.

Keep cold foods cold (at or below 40 F). Do not keep them out for longer than two hours at room temperature before eating.

Divide leftovers into small, shallow, covered containers before refrigerating, so that the food chills rapidly and evenly.

Keep your refrigerator clean, and keep the temperature at 34 F to 40 F. n Recommendations to High-Risk Individuals

Pregnant women, the elderly, people with conditions that suppress immunity, and others who may be highly susceptible to listeriosis can decrease their risk by:

Avoiding soft cheeses such as Mexican style, feta, Brie, Camembert, and blue cheese. Mexican-style cheeses are soft, white, ethnic (Hispanic-Latin American) cheeses such as Queso Blanco and Queso Fresco. There is no need to avoid hard cheese, processed slices, cottage cheese, or yogurt.

Reheating leftover foods or ready-to-eat foods such as hot dogs thoroughly until steaming hot before eating.

Although the risk of listeriosis associated with foods from delicatessen counters is relatively low, pregnant women and people with suppressed immunity may choose to avoid these foods or thoroughly reheat cold cuts before eating.

About the Author

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FDA is A United States government body that oversees medical devices, including contact lenses, intraocular lenses, excimer lasers and eyedrops. In the US, these products must be approved by the FDA before they can be marketed.