Tackling Listeria monocytogenes is everybody’s business

A guest blog by Hudaa

Listeria is a bacterium that is commonly found in the environment in air and water, on our hands and in our refrigerators. Most people are routinely exposed to Listeria with no health consequences. But one species of Listeria – Listeria monocytogenes — is a virulent strain and can lead to the very serious disease, listeriosis, particularly among some at risk populations. These include pregnant women, newborns, the very old and people who are immune-compromised (AMI, 2002)

The control of L. monocytogenes has been the subject of much discussion. The minimal infection dose is not known, but it is generally accepted in many countries and endorsed by the International Commission on Microbiological Specifications that foods containing <100 L. monocytogenes per gram are safe (ICMSF, 1994). This is supported by studies on the epidemiology of listeriosis suggesting that low levels of L. monocytogenes in foods are too frequent to be responsible for listeriosis even in susceptible humans, whereas exposure to higher doses (≥103 CFU/g) of L. monocytogenes might result in foodborne listeriosis (Pinner et al., 1992; Hitchins, 1996; McLauchlin, 1996).

L. monocytogenes is a major safety concern especially for RTE meat products which are frequently contaminated with this pathogen. If contamination of RTE meat products is to be prevented, it is essential that contamination routes for the bacteria to be elucidated. Feed may be an important source of contamination for animals (Fenlon and others 1996). Adult animals may be transiently colonized by consuming contaminated feed or water (Husu and others 1990). Hence Listeria may enter the packing plant at low levels in the intestines of recently infected animals. Some strains may persist in the processing environments, ultimately contributing to both environmental and RTE meat product contamination (Giovannacci and others 1999). The ways in which L. monocytogenes may be introduced into the processing plants are numerous. L. monocytogenes being a ubiquitous bacteria, may also be carried by man (Schuchat et al. 1991). Other possible vectors may be equipment, packing materials etc.

Whatever the initial source of contamination might be in each case, the main issue for the producer is to avoid colonization of the processing environment and spread to the product. This should be done by the systematical implementation of general hygienic precautions, including the HACCP approach.

Excellent sanitation and environmental controls in the plant are important operational guidelines to minimize the spread of Listeria in meat processing plants. These control steps are essentially proactive measures to reduce the incidence of Listeriosis. However, there is evidence that L. monocytogenes contamination occurs mainly during post-processing (Zhu, 2005). Hence additional measures are needed to control the growth of L. monocytogenes in these products. Formulating meat products with antimicrobial additives represents an effective approach to suppress the growth of contaminated L. monocytogenes during storage. However, antimicrobials cannot destroy the pathogen that existed in RTE meat but only inhibits growth of the bacteria thus reducing contamination to only a minimum.

Combining several intervention strategies such as antimicrobials and post-package decontamination methods such as in-package thermal pasteurization and low-dose irradiation represent promising technologies to ensure the safety of RTE meat products. However, irradiation is not yet approved for processed meat and poultry.

Consumers should also realize that use of preservatives to retard the growth of L. monocytogenes does not ensure the complete absence of the organism. Even if a Listeria-free product is placed in the refrigerator, if it is improperly stored, bacteria in the refrigerator can contaminate products since Listeria survives well in the cold, moist environment found in refrigerators. Consumers should adopt simple domestic practices to further reduce incidence of listeriosis. These include keeping refrigerated foods at 40°F and below, consuming perishable precooked RTE foods as soon as possible and reheating RTE meat products prior to consumption. Regular cleaning of refrigerators as well as prevention of cross-contamination of meat and vegetables are also recommended.

Hence control of L. monocytogenes is a multi-step process that requires the intervention of not only producers and plant workers but also scientists, public health agencies and consumers at large.


American Meat Institute Fact Sheet: Listeria. 2002.

Fenlon DR, Wilson J, Donachie W. 1996. The incidence and level of Listeria monocytogenes contamination of food sources at primary production and initial processing. J Appl Bacteriol. 81:641–50.

Giovannacci I, Ragimbeau C, Queguiner S, Salvat G, Vendeuvre JL, Carlier V, Ermel G. 1999. Listeria monocytogenes in pork slaughtering and cutting plants. Use of RAPD, PFGE and PCR-REA for tracing and molecular epidemiology. Int J Food Microbiol 53:127–40.

Hitchins, A.D., 1996. Assessment of alimentary exposure to Listeria monocytogenes. Int. J. Food Microbiol. 30, 71-85.

Husu JR, Beery JT, Nurmi E, Doyle MP. 1990. Fate of Listeria monocytogenes in orally dosed chicks. Int J Food Microbiol 11:259–69.

Mc Lauchlin J., 1996. The relationship between Listeria and listeriosis. Food Control. 7, 187-193.

Pinner R.W., Schuchat A., Swaminathan B., Hayes P.S., Deaver K., Weaver R.E., Plikaytis B.D., Reaves M., Broome C.V., Wenger J.D., the Listeria Study Group, 1992. Role of foods in sporadic listeriosis. II. Microbiological and epidemiological investigation. J. Am. Med. Assoc. 267, 2046-2050.

Schuchat A., Swaminathan B., Broome C. 1991. Epidemiology of human Listeriosis. Clin. Microbiol. Rev. 4, 169-183.

Zhu M., Du M., Cordray J. , and Uk Ahn D. 2005. Control of Listeria monocytogenes Contamination in Ready-to-Eat Meat Products