Foodborne diseases / Listeria monocytogenes

Listeria monocytogenes

Listeria monocytogenes is a psychrotrophic bacterium which is ubiquitous in the environment and is frequently present on foods of animal and plant origin. It can be responsible for sporadic cases and outbreaks of disease in humans. Recontamination of heat treated meat products are a greater risk to human health than contaminated raw meats.

1. Bacteriological characteristics

Listeria monocytogenes is a Gram-positive, rod shaped bacterium which is nutritionally non-fastidious. The organism can grow at low temperatures (1-3°C) and is relatively insensitive to environmental stresses.

2. Disease in humans

Listeriosis is characterised by meningitis and inter-uterine infections in pregnant women. The latter, which may give rise to only mild symptoms in the mother initially, are usually followed by abortion or premature birth. The mortality rate of the neonates lies between 20% and 40%. Immunocompromised patients, particularly those suffering from neoplasia and AIDS form another risk group. Diabetes mellitus and alcoholism are also recognised risk factors.

3. Sources of infection and epidemiology

L. monocytogenes is widely distributed in the environment and can be isolated form soil, surface water, plants and animals including man. The bacterium may enter slaughterhouses with carrier animals, but contamination of red meat and poultry often originates from the environment in which these products are processed rather than from the animals themselves. Cases of listeriosis are most often associated with the consumption of contaminated, heat treated meat products, soft cheeses and milk. In heat treated products cross-contamination after heating, rather than insufficient heat processing, is the main problem. Damp environments, such as drains and pools of water, are likely to harbour the organism. Cross-contamination and recontamination may occur (1) on transport vehicles and machinery which are difficult to dismantle and thus continue to contaminate meat despite cleaning, (2) in the ventilation system which can distribute listerias via turbulent air flows induced by forced air circulation, (3) from the hands of people handling meat and meat products and (4) during cleaning operations, particularly when pressure hoses are used and aerosols are formed thereby.

4. Growth properties in relation to meat and meat products

The reported growth range is between 0 and 45°C with an optimum of 30-37°C.
L. monocytogenes cannot grow if the pH value is <4.5 or >9.5 or the water activity is below
0.94. Modified atmosphere conditions have little or no impact on growth. L. monocytogenes survives well in frozen foods.

5. Isolation

Qualitative and quantitative methods exist for the isolation of L. monocytogenes. The qualitative approach consists of a double enrichment procedure followed by subculture onto Oxford or PALCAM agar. Various confirmatory tests are employed to differentiate the listeria species. The quantitative technique is the same but without pre-enrichment. In addition to “traditional” cultural methods, rapid methods and test kits have been developed for differentiating the species. These include the enzyme linked immunosorbent assay (ELISA), immunomagnetic separation, gene probes, impedance methods and the polymerase chain reaction (PCR).

6. Control measures in slaughterhouses and cutting plants

Strict adherence to Good Manufacturing Practice (GMP), especially the prevention of contamination from faeces and the pharynx is required to reduce risk of contamination of meat. Also, strict personal hygiene is important since workers in meat plants may be carriers of the bacterium. A wide range of disinfectants is active against L. monocytogenes although the bacterium is more resistant to disinfectants on dry surfaces compared with wet. L. monocytogenes has the ability to form biofilms on smooth surfaces making it necessary to use methods of cleaning which remove these effectively and hence prevent clones of L. monocytogenes establishing themselves in meat processing plants.
Heat treatment at 70°C for at least two minutes is sufficient to inactivate L. monocytogenes. Preservatives such as nitrite, together with reduced water activity, may have some inhibitory effect on the organism.