About Listeria monocytogenes

Main microbiological characteristics

Non-sporulated, motile, small Gram-positive rod shaped bacteria. Facultative aerobic-anaerobic, ferments numerous carbohydrates without gas. Does not produce urea and indole.
The behaviour of L. monocytogenes is relatively variable depending on the strains, irrespective of the factor studied (T°C, pH, aw) and the type of food matrix.
Growth temperature: between –2 and +45°C, optimum temperature between +30 and +39°C
Growth pH: between 4.6 and 9.6, optimum pH 7.1
Atmosphere: grows well in microaerophilic conditions but also in aerobic and anaerobic conditions. Can grow in the presence of 30% CO 2
Water activity: minimum aw 0.90 if glycerol is used to adjust the aw, 0.92 to 0.93 if NaCl (=10.5% NaCl), sucrose or meat extract are used

NB: Confinement class of L. monocytogenes : class 2.
Other species of the genus Listeria (L. innocua, L. ivanovii, L. seeligeri, L. welshimeri, and L. grayi) are not pathogenic for humans.

Human disease: listeriosis

L. monocytogenes infections are serious but fortunately “rare”. Since the bacterium can contaminate a number of different foods, many people ingest small numbers of L. monocytogenes relatively frequently without any symptoms developing.

Zoonotic character

L. monocytogenes has been isolated in numerous animal species, generally asymptomatic intestinal carriers. Cattle, sheep and goats can develop nervous and abortive clinical forms, similar to those observed in humans. In addition, encephalitis caused by Listeria has been reported in pigs and some septicaemias due to Listeria have been observed in monogastric animals (pigs, carnivores, rabbits, but also calves and lambs). However, it is important to underline that transmission to humans is via food in the majority of cases and that direct animal-to-human transmission has not much been documented.

Reservoir

Listeria is a ubiquist, telluric bacterium, very widespread in the environment and which has a high capacity for resistance in external environments (1 to 2 years in soil and several years if the sample is stored in a refrigerator). Listeria has been isolated in numerous countries in the Northern hemisphere, from samples of soil, water of various origins (lakes, rivers, sewers or coastal water) as well as from some vegetation.
Around 10% of silage contain L. monocytogenes, sometimes in high quantities in poor-quality silage, which can be a source of ruminant contamination. The environment is primarily contaminated by the excrement of healthy and ill animals: 10 to 30% of cattle, sheep, pigs and chickens naturally host this bacterium in their gastrointestinal tracts.

Foods involved (water, animal and vegetable products)

All major food categories, whether these involve milk and dairy products, raw meat and meat products, vegetables, fish or shellfish or ready-to-eat foods, can be contaminated by this bacterium, with variable contamination frequencies and rates. The frequency of contamination with L. monocytogenes, along with the level of contamination, vary depending on the food category, whether raw foods or processed foods are involved. Cooked foods can also be contaminated following inadequate heat treatment or be contaminated by cross-contamination after treatment.
Its specificability to multiply at refrigeration temperatures explains why L. monocytogenes is usually associated with refrigerated foods with a long shelf-life: in terms of L. monocytogenes, a high-risk food is a food consumed as is, able to support the growth of L. monocytogenes and stored for a certain length of time at refrigeration temperatures.