LISTERIA: HEALTH AND CONTINUITY THREAT

Listeria is the western world’s primary food safety threat and causes listeriosis in humans. Although accurate estimates are difficult to obtain, it is clear that a large number of food categories is susceptible to Listeria, including many of the popular convenience food categories with double-digit growth. Listeria related economic losses now run into the billions per year. This excludes the damage in 'brand equity' and the loss of market share and litigation costs that result for corporations when their contaminated products hit the market.

view animation of moving listeria
(source: J. Theriot & D. Portnoy; UC Berkeley)

LISTERIA , THE PATHOGEN
The genus Listeria includes 6 different species (L. monocytogenes, L. ivanovii, L. innocua, L. welshimeri, L. seegligeri, and L. grayi). Both L. ivanovii and L. monocytogenes are pathogenic for mice, but only L. monocytogenes is consistently associated with human illness (Hitchins, 1998). In the early 1900s, L. monocytogenes was recognized as a bacterium that caused illness in farm animals. Listeria monocytogenes is found in the environment (e.g., in soil, water, vegetation and on the surfaces of equipment, floors and walls) and is often carried by healthy animals (including humans). Listeria monocytogenes is spread easily by direct food contact with a contaminated surface, and can survive and grow in a refrigerated, packaged Ready to Eat product. Listeria monocytogenes grows under low-oxygen conditions and at low refrigeration temperatures and survives for long periods of time in the environment, on foods, in processing plants and in household refrigerators. Untill now it was cumbersome for food producers to secure compliance with regulatory requirements.

LISTERIA (Growth, Survival & Inactivation)

Growth

• Temperature: Optimum 37°C, range -1.5 to 45°C.
• Grows at refrigeration temperatures (4°C).
• pH: Optimum 7.0, range 4.4-9.4.
• Atmosphere: Grows optimally under microaerophilic conditions but grows well both aerobically and anaerobically.
• Can grow in relatively high (e.g. 30%) CO₂ , but is inhibited under 100% CO₂.
• Growth was not retarded by a 5-10% CO₂ atmosphere.
• Water activity: Minimum aw permitting growth = 0.92 (=11.5 % NaCl).

Survival

• Temperature: Survives freezing very well.
• Atmosphere: Not influenced by atmosphere.
• Viable but non-culturable (VNC) cells: There is some recent evidence that L. monocytogenes may become VNC.

Inactivation (CCPs and Hurdles)

• Temperature: Rapidly inactivated at temperatures above 70°C. D value at 50°C can be in the order of hours, at 60^_oC 5-10 minutes, 70^_oC approximately 10 seconds. ( Note that in microbiological terms .D (refers to a 90% (or decimal or 1 log₁₀) reduction in the number of organisms).
• pH: Inactivated at pH values less than 4.4 at rates depending on the acidulant and temperature.
• Inactivation proceeds faster at higher temperatures.
• Water activity (aw): Can remain viable in dry environments for long periods.

LISTERIOSIS
Consumption of food contaminated with Listeria monocytogenes can cause listeriosis. Listeriosis is a potentially fatal disease in newborns, the elderly and persons with weakened immune systems, such as those with chronic disease or human immunodeficiency virus (HIV) infection or those taking chemotherapy for cancer. Listeriosis is also a major concern in pregnant women. Even though symptoms may be relatively mild in the mother, the illness can be transmitted to the foetus, causing illness or foetal death (spontaneous abortion).

Microbiological criteria for L.monocytogenes Foodstuffs EU regulation 2073/2005