There are a 6 different species within the genus but only Listeria monocytogenes is considered pathogenic for humans. Listeria is ubiquitous in the environment. It is quite hardy - resisting freezing, drying and heat. The ability to grow at temperatures as low as 3°C permits multiplication in refrigerated foods.

Listeria has been isolated from products including raw milk, cheese made from unpasteurised milk, soft cheese, ice cream, meat and poultry and fish. Ready to eat meat and poultry products are of particular risk of infection with Listeria.

Listeria monocytogenes bacteria are frequently found in the food-processing environment and can form biofilms on solid surfaces. Listeria is able to survive apparently adverse conditions on smooth surfaces; it thrives in wet, dirty conditions. The presence of Listeria species is a useful hygienic indicator in all stages of the food processing chain. Strains can spread within manufacturing plants and even establish themselves as "house flora".

In susceptible persons an infective dose can be fewer than 100 organisms.

Traditionally food and environmental samples are enriched in a broth prior to subculture into a further broth and then onto selective agar.

The initial broth incubation is at 30°C for 24 hours, the subsequent broth incubation is at 35°C for 24 hours. All broth cultures are then subcultured onto agar for a further 24 hours and subsequently identified by biochemical tests.

The traditional method is labour intensive and takes up to 5 days to give a result. There are therefore now available many commercial alternatives to generate a faster result. One of these is the use of chromogenic agars following a simple 24 hour enrichment. These agars give presumptive positive results.

As an alterative to growth on agar there are a variety of technologies that provide rapid results eg. immunoassay; molecular methods such as PCR; or techniques that reduce time to result e.g. concentration. Sometimes combinations of these techniques are used to further enhance speed to result.

Molecular methods such as PCR can eliminate the need for further identification in the event of a positive and can also provide quantitative information.

The unique challenges of rapid routine environmental monitoring for the presence of Listeria in food processing have been met by specific products.

Following isolation on selective media, identification can be carried out using agglutination, biochemical and molecular techniques.

Latex agglutination allows rapid elimination of negative samples, positives can then be checked out with more targeted tests.

Biochemical profiles identify organisms phenotypically and are widely used.

Molecular methods using PCR or nucleic acid techniques are routinely used as confirmatory tests.

The European Regulation EC 2073/2005 "Microbiological Criteria for Foodstuffs" states that all "Ready-to-eat" (RTE) foods must be tested for Listeria. RTE is defined as food that is sold ready for consumption without the need for cooking or reheating.

Ready-to-eat food for infants and special medical purposes must demonstrate absence of Listeria monocytogenes in 25g during product shelf-life and other RTE eat foods must demonstrate absence of Listeria monocytogenes in 25g before product leaves the factory and must remain below 100 cfu/g throughout shelf-life.

There are as many as 2500 serious cases of Listeriosis with 500 deaths in the US each year (CDC data). In the first 8 months of 2005 the UK Health protection Agency had 118 cases of Listeria reported - it is noted that non-pregnancy associated Listeria is on the increase, especially in those aged over 60.