Listeria Detection and Identification Methods
- Gram +ve rod, individually or in short chains
- Non-spore forming
- Motile by means of flagella
- Optimum growth temperature 30-37°C
- Can grow slowly in refrigerated foods
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 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.
for these items:
broths, agars etc.
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
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
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
for these items:
molecular strain typing
Following isolation on selective media,
identification can be carried out using agglutination, biochemical and
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
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
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
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.
This guide has been prepared by Food Safety Info, scientific and technical information providers for the food industry. For more information, visit our web site at www.foodsafetywatch.com