FACTS & IDENTIFICATION
LISTERIA MONOCYTOGENES
Listeria monocytogenes is a pathogenic
microorganism that was first identified
in 1924 by Murray et. al in rabbits. A
few years later, in 1929, the first human
listeriosis outbreaks were observed,
but it was not until after many years
that this illness was associated with the
consumption of contaminated beef
and dairy products.
Listeria monocytogenes is a foodborne human
pathogen responsible for invasive infections, overall
presenting a high mortality. Despite the ubiquity of
the microorganism, the actual disease rate is quite
low and the disease is most often associated with an
underlying predisposition.
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Where is it found?
L. monocytogenes is widely distributed in nature. It
has been isolated from healthy animals and
humans, as well as soils and vegetation.
Listeria monocytogenes is a very persistent
bacterium in food processing plants that can
survive and grow at very low temperatures.
Therefore, L. monocytogenes has become one of
the most important microorganisms in the food
industry.
Special risk materials are raw or processed meat, raw
milk products, raw or smoked fish, ready prepared
salads and long stored vacuum packaged food.
Why is it a problem?
L. monocytogenes has been known to cause a
serious but rare infection known as listeriosis.
Listeriosis is one of the most relevant food-borne
diseases.
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The bacterium is relatively insensitive to high
concentrations of salts and acids and is able to
multiply at fridge temperature and inside vacuum
packaging.
Between 85 and 90% of listeriosis cases result
from the ingestion of contaminated products.
Listeriosis is a human disease with low morbidity
but high mortality rate (between 20-40%), higher
than that caused by Salmonellosis or
Campylobacteriosis.
Although some adults experience only mild
infections of the eye and skin, and gastroenteritis,
it can lead to severe blood poisoning
(septicaemia) or meningitis.
International regulations
In the US, any RTE (”ready to eat") food in which the
pathogen is detected should be considered
adulterated and therefore withdrawn from the
market.
In the European Union, RTE foods which are not
meant for at-risk-populations as well as those which
do not permit the growth of L. monocytogenes, have
an imposed limit of 102 cfu/g throughout their
shelf-life. Conversely, foods intended for groups at
risk, as well as those allowing the growth of the
pathogen, are required an absence in 25 g for each
analytical unit.
Who is especially at risk?
Pregnant women, the elderly and people with
weakened immune systems, including those
suffering from cancer, AIDS or alcoholism, are
more susceptible to listeria.
In pregnant women spontaneous abortion or
stillbirth may result.
How can we identify it?
How can we reduce the risk?
Cleaning and disinfection in food processing
plants is essential, as well as the use of measures
and good practices in handling and the Hazard
Analysis and Critical Control Points (HACCP).
To slow the growth of L. monocytogenes, high
risk foods should be kept below 4°C and for the
shortest time possible.
Avoiding cross contamination of food through
good food hygiene and proper food handling can
also reduce the risk of infection.
Susceptible individuals can further minimize the
risk of listeriosis by taking special dietary
precautions, which include avoiding high risk
foods.
A variety of culture and rapid methodsbare
available for the detection of Listeria spp. in foods.
Although the presence of L. innocua may indicate
potential contamination with L. monocytogenes,
only the latter species is pathogenic for humans.
Therefore, the most adequate tests are those
which specifically detect L. monocytogenes.
There is not a single marker available to test
the virulence of field isolates for this species.
In the future, the combination of different
molecular markers will probably allow the
screening of food contamination by only the
virulent clones of L. monocytogenes, thus
improving the prevention of foodborne
human listeriosis.
most
commonly
used
culture
The
reference
methods world-wide for the
detection of Listeria in foods are the ISO
11290 -1/ISO 11290 -2 standards (ISO, 1996;
EC, 1999).
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ISO 11290 1/ ISO 11290 - 1 Steps
PREENRICHEMENT
LISTERIA HALF FRASER BROTH BASE
( ISO 11290 1) CAT.1183
SELECTIVE ENRICHEMENT
LISTERIA FRASER BROTH BASE
( ISO 11290 1) CAT.1182
LISTERIA AGAR BASE OXFORD CAT.1133
LISTERIA CHROMOGENIC AGAR BASE ( ISO 11290 1) CAT.1345
SELECTIVE PLATING
GENOTYPE & PHENOTYPE TEST
CONFIRMATION
BIOCHEMICAL PROFILE
Listeria spp. Gram- positive, catalase-positive, oxidase- negative, non-spore forming, short
rhods, motile at 30ºC or less.
HEMOLYSIS
-
+
Rhamnose Fermentation
Mannitol Fermentation
-
-
+
Xylose Fermentation
Xylose Fermentation
Xylose Fermentation
+
+
-
-
+
+
-
Listeria gravi
Listeria
welshimere
Listeria
innocua
Listeria ivanovii
londoniensis
Listeria
ivanovii
Listeria seeligeri
Listeria
monocytogens
BIBLIOGRAPHY
Food-Borne Pathogenic Microorganisms and Natural Toxins Handbook:
“The Bad Bug Book” U.S. FDA/CFSAN. Center for Food Safety and
Applied Nutrition, Food and Drug Administration, College Park ,2003.
Slutsker L, Schuchat A. Listeriosis in humans. “Listeria, listeriosis and
food safety”, 2nd ed. Ryser ET, Marth EH, editors.New York: Marcel
Dekker; 1999. p. 75-95
Antle, J.M., 1995. Choice and Efficiency in Food Safety Policy.The AEI
Press, Washington, DC.
Ottaviani, F., Ottaviani, M. and Agosti, M (1987) Quimper Froid
Symposium Proceedings, P6 A.D.R.I.A Quimper (F) 16-18 June ISO
11290-1:2004 Horizontal method for the detection and enumeration of
Listeria monocytogenes Part 1: Detection Method.
Elliot T. Riser, Elmer H. Marth “Listeria, Listeriosis and Food Safety”
Kathariou S.”Listeria monocytogenesvirulence and pathogenicity,a food
safety perspective.” J Food Prot 2002;65: 1811-29
Rocourt J. “Risk factors for listeriosis.Food Control “.1996; 7: 195-202.
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