Listeria monocytogenes remains to be one of the most leading causes of food-borne
illnesses. It has become a serious problem in food manufacturing plants owing to its food
poisoning capability which threatens the health of food products' consumers, especially in the
United States where food processing occur at a high magnitude compared to other parts of the
world. Stephan and Jemmi (2006) report "listeriosis ranks among the most frequent causes of
death due to food-borne illness. L. monocytogenes infections are responsible for the highest
hospitalization rates (91%) amongst known food-borne pathogens and have been linked to
sporadic episodes and large outbreaks of human illness worldwide" (p. 571). This is attributable
to its high case fatality, and this is probably the principal reason as to why L. monocytogenes is
regarded as one of the most significant food-associated pathogen. This Gram-positive bacterium
causes human listeriosis and contamination with L. monocytogenes has been one of the principal
microbiological causes of processed food recalls, primary in regard to seafood, poultry, meat and
dairy products such as milk and cheese. Research report indicates that the increased
pathogenicity of L. monocytogenes is enhanced by its adaptability to food-processing
environments. For instance, this bacterium can thrive and multiply under refrigeration
conditions. It has also been found to thrive in drains in food processing plants. Moreover, L.
monocytogenes ability to form biofilms enhances its colonization, distribution and adaptation to
a wide range of environmental conditions including adverse temperatures and PH ranges
(Adriana et al. 2008). Therefore, this paper will discuss the detection and identification of L.
monocytogenes, and present comprehensive implementation of Listeria intervention strategies to
control contamination of food products with the food-borne pathogen.
Detection and Identification of L. monocytogenes
Detection and identification of L. monocytogenes involves diverse diagnostic procedures,
some of which are quite complicated, although others are simple to perform in an ordinary
laboratory. In food manufacturing plants as well as diagnostic centers such as Food and Drug
Agency of the U.S which deals with approval of processed food products, detection of L.
monocytogenes is based on morphological features of the pathogen to distinguish it from non-
pathogenic listeria species which flourish freely in the environment. This procedure involves
growing colonies in selective bacterial cultures in which pure cultures are isolated for
identification. In appearance, L. monocytogenes is a Gram positive coccoid rod. When cultured
at temperatures below 30
C, these bacteria are motile, and they are known to be non-spore
forming. Its non-spore forming characteristic helps in distinguishes it from an array of other
gram-positive bacteria such as Salmonella sp, which form spores as one of their adaptive
features, especially under adverse environmental conditions. In regard to colony formation, L.
monocytogenes forms smooth bluish gray colonies which ranges between 0.5 to 1.5 mm in
diameter when grown in bacteriological culture media and it is usually characterized with
hemolytic reactions on blood agar media. Another significant characteristic of L. monocytogenes
is that, it is oxidase negative, catalase positive and it is known to produce acid from rhamnose.
However, it does not produce acid from xylose; thus, this serves as one of the most reliable
feature in the detection of L. monocytogenes. On the other hand, L. monocytogenes is negative
with Rhodococcus equi and CAMP positive with S. aureus. This bacterium grows at an optimal
temperature of between 30
C to 37
C, although it can thrive in temperatures ranging between
C and 45
C. However, it is worth noting that L. monocytogenes grows in both aerobic and
anaerobic conditions, and this serves as one of its adaptive features which enhance its virulence
(McLauchlin & Rees 2009).
Ordinarily, biochemical, phenotypic markers and culture methods are used for the
identification of L. monocytogenes. Despite their routine use in laboratory diagnostics, DNA-
based diagnostic methods are gaining unprecedented popularity in diagnostic laboratories owing
to their sensitivity and reproducibility. It is believed that DNA-based diagnostic methods present
high specificity and rapidity compared to the conventional methods. In practice, routine
laboratory diagnostic procedures rely on the expression of antigenic features whereas DNA-
based methods rely on the genome; thus, enhancing efficient identification. Identification of L.
monocytogenes can also be performed using molecular methods. However, most molecular
diagnostic procedures are highly costly because they require highly trained personnel, in addition
to the involvement of different equipments and reagents.
Ideally, enrichment cultures are required for the isolation of L. monocytogenes because
these bacteria exist in small numbers in environmental and food samples. In most cases, small
numbers of bacteria are found in the presence of large numbers of other bacteria, especially
competitor species. Ordinarily, cold enrichment procedure is used for the isolation of L.
monocytogenes in which the enrichment procedure takes several weeks. This is followed by
selective enrichment and plating to isolate pure cultures. Isolation of L. monocytogenes using
cold enrichment methods serves as the most reliable method because L. monocytogenes has the
ability of growing at low temperatures.
Plating involves the use of selective plates such as LMBA, PALCAM, Oxford and
ALOA. These media contain indicator substrates and selective agents such as antibiotics to
enhance the recognition of L. monocytogenes from other bacteria, especially gram positive
bacteria species such as Salmonella and Clostridium. For instance, ferric ammonium citrate
added to Oxford or PALCAM plates helps in distinguishing Listeria spp from other bacteria. On
the other hand, chromogenic agars such as ALOA enable identification of Listeria spp because
they form colored colonies.
It is believed that enrichment, selective and identification procedures which are used in
the traditional detection methods are laborious and time-consuming. Therefore, several rapid
diagnostic methods have been developed for the detection of L. monocytogenes because they are
easy to perform in the local laboratory setup without complications. For instance, L.
monocytogenes can be detected from enrichment broth, environmental or food samples with
species-specific primers, which have been developed using PCR-based methods. These
diagnostic procedures do not involve isolation stages and their results have been found to be
comparable with the traditional culturing methods.
Currently, antibody-based commercial test kits are used for detecting L. monocytogenes
in food samples and this process takes an approximation of 50 hours, especially when LMO and
Vidas test kits are used (Hagens & Loessner 2007). Therefore, the detection of Listeria cells in
contaminated food or food manufacturing environment is performed using rapid methods rather
than the traditional methods. However, isolation of bacteria colonies remains one of the most
fundamental techniques in the detection and identification of Listeria spp, especially in
epidemiological studies (Hellström 2011).
Implementation of Listeria Intervention Strategies
Implementation of Listeria intervention strategies involves three principal initiatives
which aim at controlling the spread of the organism. The first strategy for preventing
contamination with L. monocytogenes is incorporating Listeria growth inhibitor on ready-to-eat
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- PATRICK KIMUYU (Author), 2017, Detection of Listeria Monocytogenes and Implementation of Listeria Intervention Strategies to Control the Spread of the Organism, Munich, GRIN Verlag, https://www.grin.com/document/381290