Detection of Listeria Monocytogenes and Implementation of Listeria Intervention Strategies to Control the Spread of the Organism

Travail de Recherche , 2017 , 8 Pages , Note: 1

Médecine - Épidémiologie

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”. 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.
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.

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Introduction

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.

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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

0

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

0

C to 37

0

C, although it can thrive in temperatures ranging between

0

0

C and 45

o

C. However, it is worth noting that L. monocytogenes grows in both aerobic and

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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

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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

• Université: Egerton University
• Note: 1
• Auteur: PATRICK KIMUYU (Auteur)
• Année de publication: 2017
• Pages: 8
• N° de catalogue: V381290
• ISBN (ebook): 9783668618039
• ISBN (Livre): 9783668618046
• Langue: anglais
• mots-clé: detection, listeria, monocytogenes, implementation, intervention, strategies, control, spread, organism