the study of listeria monocytogenes


Listeria is considered a rare but potentially life-threatening disease. Furthermore, it is an aberrant bacterium since it can grow at low temperatures of below 5. This information is supported by the work of Health Protection Agency (2010). Listeria genus consists of gram positive rods, non-spore forming, facultative anaerobic, regular rod shaped bacteria. Seegler and Jones (1986) agree with these characteristics and add that Listeria genus is non-capsulate in nature. It was further stipulated that they (Listeria) are catalase-positive, oxidase-negative and motile at 20-25 based on the peritrichous flagella but non-motile at 37. This information is supported by the work of Winn and Koneman (2006). Furthermore, it has been stipulated that the genus is defined by a G + C DNA content of 36-38%, a typical gram positive cell wall which has a peptidoglycan murein layer containing meso-diaminopimelic acid (meso-DAP) attached to the cell membrane by lipoteichoic acid, and membrane associated polyribitol teichoic acids. Listeria belongs to the family, Listeriaceae.

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With respect to its pathogenicity, Listeria is mainly restricted to two species. These species are L. monocytogenes and L. ivanovii. This information is supported by the work of Downes and Ito (2001). It was stated further that one of the species (strains), Listeria monocytogenes, is responsible of almost all issues related to human listeriosis and in approximate, 90% of listeriosis cases involving animals, with L. ivanovii seen as the less virulent and of most concern in animals.

The genus, Listeria is believed to be made up of six species. These species are Listeria monocytogenes, Listeria ivanovii, Listeria welshimeri, Listeria innocua, Listeria seeligeri and Listeria grayi. This information is supported by the work of Racourt (1999); Vazquez-Boland et al., (2001). Furthermore, it was stipulated that apart from Listeria monocytogenes and Listeria ivanovii which are known to be pathogenic, the remaining species are considered non-pathogenic.

Listeria monocytogenes is a gram positive facultative intracellular pathogen that has the ability of causing chronic food-borne infections in humans, thus resulting to 20 to 50% mortality in vulnerable populations, such as toddlers, the aged and immuno-compromised persons. This information is derived from the work of Huang et al., (2006). This organism was previously known as Bacterium monocytogenes. This information is derived from the work of Nyfeldt (1929). Fischetti (2006) concurred with the fact that Listeria monocytogenes are gram positive bacteria but added that they are subsequently rod-shaped and non-sporulating in nature with the ability to usurp different mammalian cells in order to thrive in the cytosol of these host cells and to propagate into near-by host cells. The dimension of the non-sporing rods is about 0.5m in diameter and 0.5-2.0m in length. This information was derived from the work of Seeliger and Jones (1986). According to Fang et al., (2004) it is claimed that Listeria monocytogenes are normally found in food products such as fresh meat and poultry, processed ready-to-eat meats, sea food, soft-style cheeses and butter. Furthermore it is stipulated that their existence is as a result of their ability to thrive well and grow at circumstances related to normal processing and distribution of food, such as refrigeration temperatures, enormous sodium chloride concentrations or water activities as low as 0.92. Apart from being able to develop at refrigeration temperature, L. monocytogenes has the capacity to withstand heat and salt effectively. Apart from foods, Listeria monocytogenes were first isolated from tissue of patients in Germany in 1891. Subsequently other hosts of Listeria monocytogenes have been liver of rabbit and spinal fluid of meningitis patients sometime ago thought to be in the year 1911 and 1917 respectively. This information was sourced from the work of Reed (1958); McCarthy (1990) as cited in World Health Organization (2004). According to Murray et al., (1926), Listeria monocytogenes was not only isolated from rabbits but also guinea pigs of which the organism (L. monocytogenes) was observed to cause monocytosis in the infected animals.

According to Schuchat et al., (1991) as cited in Goldfine and Shen (2007), Listeria monocytogenes are relevant causes of zoonoses, especially in herb animals. It was further stipulated that apart from these microbes, Listeria monocytogenes, being found in food products, they can subsequently be found in soils. These microbes can also be found in decaying vegetation.

World Health Organization (2004) claims that Listeria monocytogenes is an opportunistic pathogen which mainly affects those with a chronic underlying infirmity or condition (e.g. immunosuppression, HIV/AIDS, chronic conditions such as cirrhosis that defect the immune system); pregnant women; unborn or newly born delivered infants; and the elderly. Unlike Listeria innocua, which are seen as the non-pathogenic specie of the genus, Listeria, they are utilised as surrogate micro organisms in observing and studying the growth and survival characteristics of Listeria monocytogenes. This information is supported by the work of Glaser et al., (2001) as cited in World Health Organization (2004).

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Listeria monocytogenes is known to contain a cell wall formed by a multi-layered cross linked peptidoglycan decorated with teichoic and lipoteichoic acids. The cell wall of Listeria monocytogenes has an enormous responsibility, physiology wise, in ensuring integrity of the cell while maintaining a high internal osmotic pressure. Furthermore, the cell wall is known to give the cell a precise cell shape and provide protection against mechanical stress. The cell wall equally acts as a scaffold which allows different proteins to articulate to it. This information is supported by the work of Goldfine and Shen (2007).

According to Fischetti (2006), based on the pathogenicity of Listeria monocytogenes, they are known to infect domestic animals and humans by gaining entry into the intestine; hence spreading rapidly to deeper organs which could lead to infrequent, but seldom deadly infirmity. According to Downes and Ito (2001), the sequence in which Listeria monocytogenes causes disease can be categorised into three phases. The first phase involves the ingestion of organisms; while the second stage involves the infusion of the gastrointestinal lining, this is followed by replication after undergoing phagocytosis through macrophages. The third phase involves the lysis of the macrophage, which advances to the septicaemia and usurping of various target organs, including the placenta and central nervous system (CNS). However, for L. monocytogenes to be able to cause illness, it must thrive in the stomach which has an acidic encompassment. Apart from surviving the acidity of the stomach, it also should have the ability to penetrate the intestinal lining and grow in the host.

Chilled foods are termed as ready-to-eat foods which contain raw or uncooked condiments. These condiments are the likes of cheese or salad. This information is supported by the work of Stringer and Dennis (2000). Furthermore, their mode of preparation is such that the food is either apparent to ingest or requires partial cooking rather than full cooking before usage. In most cases these particular food types have the tendency to be hazardous and lethal since it requires partial cooking bearing in mind that Listeria monocytogenes which is the causative agent can resist cold but heat effectively. On the other hand, ready-to-eat foods can equally be referred to as cooked foods meaning that the whole food condiments are cooked which should lack the presence of Listeria monocytogenes as a result of their (L. monocytogenes) non-heat resistant status.

With respect to the examples of chilled food, they are as follow:

Butters and Spread



Jacket potatoes


Sausage rolls



This information was derived from the work of 'Rew Valley Dairies' (2010).

Different storage temperatures are required in storing ready-to-eat chilled foods. In tabular form, different storage temperatures at which most influential European countries adopt are as follows:


Chilled Storage


Max 7




Meat based products: 6; other chilled products: 8


Storage at 4


Meat products: -1to 7; fish products: 0 to 4





The Netherlands

Max 7

United Kingdom

Max 8

This table was sourced from Stringer and Dennis (2000).

Sandwich is regarded as a simple and uncomplicated food that in time immemorial was discovered first in the Middle East, around 9000 BC. Its discovery was about the time civilization kicked into existence. This information is derived from the work of Mercuri (). It was further stipulated that the discovery period is still under debate as the English claim of discovering sandwich.

Sandwich consists of one or more slices of bread with delicious and nutritious filling between them. With respect to the bread-type, bread, cream or loaf bread, rolls and buns will make a good sandwich. This information was derived from the work of 'Hub pages' (2010). It was further stipulated that the filling may be slices of cold meat, chopped meat, eggs, chicken, ham and cheese with creamed butter, pickles, tomato catsup or mayonnaise. Furthermore, sandwiches are divided into six types. These are:

Open Sandwiches: Here, this sandwich-type consists of one bread-kind with filling on top. The shape of the bread in question is usually square, triangular or round. This sandwich type is usually garnished with slices of carrots, raisins, pickles, as the main fillings are butter, pieces of cheese and meat fillings.

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Plain Sandwiches: A plain sandwich is made up of two slices of bread, mainly a day-old bread, of which butter or mayonnaise is used as spread to not just to add flavour on the sandwich but also to make the sandwich more nutritive.

Pinwheels Sandwiches: These sandwich types are made of cream bread sliced length wise, about 3/8 inch thick. Fresh cream breads are preferred to non-fresh cream breads since they are easy to roll and will not crack. The spreads are mainly softened butter or margarine, creamed cheese, marmalade, cheese pimiento, peanut butter, jams and jellies.

Closed Tea Sandwiches: This type of sandwich requires tremendous amount of refrigeration. They are normally removed from the freezer three hours before they are served. The shape of the sandwich is could be square, rectangular or oblong. Here, butter is spread on one side of the bread and the fillings are placed on the other half. This sandwich type is normally placed in waxed paper or in a clean damp towel and placed in a vacuum before frozen.

Ribbon Sandwiches: This sandwich type is made up of coloured cream bread. Here 3 slice each of pink and green bread with one or more feelings spread between slices. The stack of slices is compressed together and the crusts are trimmed.

Mosaic Sandwiches: The initial step is synonymous to that of the ribbon sandwiches but here the 3 ribbon sandwiches are stacked so that green and pink sections alternate themselves entirely.

In regards to the legal requirements of food samples containing Listeria, the microbe is categorised safe if a certain amount of it is present in a food sample. Depending on the amount of food (in grams), the colony forming units of Listeria differ. In other words, for every gram, there is an acceptable level of colony forming units. For example, numerous levels of Listeria monocytogenes ( cfu per gram) show a failure with food handling controls and with regards to present epidemiological evidence are considered a public health risk. This information is derived from the work of ICMSF (2001). It was further stipulated that the presence of Listeria on food is as a result of inadequacy in the cooking technique adopted or as a result of abysmal post preparation practises adopted. According to Toldra (), the aim of the commission regulation is to alleviate Listeria amount cases in humans. Not only Listeria is aimed at but also Enterobacter sakazakii and Salmonella. Furthermore, it was stated that the key component of the Regulation is to set two different types of criteria for food materials which need to be documented by food business operator. These criteria include:

A food safety criterion defining safety of a product or a batch applicable to products placed on the market.

A process hygiene criterion showing the correct functioning of the manufacturing process.

The microbiological Criteria Regulations are legal regulations that set specific rules for the testing of certain foods for relevant disease organisms. This information is derived from the work of alcontrol laboratories (2010). Tompkin (2002), microbiological criterion is defined as the acceptability of a product or a food lot, based on the absence or presence or amount of micro organisms, including parasites, and/or quantity of their toxins/metabolites, per unit(s) of mass, volume, area, or lot.


Microbiological criteria can be categorized into three even though Codex recognizes only one category.

Microbiological Standard: This is seen as a compulsory criterion that is added into a law or ordinance.

Microbiological Guideline: This is seen as an advisory criterion utilized to notify food operators and others of the microbial content that can be expected in a food when best practices are applied.

Microbiological Specification: This criterion may be mandatory or advisory depending on use. It is seen as part of a purchasing concord between a buyer and a supplier of food.

According to National Research Council (), the microbiological criteria were detailed. These include:

For Standard, a microbiological standard is a microbiological criterion that is a part of law, ordinance, or administrative regulation. Since a standard is a compulsory criterion, failure to comply with it results to law, ordinance, or regulation disobedience and this will be subject to the enforcement policy of the regulatory agency having jurisdiction.

For a microbiological guideline, this criterion is often utilized by the food industry or a regulatory agency to supervise a manufacturing process. Guidelines operate as alert mechanisms which gives a precursor as to whether microbiological condition succeeding at critical control points or in the finished product are within the normal range. Subsequently, microbiological guidelines are utilized to assess processing efficiency at critical control points and conformity with GMP (Good Manufacturing Practices). Also, microbiological guideline is an advisory criterion in that a given lot of food exceeding the limit for a non pathogenic organism would not be taken off the market or even downsized. Guidelines sometimes could be compulsory just like standard criterion in the sense that food company management and regulatory agencies may demand that the conditions responsible for continuous microbiological deficiencies be corrected without procrastination.




















1 LISTERIA- FOCUS L.monocytogenes

What it is

Illnesses(Including epidemiological data

Food linked with L. monoctytogenes(

2 Legal requirements

-Micro criteria regulstions

Responsibility of manufacturer to monitor

Ph / water activity limit to l.mono growth

3 sandwiches

Components- tomatoes- ph


Water activity

Temperature- chilled

Short shelf life

Aims and objectives