Listeria monocytogenes

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Listeria monocytogenes
Listeria monocytogenes.jpg
Scientific classification
Kingdom: Bacteria
Phylum: Firmicutes
Class: Bacilli
Order: Bacillales
Family: Listeriaceae
Genus: Listeria
Species: monocytogenes
Binomial name
Listeria monocytogenes

Description and significance

Listeria monocytogenes is a Gram-positive, rod shaped bacterium that is usually arranged in short chains. When smeared L. monocytogenes may be coccoid and mistaken for Streptococci and larger cells tend to resemble corynebacteria. Flagella are present at room temperature. Are motile due to flagella and can also use obtain motility by using the host cell actin filaments to move within and between cells. [1] The bacterium L. monocytogenesis nearly found everywhere in the environment and can be isolated from wild and domestic animals, birds, insects, soil, wastewater, and vegetation. L. monocytogenes are able to survive in a wide range of varying temperatures from below freezing to body temperature and typically grows best at 0 degrees Fahrenheit to 50 degrees Fahrenheit. In fact this very range of temperature is used for freezing and refrigeration! It is resistant to freezing, drying, and heat. Which is extremely well for a bacterium that does not form spores. [2]

Listeria monocytogenes.

L. monocytogenes is an intracellular pathogen that is the cause of the major food born infection Listeriosis.[3] The disease is transmitted from the consumption of mainly contaminated dairy and meat products. L. monocytogenes is carried in the intestinal tract and 5-10% of the human population who carry no apparent symptoms of the disease. [4] The individuals that are highly susceptible to this disease are pregnant women, the elderly, and individuals with deficient immunity due to illness or medication. [5]

In the United States an estimated 2,500 persons become seriously ill with Listeriosis and 500 of these individuals die. [6]

Sequencing the genome of this deadly bacterium is vital as a basis for improving food safety and is one that is most highly researched.[7] By using comparative analysis and gene arrays researchers are able to study the traits of evolution and new traits.[8]

The bacterium Listeria monocytogenes is named after Joseph Lister who introduced sterilization to the realm of medicine.[9]

Since the 1960s the bacterium L.monocytogenes was thought to only cause infection in animals. It was first discovered ninety years ago in rabbits and pigs. It's lethal effects as a foodborne pathogen wasn't apparent until 1981 after a major outbreak. [10] However, it soon came to light that both animals and humans carry this disease and that it can be isolated from numerous sources in the environment.[11]

Genome structure

Listeria Monocytogenes has two strains of sequences that have been sequenced. Several others are currently being sequenced. The size of these strains is 2.9 Mb and the percentage content of %GC is 37- 39%. [12]. The only two strains that have been sequenced are L. monocytogenes strain EGD-e (serotype 1/2a) and Listeria innocua CLIP 11262 (serotype 6a).[13] The strand EGC-e is 2,944,523 base pairs long with 2,853 open reading frames and a %GC percent content of 39%.[14] One of the currently researched strands is F236 (serotype 4b, cheese isolate) and it is 2,905,187 base pairs long. The percent content of AT is 62.0%. The number of genes present in this strain is 2, 821 genes, 1,030 base pairs per gene.[15] About 50% of the sporadic Listeria infection outbreaks have been determined to be from the strain F236 serotype 4b. [16].

Listeria monocytogenes has a single circular chromosome. The ability for Listeria Monocytogenes to be able to survive in a wide range of environments is made possible by the 331 genes encoding different transport proteins.[17]

In a recent study, the plasmid content of 30 isolates of Listeria monocytogenes was obtained from short-ripened cheeses and further analyzed. “The isolates of L. monocytogenes seagroup harbored a single plasmid, pLM33 (33.2 kbp).” [18]

Cell structure and metabolism

Listeria Monocytogenes is a Gram-positive bacterium. Like several other Gram-positive bacterium many of its surface proteins are harbored to the cell wall by a mechanism requiring a COOH-terminal sorting signal with a conserved LPXTG motif. [19].

Listeria Monocytogenes is a pathogenic bacteria that invades human epithelial cells. It is motile due to its flagellum and can move within eukaryotic cells by explosive polymerization of actin filaments. [20]. "It's flagellum strucutures allows motility and by far improves L. monocytogenes infectivity soon after bacterial ingestion in vivo." [21]

Listeria Monocytogenes is a aerobic, facultative intracellular parasite and is non-spore forming. It is different from other Gram-positive bacteria in that it produces a endotoxiin. It is often difficult to diagnose Listeria Monocytogenes and certain tests are normally run to test it. Such tests must be positive to conclude that the bacterium is Listeria Monocytogenes. These tests are Gram stain, catalase, hydrolysis of sodium hippurate and esculin, motility, alpha-hemolysis and it must test negative for H2S. Listeria Monocytogenes is dependent upon its host to disperse and has little to no motility in a human host. [22]

This bacterium is able to utilize glucose-1-phosphate in a PrfA-dependent way. It can also use glucose-, fructose-, and mannose-6-phosphate efficiently for energy source. The other way it acquires energy is through mediating the import of glucose-6-phosphate and other such sugars from the cytoplasm of the infected cell into the bacterium. Thereon, these sugars can be used as carbon and such energy sources that are important for bacterial survival and for rapid repliction within the host cell. [23]


Listeria Monocytogenes is found nearly everywhere in the environment. This bacterium has been found in at least 37 mammalian species. [24] It is very dependent upon its host for dispersal. This bacterium has also been known to be found in fecal matter and the soil. It is also a food-borne pathogen that affects both animals and humans. The symptoms have been known to range from mild to severe death. This bacterium is sufficient in surviving a ranging wide of various environmental conditions. Such harsh environmental conditions include high salt concentrations, high pH, and high temperature. Another known interesting characterisic of this bacterium is that it can form biofilms which allow it to attach to a solid surface as a community. From thereon they proliferate and their removal is difficult. [25] In all its existence is parasitic since it is known to cause illness to its host in specific cases. It has been found that a strain of this bacterium is found in healthy individuals who experience no harmful effect from its presence.


L. monocytogenes is an intracellular pathogen that is found to cause illness and infection in both humans and animals. “Most strains of L. monocytogenes are found to be pathogenic. The infection Listeriosis is caused by eating food contaminated with the bacterium. The infection typically targets people of advanced age, pregnant women, newborns, and adults with weakened immune systems. [26]

L. monocytogenes secretes an invasin that allows the listeriae to penetrate host cells of the epithelial lining. In a healthy immune system the infection is eliminated. However, this is not the case with individuals with a weak immune system who are by far the most susceptible to the disease. The bacterium is able to multiply intracellularly as well as extracellularly. It is able to multiply within macrophages after phagocytosis or enter within parenchymal cells by induced phagocytosis. An interesting fact about this bacterium is that unlike other bacterial pathogens it is able to infect the fetus by penetrating the endothelial layer of the placenta. [27]

It must be noted that the infective dose of this bacterium is not yet known. Also the degree of the infection comes to vary with the susceptibility of the victim and the particular infecting strain. [28] The disease is detected through isolation of the bacterium from blood, cerebrospinal fluid, placenta and fetus. [29]

Again the symptoms vary with the individual. The onset of this bacterium is noted by influenza like symptoms such as a persistent fever. Other symptoms include fever, muscle aches, and at time gastrointestinal symptoms such as nausea or diarrhea. In some cases where the infection spread to the nervous system symptoms such as headache, stiff neck, confusion, loss of balance may come to occur. When pregnant women are infected they only come to experience a mild, flu-like illness. Greater damage is placed on the newborn. Such an infection can cause miscarriage, stillbirth, premature delivery, or even infection of the newborn. . [30] Outbreaks of the disease are found to be caused by consumption of contaminated milk, soft cheese, and meat products. Unlike the cases of other food-born pathogens which cause gastrointestinal illness, L. monocytogenes is causes serious disease such as meningitis, sepsis, encephalitis, corneal ulcer, pneumonia, and stillbirth. Treatment methods of this disease include intravenous administration of either ampicillin or penicillin.[31]

Application to Biotechnology

Up to this date no known useful compounds and enzymes have been found to be produced by Listeria Monocytogenes.

Current Research

1.) L. Liu, P. O’ Conner, P.D. Cotter, C. Hill and R.P. Ross. Controlling Listeria monocytogenes in Cottage cheese through heterologous production of entercocin A by Lactococcus lactis. Journal of Applied Microbiology. 2007 Jul, 1032.

The purpose of this study was to attempt to reduce the levels of Listeria Monocytogenes production in food. L. monocytogenes is found in a vast number of different foods. However, the number one food source that L. monocytogenes is most isolated from is soft cheeses which have been found to cause the greatest threat. In this study natural antimicrobials (bacteriocins) were used. There is a great demand and advantage for them. For one, they are natural and help to stray away from chemical preservatives. They also contribute to the cheese flavor by producing enzymes as well as controlling the growth levels of L. monocytogenes on cheese. In this particular study, a plasmid Ent02 was introduced into a Lactococcus lactis strain. The Lactococcus strain is capable of fermenting lactose. In combination with this bacteriocin-producing starter was placed a non-enterocin A producer. The results obtained from this study showed a decrease in L. monocytogenes levels. This study also came to demonstrate the vast benefits that enterocins place as a biocontrol agent in food.

2.) N. Dreux, C. Albagnac, R.D. Sleator, C. Hill, F. Carlin, C.E. Morris and C. Nguyen-the Glycine betaine improves Listeria monocytogenes tolerance to desiccation on parsley leaves independent of the osmolyte transporters BetL, Gbu, and OpuC. Journal of Applied Microbiology. 2007 April;

In this study observations were made on the effect of glycine betaine (GB) on the survival of Listeria monocytogenes on leaf structures that were placed under low relative humidity. It was observed that the addition of GB came to vastly improve the survival of L. monocytogenes under low RH on parsley leaves. It was found that exogenous GB had a positive protective effect on L. monocytogenes cells. That this positive effect was not dependent of intracellular GB accumulation.

3.) A.A. Lathrop, P.P. Banada, and A.K. Bhunia. Differential expression of InIB and ActA in Listeria monocytogenes in selective and non-selective enrichment broths. Journal of Applied Microbiology. 2007 May;

This study was aimed at investigating the expression of ActA and InIB proteins in Listeria monocytogenes when they were placed in selective and nonselective media. Selective media is one that carries little nutrient and nonselective media carries the nutrients for the bacteria to grow. It was concluded in this study that selective enrichment broths came to promote ActA while nonselective broths came to promote InIB expression.


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