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Listeria monocytogenes is a gram positive bacterium that causes many of food borne diseases in humans and animals (Bakker et al, 2010). This rod shaped bacterium was first isolated in 1924 by E.G.D Murray from the blood of laboratory animals. Murray was not able to categorize this pathogen to a bacterial genus at the time. He decided to name this newly discovered bacteria Bacterium monocytogenes. In 1940, researcher Harvey Pirie renamed the bacteria Listeria monocytogenes in honor of the British surgeon, Joseph Lister. This bacteria was not recognized as a pathogen until the listeriosis epidemic in 1949. This epidemic occurred in Germany and mostly effected newborns (Hof, 2003).  

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Listeria monocytogenes has a high incidence rate in meat, poultry, seafood, and dairy products (Oliver et al, 2010). According to the CDC, Listeria outbreaks in the early 1990’s were linked to deli meats and hot dogs. Today, Listeria outbreaks have been traced back to cheeses, celery, sprouts, and cantaloupe. Listeria monocytogenes can lead to mild or severe infections (Kazmierczak et al, 2003). One disease in particular that this bacteria causes is Listeriosis.

It is fairly common for Listeria monocytogenes to make its way into a host. The adult immune system will attack the pathogen and after a few days, Listeriae will be excreted by the body in the feces (Hof, 2003). Rarely does L. monocytogenes cause listeriosis in healthy individuals, but the consumption can lead to mortality in those with compromised immune systems (Oliver et al, 2010). Individuals that are most at risk are the elderly, pregnant women, and infants. Infections can lead to fever, muscle aches, diarrhea, meningitis or encephalitis. This infection can also spread to the nervous system (Disson, 2012). These symptoms can include stiff neck, loss of balance, confusion, and convulsions. Pregnant women have an increase in risk of acquiring listeriosis compared to the normal population. This pathogen can spread from the mother to the fetus through the placenta. Although the mother will only experience mild  symptoms. These symptoms usually present themselves like the flu, although this could be detrimental for the fetus (Hof, 2003). Listeriosis in pregnant women can lead to stillbirth, premature delivery, or neonatal meningitis (Dowd et al, 2011). Listeriosis causes about 10 percent of deaths related to food borne illnesses in the United States each year (Palmer et al. 2011). 

Listeria monocytogenes strains come from different lineages which affects the bacteria’s ability to be transmitted to the host. This bacterium belongs to two distinct lineages: I and II. Lineage I has higher pathogenic potential and is also associated with human cases of listeriosis. Its pathogenic potential is shown by its ability to spread to neighboring cells in the host. Linage II is more common in food isolates.  (Oliver et al, 2010).

Listeria monocytogenes is able to break down and digest decayed organic matter. This process makes Listeria monocytogenes saprotrophic. This bacteria is able to convert from a saprotroph into an intracellular pathogen  (Palmer et al. 2011). This occurs through regulatory factors that allows for the survival of the bacteria in different environments (Kazmierczak et al, 2003). 

Listeria monocytogenes has the ability to adapt to different environmental conditions which allows it the ability to successfully be transferred from food to its host. This bacteria can survive in a wide range of environments. Listeria monocytogenes is able to grow in temperatures that range from 0 to 45°C and in environments of pH ranging from 4.4-9.4 (Oliver et al, 2010). This pathogen has to overcome many barriers in the intestinal system to have an effect on the host. The largest barrier that this organism encounters is the bile released from the liver. L. monocytogenes has the ability to be bile resistant which is essential for its survival and colonization in the human gallbladder. 

Bile is an aqueous solution that is mostly composed of bile acids, cholesterol, phospholipids, and biliverdin. It is synthesized in the hepatocyte of the liver and stored in the gallbladder. When food in consumed, the gallbladder is stimulated to contract and bile is secreted from the gallbladder into the small intestine. Its main function is to act as a detergent to emulsify and solubilizes lipids for fat digestion (Begley et al, 2006). Bile has many properties which allows it to cause damage to DNA, RNA, and macromolecules (Dowd et al, 2011). Listeria monocytogenes is able to be pathogenic due to its ability to resist the damaging properties of bile.