While researching different organisms to write on I was immediately drawn and intrigued by Listeria monocytogenes after a close encounter I experienced with it this summer, from ingesting contaminated chicken. This peaked my interest as I wanted to expand my knowledge on this bacterium and further investigate signs, symptoms and the different affects it has on the human body.
L. monocytogenes’ is gram positive bacillus, facultatively anaerobic, motile, bacteria and incapable of the production of endospores3. L. monocytogenes is usually found in soil or water, but can also be traced back to animals, such as poultry, cattle, and fish7. It can be transmitted fecal-orally to humans, by ingesting raw, or unpasteurized contaminated food2. Although, it can also be transmitted to the fetus in a pregnant woman through the placenta2. It is a significant zoonotic pathogen that contaminates a wide variety of food6. These foods can consist of uncooked vegetables and meats, coleslaw, unpasteurized dairy products, cold cuts, turkey wieners, and even washed, prepackaged salads7. L. monocytogenes has a wide temperature (0.5-45 degrees Celsius), pH (4.7-9.2), and osmotic range8. This allows the bacteria to survive at refrigerator like temperatures, meaning that although your food is in the refrigerator it is not guaranteed safe, as it is susceptible to grow in a broad span of environmental conditions3. There are seventeen different Listeria species, but only two, one being L. monocytogenes, is pathogenic when regarding human beings8.
It is the third leading cause of food poisoning in the world when talking about foodborne sicknesses9. Annually 250 people die a year worldwide from this, and it has a 20-30% mortality rate in humans, if the infections are CNS infections in the host4. The Incubation period lasts from 3-70 days, meaning it can persist in the body for months without being noticed7. This long duration makes it a difficult task to determine what was ingested that contributed to the illness7. Individuals that have Listeria monocytogenes bacteria are said to have the disease listerioisis5.
This disease only affects a small range of people, so the average human is not a susceptible host and will rarely become ill, although the possibility can still occur4. People who are most prone to becoming infected are pregnant women, neonates, adults above the age of sixty-five and immunocompromised patients7. Some specifics may be those with cancer, kidney or liver failure, diabetes, alcoholism, HIV/AIDS, and those who are currently treated with radiation or chemotherapy7. Resistance to infection depends primarily on the activation within macrophages of T-cell lymphokines, as they are protein mediators in charge of directing the immune response2. It involves both adaptive and innate immune responses, with adaptive being primarily cell mediated2.
Women who are pregnant are ten times more susceptible to infection than the average person7. Complications of the pregnancy can arise as L. monocytogenes has the ability to cross the placenta barrier towards the neonate, as well as invading the fetus or newborns blood-cerebral spinal barrier, ultimately leading to central nervous system infections9. These complications may cause premature delivery, maternal intrauterine infections, stillbirth, abortion, miscarriage and other health problems that can physically and mentally affect the baby and the mother7. Central nervous system infections are significant as they can lead to death of the baby or the mother3.
In the digestive system the gastric fluids and phagosomes in an infective individual can be controlled through the wide pH range in the stomach that characterizes Listeria monocytogenes9. If a person is immunocompromised the bacteria causing listeriosis is able to enter the bloodstream by invading the epithelial barrier, which can lead to bacteremia, arthritis, gastroenteritis, hepatitis, peritonitis and severe systemic infections such as septicemia, and meningitis9.
Symptoms that may arise as a precursor for diagnosing Listeria monocytogenes are fever, confusion, weakness, vomiting, diarrhea, and nuchal rigidity which is the inability to flex the neck7. Treatments vary on the severity of the different signs and symptoms addressed7. Most commonly the treatment used involves antibiotic therapy through the use of ampicillin (2 g through IV every 4 hours), as long as the patient has no allergies to this 2 . Tetracycline, erythromycin, penicillin, choramphenol, gentamicin, or trimethoprim with sulfamethoxazole are other drugs that can be used if an allergy to ampicillin is present5 . They work by destroying or slowing down the growth of L. monocytogenes within the macrophage’s cytoplasm, making sure enzymes such as listeriolysin O do not allow for multiplication of the bacteria5. However, some people may become resistant to certain antibiotics making it most effective to use the antibiotics in combination with one another in case resistance occurs5. Those with mild signs and symptoms require little to no treatment7. An individual with gastroenteritis is only treated routinely if pregnant, and then oral amoxicillin or trimethoprim/sulfamethoxazole is the recommended treatment7. For severe infections I.V. Ampicillin is the most effective form of treatment7.
Three known mechanisms are used to aid L. monocytogenes invasion of the central nervous system9. The first one involves bloodborn bacteria’s direct intrudment of endothelial cells within the blood- brain barrier2. Secondly, it uses a phagocyte facilitated mechanism to move bacteria towards the central nervous system in leukocytes that are circulating around 2. The last known mechanism invades cranial nerves through a neural pathway, when abrasive food is chewed, which allows the fusion of bacteria into oral tissues causing tissue phagocytosis of the bacteria within the macrophage2.
Listeria monocytogenes has the ability to invade and survive in both nonphagocytic and phagocytic cells, including epithelial cells, hepatocytes and endothelial cells6. L. monoctyogenes attaches to mucosal surfaces by the help of protein, LPXTG2. The bacterium itself uses its own protein, internalin presented on its surface to interact with a receptor E-cadherin, found on macrophages and epithelial cells to initiate phagocytosis2. Listeriolysin O, is a major virulence factor, protein and toxin in L. monocytogenes that produces pores, that causes the inactivation of T-cell receptors2. This protein, allows for Listeria to break free of the macrophage’s vacuole and avoid being phagocytized2. The bacteria uses Listerolysin O to multiply within the cytoplasm and uses actin polymerization, that allow for the reorganizization of the cytoskeleton forming a “tail” to power itself towards the cell membrane3. Cells adjacent to the host cell, consisting of macrophages, hepatocytes and enterocytes ingest filopods that have been formed from the bacteria adhearing to the host cell membrane2. Act A establishes the spread from cell to cell, making it another virulence factor 2. Through these steps L. monotcytogenes is able to spread from cell to cell, without coming in contact with antibodies, complement proteins, and neutrophils, that would attack the bacterium and kill it2. This presents the cell with the ability to have intracellular growth, increasing the bacterium’s pathogenicity, which will help it to infect the immune system3 . The bacteria within the infected macrophages uses a set of proteins such as, phosphatidylinositol- specific phospholipase, phosphatidylcholine- phospholipase C, and an actin- assembly inducing protein to aid listeriolysin O and contribute to the virulence factors by attaching to the host cell membrane to spread and divide rapidly within the cell’s cytoplasm6
Currently, there are no vaccines administered that can prevent listeriosis, but there are things to do to help prevent it and other foodborne illnesses7. Firstly, make sure to completely cook raw meat, and seafood at a safe temperature to kill all dangerous organisms internally7. Store food that need a refrigerator or freezer at proper temperatures7. Wash raw vegetables before ingestion, and eat only pasteurized dairy products7. Heat hot dogs and cold cuts before eating and most importantly throw out food if it is expired or past it’s best before date7.