Listeria monocytogenes is a species of bacteria belonging to the Firmicutes division. The germ belongs to the genus Listeria. The genus name Listeria was named after the English surgeon Joseph Lister. The species name Monocytogenes was chosen because of monocytosis, which is often caused by Listeria monocytogenes.
What is Listeria monocytogenes?
The bacterium has a rod-like shape and is motile (motile) because of the flagella it forms. It has an approximate diameter of 0.4 to 0.5 micrometers and is 0.5 to 0.2 micrometers long. The flagella or flagella are present in a polar or peritrichous fashion, i.e., they can occur at one or both ends or be scattered throughout the cell. The bacterium shows positive Gram staining and is not a spore-forming organism. There is exceptional resistance to adverse environmental conditions. The bacterium can survive extended dry periods as well as high temperatures unscathed. Elevated salt concentrations and intense cold also pose no threat to the germ. A high PH value <4.4 prevents the pathogen from colonizing. PH values in the range of 4.4, to 9.8, i.e. in both acidic and basic environments, are suitable for Listeria monocytogenes to multiply. Temperatures of 30 to 37 degrees Celsius lend themselves to rapid development of the germ, but even normal refrigerator temperatures of up to 4 degrees Celsius can only halt the growth of the pathogen to a limited extent. However, very high temperatures will safely kill the bacterium. Pasteurization and sterilization as well as conventional frying and cooking processes can thus render the germ harmless. Colony morphology shows a similarity with Streptococcus agalactiae. The large, round, and blue-gray colonies of the two germ species can be easily confused on agar. Mild ß-hemolysis is also present on Columbia blood agar in both bacterial species.
Occurrence, distribution, and characteristics
The germ is facultative anaerobic and ubiquitously present. It is not restricted to specific host organisms or specific habitats. Listeria monocytogenes has been found in 37 mammalian species and 17 avian species. The germ can even be detected in marine organisms such as fish and shellfish. Infections are estimated to be 1-10% in humans. The high virulence of Listeria monocytogenes is mainly due to the sophisticated defense strategies of the pathogen. The toxin Listeriolysin 0 (LL0) enables the pathogen to escape from phagocytosis and pass unhindered through all blood barriers of the body with the help of the surrounding phagocytes. Furthermore, the pathogen can also pass through cell walls unnoticed without having to expose itself to extracellular defenses. However, despite their facultative intracellular parasitism, Listeria are not dependent on host organisms and can also survive in soil, water and on various plants. With the ability to form a biofilm on numerous different surfaces, Listeria monocytogenes is a true survivor and can be detected in a wide variety of terrains.
Diseases and ailments
Listeria monocytogenes is considered a facultative pathogen that can cause a variety of diseases. The diseases are grouped together as listeriosis and can occur in humans and animals. Typical routes of infection are contaminated food and animal foods with non-existent or poorly performed sterilization or pasteurization. However, Listeria can also be transmitted by skin contact from human to human, human to animal, etc. A typical Listeria infection goes unnoticed and without clear symptoms. The infection can lead to an acute illness due to other favoring factors such as immunosuppression. Thus, other viral, bacterial, and parasitic infections may contribute to listeriosis. This process has an incidence of 2 to 15 cases per million people per year and is therefore extremely rare. The clinical manifestation is initially manifested by influenza-like symptoms such as fever as well as nausea, vomiting, and diarrhea. The course is rather inconspicuous in immunocompetent people and symptoms in the gastrointestinal tract remain the only complaints. Complications can occur in immunocompromised individuals. Inflammation, swelling of the lymph nodes, encephalitis and meningitis can occur.Especially inflammations of the eye (keratitis, uveitis), throat, pharynx, urinary bladder and the renal pelvis have been observed. Severe cases of encephalitis and meningitis have been recorded especially in the elderly. As a result, there is a mortality rate of about 70%. The disease can also have severe consequences in pregnant women. The infection of the reproductive organs can lead to miscarriages and stillbirths. Newborns affected by listeriosis have a high mortality rate. After a successful cure, developmental disorders are frequently observed. Because of the severe symptoms that Listeria monocytogenes can cause in immunocompromised individuals and pregnant women, detection of the pathogen is reportable. Various antibiotics are administered to treat listeriosis. Particularly recommended is the β-lactam antibiotic ampicillin, which in severe cases can be
combined with aminoglycoside. Cotrimoxazole may be considered as an alternative in cases of intolerance. Natural resistance of the pathogen is present especially against cephalosporins. Appropriate hygiene measures, especially in the processing and preparation of food, are always appropriate for pure prophylaxis. Thus, even proper frying and cooking can destroy numerous pathogenic strains such as Listeria monocytogenes. Delicacies such as bloody steak should only be prepared if accurate information about the animal’s origin and good health is available. However, since healthy animals without obvious symptoms can also be carriers of Listeria monocytogenes, avoidance of bloody and undercooked delicacies seems reasonable.
