Listeria monocytogenes is a Gram-positive, catalase-positive, non-spore forming, aerobic or anaerobic intracellular rod bacterium in the genus Listeria and family Listeriaceae. L. monocytogenes can also exist as coccobacilli or short chains (resembling Streptococcus and Corynebacteria species) and as saprophytes in the environment. Its motility varies with temperature. L. monocytogenes is motile at 18-20oC with a tumbling motility in liquid media but at 37oC, the bacterium is weakly motile or non-motile. L. monocytogenes is the main causative agent of listeriosis, a food borne illness (gastroenteritis) in humans especially the immunocompromised, the elderly, transplant patients, infants, pregnant women and those taking immune suppressive drugs.
Listeria monocytogenes is a Gram positive bacterium; and thus the organsim appears purple under the microscope after Gram staining.
It causes neonatal sepsis and meningitis in neonates and newborns; and it is also implicated in causing sepsis, meningoencephalitis and bacteraemia in the elderly, pregnant mothers and the immunocompromised who are the most people at risk of an infection with L. monocytogenes. L. monocytogenes is a commonly encountered bacterial pathogen in food products especially vegetables, meat and milk products which they ubiquitously inhabit. The bacterium is a psychrotolerant organism i.e. it can grow at very low temperature such as the refrigerator temperature. Growth at low temperatures allows L. monocytogenes to thrive in refrigerated stored-food products via which likely human infection may occur after consumption.
PATHOGENESIS OF LISTERIA MONOCYTOGENES INFECTION
L. monocytogenes is an environmentally ubiquitous bacterium that commonly contaminates food products including those stored in the refrigerator (e.g. cheese, milk, vegetables and meat). The main route via which the pathogen enters the body is via the consumption of food contaminated with L. monocytogenes. After ingestion, the pathogen moves to the gastrointestinal tract where it adheres and is taken up by macrophages and other non-phagocytic cells (through endocytosis). The attachment and ingestion of L. monocytogenes by phagocytic cells and/or the epithelial cells of the gastrointestinal tract is facilitated by internalins (cell wall surface proteins) produced by the pathogen.
L. monocytogenes is an intracellular bacterium, and it produces listeriolysin O (a membrane liquefying and pore-forming enzyme) within the phagolysosome where it is enclosed. Listeriolysin O (LLO) creates holes on the phagolysosome and promotes rupture, and thus releasing L. monocytogenes into the cytosol of the cytoplasm where further multiplication occurs. Escape of L. monocytogenes from the phagolysosome which is mediated by listeriolysin O prevents damage of the bacterium within the phagosome. This phenomenon allows the bacterium to spread to other nearby cells and even from one epithelial cell to another.
The ability of L. monocytogenes to multiply at low temperatures coupled with the virulence factors it produces (e.g. siderophores, listeriolysin O, phospholipase enzyme and internalins) increases the virulence and/or pathogenicity of the bacterium in human hosts. Complications with L. monocytogenes infection in humans usually occur in pregnant women, and this can lead to stillbirth, premature birth or abortion due to intrauterine infection. L. monocytogenes crosses the placenta and the blood-brain-barrier (BBB), thus causing bacteraemia, sepsis, meningitis and other disseminated infection in neonates or newborns. L. monocytogenes can also be acquired by newborns from the vagina (birth canal) of an infected mother who has a disseminated infection; and manifestation or clinical episodes of the disease usually occur at a later time in the affected infants.
LABORATORY DIAGNOSIS OF LISTERIA MONOCYTOGENES INFECTION
The laboratory diagnosis of L. monocytogenes requires the identification of the pathogen by culture and microscopy. Blood, cerebrospinal fluid (CSF) and focal lesions are the primary specimens obtained from infected patients for analysis. Mueller-Hinton agar and blood agar (BA) are preferably used for the cultivation of L. monocytogenes in the laboratory. L monocytogenes produces small, grayish, haemolytic and translucent colonies on BA. Culture plates are incubated for 48 h at temperature range of 3-40oC. Selective media (e.g. Listeria oxford agar base) also exist for the isolation of L. monocytogenes from clinical and environmental samples.
Growth of L. monocytogenes on chromogenic agar.
L. monocytogenes also grow on Oxoid Brilliance Listeria agar (OBLA) plate causing haemolysis. L. monocytogenes forms beta-haemolysis on blood agar, and its rods resemble corynebacteria. The isolation of L. monocytogenes is enhanced if the specimen is kept for some number of days in the refrigerator (at 4oC) before culturing. L. monocytogenes is Gram-positive, catalase positive, and motile; and its colonies forms beta-haemolysis on blood agar. The pathogen ferments carbohydrates (e.g. glucose and maltose) to produce acid but not gas. Polymerase chain reaction (PCR) and ribotyping are the two molecular techniques employed in the detection of the parasite; and the molecular subtypes of the organism can be detected by pulse-field gel electrophoresis (PFGE).
TREATMENT OF LISTERIA MONOCYTOGENES INFECTION
Sulphamethoxazole-trimethoprim, penicillin G, erythromycin, fluoroquinolones, vancomycin and ampicillin are the drugs of choice for the treatment of listeriosis. Early treatment of infected individuals (including the elderly, pregnant women and the immunocompromised) with any of these agents or a combination of them (according to the guidelines of a physician) is effective in aborting complications due to L. monocytogenes infection. No vaccine currently exists for listeriosis.
PREVENTION AND CONTROL OF LISTERIA MONOCYTOGENES INFECTION
Incidence of L. monocytogenes infections amongst humans could be reduced by taking precautions when consuming some foods. Some type of food meant for human consumption (e.g. salads, sandwiches, vegetables, pork meat, cheeses, milk, and other dairy and fermented food products) can become infected by L. monocytogenes at anytime during food processing and food cultivation. L. monocytogenes is a psychrotolerant bacterial pathogen, and thus refrigeration (i.e. cold temperature) which normally inhibits the growth of most bacterial pathogens is ineffective in limiting its growth.
Proper care should be taken in the handling and processing of food in order to reduce the rate of contamination. As a way of prevention and control of the disease, immunocompromised individuals, the elderly and pregnant mothers should be cautious and avoid unpasteurized dairy and ready-to-eat meats and foods as much as possible since these food sources are the main route via which the pathogen gets transmitted to humans. Food processing companies and food vendors should endeavour to always abide and observe the hygienic principles associated with their businesses so that the rate of transmission of L. monocytogenes could be reduced.
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