Listeria monocytogenes - Microbial Food Poisoning

Listeria monocytogenes

Listeria monocytogenes is a pathogenic bacterium that isfood-borne, and causes an illness called listeriosis. This micro-organism, which is a facultative intracellular bacterium causes illness primarily in pregnant women, neonates, the elderly, and immuno-compromised individuals. Acidity in the stomach decreases the chance of survival of Listeria. Antacids and cimetidine may increase the risk of acquiring this infection in hospitalised patients.

Listeria monocytogenes is a gram-positive, non-acid fast,non-sporulating rod facultative anaerobe, and is indistinguish-able from diphtheroids. It can grow at temperatures from 1 to 45°C, and thrive on foods at refrigeration temperatures. There are more than 16 serotypes of L. monocytogenes, but only 3 are generally responsible for more than 90% of human infections. These include 4b, 1/2b, 1/2a.

Listeria is commonly found in the female genital tract without causing harm. It is much less common as a male genital contaminant. Thus, it is possible that females have slight immu-nity against more serious infection.

Source

·      Unpasteurised milk,* and milk products (ice cream, butter, soft cheese, e.g. Camembert cheese).

·      Uncooked meat, especially chicken.

·      Raw vegetables (lettuce).

·      Coleslaw, which is a salad of finely sliced or chopped raw cabbage, usually moistened with a mayonnaise dressing. Listeria monocytogenes is also found in soil, dust, sewage, water, animal feed, and decaying organic matter. It is speculated that listeriosis is a possible occupational disease of slaughter-house workers.

Incubation Period

·              Few days to three weeks.

Clinical Features

Listeria monocytogenes spreads through the body by incor-poration into lymphocytes and monocytes, and can cross the blood-brain barrier to produce meningitis. Transplacental transfer is also believed to occur. Successful infection is linked to replication in the cells which phagocytise the bacteria, despite the presence of lysosomal enzymes. The phagocyte is eventually damaged.

·     Perinatal infection can manifest as foetal meningitis or intrauterine death. Especially during the third trimester, listeriosis can cause mild illness in the pregnant female, with malaise, chills, fever, and back pain. However, the foetus may suffer severe uterine infection, abortion, prema-ture delivery, or stillbirth.

·     In healthy adults and children, the illness usually takes the form of a self-limiting, mild flu-like syndrome with GI disturbances and myalgia.

·              Immuno-compromised individuals may suffer from meningitis and sepsis (with vomiting, headache and fever).

However, the usual signs of a positive Kernig’s sign and meningismus seen with meningitis are not generally present in these patients. Endocarditis and pericarditis may rarely occur.

Overall mortality in Listeria monocytogenes infections averages 30 to 50% with ranges from 13 to 83%, depending on susceptibility. The case fatality rate for newborn infants of infected mothers is 30 to 50%, depending on the onset of illness in the neonatal period.

Diagnosis

·      Listeria being rod-shaped bacteria, can be mistaken as a contaminant due to the resemblance to diphtheroids. A CSF gram-stain frequently fails to demonstrate the causative organism. Growth in culture is improved when incubated with reduced oxygen and 5 to 10% carbon dioxide.

·              Detection of antibodies against listeriolysin O (LLO), an extracellular haemolysin, can help in the diagnosis of listeri-osis, when bacteria cannot be isolated.

Treatment

■■  Control vomiting and diarrhoea, provide fluids for dehy-dration, and undertake measures for prevention of shock.

■■  Intravenous administration of antibiotics such as ampi-cillin, penicillin G, gentamicin, erythromycin, tetracycline, doxycycline, tobramycin, cotrimoxazole, or vancomycin is usually necessary for serious listeriosis.

■■  Chloramphenicol has been successfully used in some meningitis patients who did not respond to other antibiotics.