Listeria monocytogenes

Habitat:  
The genus Listeria consists of at least six species, of which L.monocytogenes is the most commonly isolated pathogenic member. It is found worldwide in the environment and has been found in the gut of at least 37 mammalian species as well as at least 17 species of birds and possibly some species of fish and shellfish. It can be isolated from soil, vegetation and water. 1-10% of humans may be intestinal carriers.

Morphology:  
It is short gram-positive, non-acid-fast, noncapsulated, nonsporulating, motile bacilli that may appear coccobacillary and often grows in short chains. The motility is characteristically slow and is described as "heads-over-tails" or tumbling type of motility. It exhibits motility only at 22^oC but not at 37^oC, since the flagella are formed only at lower temperature.

Cultural characteristics: 
It is a psychrophile that grows slowly at 0-8^oC. It is aerobic and grows well in the presence of 5-10% CO₂. It grows on ordinary media at a wide temperature ranging from 4-42^oC. It forms beta hemolytic colonies on blood agar plates. It is unusually heat resistant at the same time thrives well in cold, multiplying in food stored in refrigerator. It is quite hardy and resists the deleterious effects of freezing, drying, and heat remarkably well.

Antigenic structure: 
It is divided into eleven serotypes based on somatic (O) and flagellar (H) antigens. Most human infections are caused by serotype 1b and 4b.

Human infections: 
Listeriosis is the name of the general group of disorders caused by L monocytogenes. Persons at risk includes immunocompromised persons (transplant recipients, HIV-infected, cancer patients), elderly or pregnant, foetus and neonates

Mode of infection:
       a. Ingestion of contaminated dairy products and raw vegetables (food poisoning)
       b. Direct contact, antepartum and intrapartum from mother to child, especially during abortions 
       c. From infected animals to butchers and slaughterhouse workers. 

Pathogenesis:
Route of infection is usually ingestion of contaminate vegetables, milk and meat, although direct inoculation on to skin and conjunctiva and transmission to foetus in-utero or post-partum can occur. It enters intestinal cells via invasins and spreads to adjacent cells by actin-based motility. It is facultative intracellular pathogen, living inside a macrophage. It escapes from the host vacuole (phagosome) by means or Listeriolysin O, which is a �-hemolysin and undergoes rapid division in the cytoplasm of the host cell. Since there is cell-cell transmission, the organisms are never extracellular and not exposed to humoral antibacterial agents such as complement or antibody. Immunity is cell-mediated.

Neonatal Disease can occur in two forms:  
       a. Early onset disease, which is acquired transplacentally (in-utero). In-utero acquired infection causes abscesses and granuloma in multiple organs and very frequently results in abortion. This condition is known as granulomatosus infantiseptica.
       b. Late onset disease, which is acquired at birth or soon after birth. Exposure on vaginal delivery results in the late onset disease resulting in meningitis or meningo-encephalitis with sepsis within 2 to 3 weeks.

Adult Disease:  
Exposure to the organism can lead to asymptomatic miscarriage or disease in humans. Infection in normal adults results in self-resolving flu-like symptoms and/or mild gastrointestinal disturbance. Chills and fever are due to bacteremia. In immunosuppressed individuals it can produce serious illness, leading to meningitis. It is one of the leading causes of bacterial meningitis in patients with cancer and in renal transplant recipients. A complication of the bacteremia is endocarditis. Oculoglandular listeriosis may follow conjunctival inoculation with regional lymph node involvement. Listerial dermatitis may follow direct contact with infected tissues. 

Laboratory diagnosis:
       a. Speciemen collected are blood, CSF, lochia, mechonium, cord blood, vaginal secretions, amniotic fluid, pus and biopsy from skin lesions.
       a. Microscopy: Blood and CSF show monocytosis. Gram positive coccobacillary forms may be seen in CSF.
       a. Culture: Specimen is inoculated onto sheep blood agar and incubated at 37^oC overnight. Colonies appear small, smooth and beta hemolytic. On potassium tellurite agar, it forms black colonies. It may be isolated from contaminated specimens, feces, tissue or animal sources by cold enrichment technique where the specimen is incubated in typtose soy broth at 4^oC and sub-cultured weekly for a month.

Treatment: 
Penicillin (ampicillin) alone or in combination with gentamicin has been effective. Cephalosporins are not effective. Oculoglandular listeriosis and listerial dermatitis can be treated with erythromycin. Co-trimoxazole can be used as an alternative.