L. monocytogenes principally causes meningitis, septicaemia, intra-uterine infection and Gastroenteritis.
Meningitis and septicaemia are mainly occur in newborns, pregnant women, elderly and immunocompromised persons.
Listeriosis in pregnancy can be lead to abortion and stillbirth.
The incubation period differs between individuals.
It varies from 1 to 90 days. Intra-uterine infections occur around 30 days.
Organisms mainly colonize the female genital tract and gastrointestinal tract.
L. monocytogenes also found in nature.
They mainly found in animals, plants, and soil.
The main sources of Listeria infection are contaminated meats, chicken, soft cheeses and vegetables.
Outbreaks of sepsis in newborns and infections related to the gastrointestinal tract mainly occurred due to the ingestion of unpasteurized milk products.
Transmission occurs across the placenta and during delivery.
The infection rate of most western countries report as 1-10 cases per million of population per year.
About 10% of cases are account for pregnancy and neonatal disease.
Among them 15-25% of cases lead to abortion and stillbirth, 70% cases are neonatal infections and 5% of cases are infections related to the mother but foetus is not affected.Â
Pathogenesis
Listeriolysin produced by L. monocytogenes is an exotoxin.
It degrades cell membranes. Reduced cell mediated immunity and immaturity of immune system in newborns lead to disease.
There are two types of infections by this organisms, as early infection and late infection.
The characteristics are differ in early infection and late infection.
In early infection onset occurs less than 2 days after delivery.
In late infection onset of disease occurs after 5 days. For the early infection maternal factors are the common reason and for late infection maternal factors influence rarely.
The source of early infection is intra uterine infection acquired from mother.
Late infection may be hospital acquired from early onset case or acquired from mother during delivery.
In early neonatal infection signs and symptoms are disseminated infection, cardiopulmonary distress, infections in central nervous system like diarrhea and vomiting, skin rashes, splenomegaly, and hepatomegaly.
In late neonatal infection meningitis, poor appetite, fever and irritability are the signs and symptoms.
In early infection mortality rate is 30-60% and in late infection it is about 10-12%.
Laboratory Findings
L. monocytogenes is an aerobic or facultative anaerobic, non-spore forming, non-capsulated bacteria.
In gram stained slides it can be seen as gram positive small rod or coccobacillus.
It has an ability to grow in refrigeration temperatures.
It is one of unusual characteristic of L. monocytogenes.
The temperature range for growth of this bacterium is 3–45ºC. The optimum growth can be seen in 30ºC.
In 35–37ºC L. monocytogenes is non-motile or weakly motile.
However at 18–22ºC it is motile with a characteristic tumbling motility in broth cultures.
Main laboratory specimen are cerebrospinal fluid and blood for culture.
In Listeria meningitis only few bacteria may present in cerebrospinal fluid.
Usually contain lymphocytes and polymorphs.
Cerebrospinal fluid proteins also increased in Listeria meningitis.
Colony morphology
Blood agar
It produces grey colour, small, translucent colonies.
They are drop like colonies.
And shows beta haemolysis around the colony.
It should be incubated up to 48 hours for a visible growth. Â
Mueller Hinton agar
Pale blue colour colonies can be observed when viewed from side with white light.
Biochemical tests
- This organism is catalase positive
- Indole, oxidase, and urease negative
- Ferment maltose and glucose and produce acid
The characteristic tumbling motility and colony characteristics in cultures are usually sufficient to identify the organism without using many biochemical tests.
Serological tests are not much useful for the diagnosis.
However in outbreaks PCR based procedures have been reported for the amplification of DNA in sequencing.
Treatment
monocytogenes is sensitive to many antibiotics in vitro, like ampicillin, penicillin, tetracyclines, vancomycin, chloramphenicol, aminoglycosides and co-trimoxazole.
Ampicillin with or without gentamicin is the most successfully used drug for the treatment.
Cephalosporins are ineffective.
Prevention
Ingestion of unpasteurized milk products or raw vegetables by pregnant women and immunocompromised patients should be reduced.
For immunocompromised patients
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