Listeria monocytogenes. Molecular serotyping of seven major serotypes 1/ 2a, 1/2b, 1/2c, 3a, 3b, 3c and 4b.

Listeria is a genus of Gram - positive bacilli, facultative anaerobic, which includes six species: Listeria monocytogenes, L. ivanovii, L. seeligeri, L. innocua, L. and L. grayi weishimeri. Of these species, L. monocytogenes is pathogenic for humans and animals. L ivanovii (formerly L. monocytogenes be. 5) is only pathogenic for sheep and cattle hoofed animals like. The other 4 species are saprophytes and are adapted to live in soil and decaying vegetation. For tolerance to changes in pH, extreme temperatures and salt concentrations, they are able to survive in many processed foods. For its ubiquity can contaminate foods of different types and sources such as vegetables, milk, cheese, meat products, fish and ready meals ready to eat or heat (Ready-to-Eat or Heat-to-Eat).

Ingestion of contaminated food is the main source of infection for both sporadic cases and outbreaks for.

Human pathogenic species, L. monocytogenes, preferably affects a population group most susceptible: pregnant women, infants, the elderly or immunocompromised, which can cause serious infections such as septicemia, meningitis, encephalitis, abortions and occasionally death (20 to 30% of the cases). In addition, it may result in the general population sporadic outbreaks of febrile gastroenteritis, non - invasive, self - limiting, also manifested by fever, for nausea and diarrhea.

Most cases are sporadic, but outbreaks have also been described by some very diverse foods such as cabbage salad (coleslaw) and cold meats (deli meat) in Canada; Swiss white cheeses; comminuted meat pates (rilletes) in France; cantaloupe or chocolate milk in USA .; butter or smoked trout in Finland; rice salad or sweet corn in Italy; gelatinized pork in Austria; among others.

L. monocytogenes includes a broad spectrum of strains differ in their virulence and pathogenicity. Many strains of L. monocytogenes are virulent and capable of causing morbidity and mortality, but yet others are avirulent and unable to cause infection in mammals. To differentiate strains traditionally been used somatic antigens (O) and flagellar antigens (H), with which were defined as 13 serotypes, the most common being 1 / 2a, 1 / 2b, 1 / 2c, 3 , 3b, 3c, 4a, 4b, 4c, 4d, and 4e. These serotypes have formed the basis of their differentiation, and thus described serotypes involved in various infections and outbreaks. Of these serotypes, about 98% of cases of listeriosis are caused by four of them: 4b, 1 / 2a, 1 / 2b and 1 / 2c. Nor serotypes are equally distributed in food, environment and clinical samples. In human clinical samples 49% of isolates are serotype 4b, while in food only 8% of the isolates are serotype 4b. Likewise, the same serotype may cause two types of processes: invasive disease or gastrointestinal fever. Some serotypes such as 4a, are found in foods and animals, but is rare in cases of human listeriosis.

The conventional serotyping using specific antisera somatic antigens (O) and flagellar (H), generally by agglutination tests, was the first line subtyping be used in food safety laboratories. This method of serotyping is based on an antigen-antibody reaction, and has some drawbacks, such as: laborious and subjective that requires experience, poor reproducibility due to the variability in the quality of the antisera used, limited by cost, availability and cost of antisera and typing of the isolates. Subjectivity in reading the results and reproducibility tried obviated with ELISA methods, but neither has been the solution.

To differentiate pathogenic serotypes have been conducted various kinds of tests and bioassays in vivo by inoculating laboratory animals; vitro bioassays inoculating cell cultures; testing for certain proteins; characterization of DNA by electrophoresis pulsed field (PFGE: Pulsed-field gel electrophoresis), etc.

Thanks to the complete sequencing of the genome of L. monocytogenes several lineages, including in each lineage some serotypes identified by conventional serotyping established. This tried to be applied in methods for detecting macroarrays probes sequenced genomes of some serotypes. Have established several lineages (I, II, III), in each of which several serotypes (cited by Datta, AR, et al included. Recent Developments in molecular sub-typing of Listeria monocytogenes. Food Additives & contaminants, Part A, 2013, 30:.. 1437-1445), or four lines (I, II, III, IV) (cited by Shen J., et al Molecular subtyping and virulence gene analysis of Listeria monocytogenes isolated from food Food Microbiol 2013. 35: 58-64). The initial three lineages were subdivided into five phylogenetic groups were:. I.1, I.2, II.1, II.2, and III (cited by Doumith M., et al Differentiation of the major Listeria monocytogenes serovars by multiplex PCR . J. Clin Microbiol 2004, 42:.. 3819-3822). We believe that the identification of lineages or phylogenetic groups, but have served to group in each of several different serotypes by their genomic similarity not allow epidemiological monitoring of each serotype and its involvement in a sporadic case or epidemic outbreak .

Today, it is possible to differentiate serogroups and serotypes of Listeria monocytogenes by PCR, which identifies the existing different genes in most serotypes pathogenic and epidemiological interest, and thus their differentiation epidemiologically relevant.            

Tests in IVAMI:

  • Molecular identification of Listeria, and species L. monocytogenes by corresponding specific genes.
  • Molecular serotyping to differentiate, by PCR amplification of the corresponding genes, major serotypes 1 / 2a, 1 / 2b, 1 / 2c, 3a, 3b, 3c and 4b.  

samples required  

  • Isolate (strain) of Listeria monocytogenes in culture.

Sample preservation

  • Room temperature or refrigerated.

Delivery of results

  • 48 hours.

Cost of the test