test for Listeria monocytogenes

Listeria monocytogenes

Listeria is a bacterium that is found in soil and water with some individuals acting as carriers. Listeria monocytogenes is a species of Listeria that is a food-borne pathogen capable of surviving in the presence and absence of oxygen.  Its ability to grow at temperatures as low as 0 °C permits multiplication at typical refrigeration temperatures. This increases its ability to evade control in food.

Of the ten species of Listeria, L. monocytogenes has been the only species on record that affects humans.

  • New-borns
  • Pregnant moms
  • Immunocompromised

L. monocytogenes is the bacterium that causes Listeriosis and can be fatal if left untreated or undetected.

Individuals that are at a higher risk of contracting the infection should avoid:

  • Raw or unpasteurised milk and dairy products containing unpasteurised milk
  • Soft cheeses such as feta, camembert, brie and goats cheese
  • Deli foods such as cold meat,
  • Prepared salads- food that may not have been heated or reheated sufficiently
  • Pâtés

Ensure that raw meat (beef, pork, lamb and poultry) , raw fruit and vegetables are thoroughly cooked or washed before consuming. Surfaces on which meat, vegetables and other food products are handled should be sanitised and cleaned regularly.

Through education, good hygiene practices, and the safe handling of food, it’s preparation and storage, Listeriosis can be prevented.

Quantitative and qualitative methodology to detect and confirm Listeria monocytogenes

Food Consulting Services makes use of two types of test for L. monocytogenes; a qualitative and a quantitative test.

Qualitative test for Listeria monocytogenes

This method involves a presence or absence (detection) analysis for a particular micro-organism in 25g of the test sample. It involves a two-step enrichment process; the sample is initially weighed and incubated overnight in an enrichment broth followed by the second enrichment step after 24 hours. During this process, the bacteria, if present, are able to multiply and increase the sensitivity for L. monocytogenes detection. Qualitative analysis is therefore more sensitive in detecting lower counts of the desired microorganism.

At Food Consulting Services, VIDAS® , a multi-parametric immunoassay system, based on the Enzyme Linked Fluorescent Assay (ELFA) technology is used for the detection of L. monocytogenes in 25g of sample. It works on the principle of an antibody-antigen reaction.

Quantitative test for Listeria monocytogenes

Involves the direct enumeration of microorganisms, by determining the number of colony-forming units present in 25g of sample. No enrichment step is involved. Enumeration allows for the analysis of L. monocytogenes at levels higher than 10 cfu/g.

Importance of testing particular food groups/ products

l.monocytogenes is capable of surviving long periods under various stresses and has the ability to form biofilms, this combined with its natural occurrence in the environment makes it difficult to eliminate once contamination occurs. The food industry is ever evolving and growing to accommodate the needs of the growing world population, due to this increased demand for food products, factories are required to keep up with mass production. Within these food factories five common places (drains, equipment, floors, freezer/ cooling systems and the air system) have given rise to L. monocytogenes survival and replication.

Due to the fast-paced lifestyle of many individuals, the focus has turned to convenient ready to eat meals (RTE) and items from the deli. These RTE food products and deli items are a perfect medium for L.monocytogenes to flourish as most have long shelf lives under refrigeration temperatures and do not require any further preparation (cooking/ heating) which is required to kill off the bacteria.  RTE meals, cold meats, prepared salads, and fruit salads are some of the examples of food products that should be tested regularly and eaten with precaution if an individual falls into one of the aforementioned risk categories.

Meat products, poultry, milk/ milk products and seafood all undergo some process in order to make them readily available to consumers. These process occur in factories which have the potential to harbour the bacterium. Raw vegetables and fruit are fresh produce and may contain trace amounts of soil which provides the perfect niche for the transmission of L. monocytogenes. Due to its origins and processes undergone it is important to test these raw materials and RTE food products regularly in order to detect, control or prevent L. monocytogenes contamination.

Soil / Water tests

Food Consulting Services focuses on the testing of food products, such as raw materials and ready-to-eat products, where it is undesirable to find L. monocytogenes on these particular samples, due to the high risk involved. Soil is a primary habitat for L. monocytogenes and therefore highly likely to contain Listeria spp..  Since food products rarely get consumed straight from the soil but get processed in some form in a plant or facility, it is not feasible to routinely test for Listeria monocytogenes in soil, except to eliminate it as a source of origin where contamination has already been identified.

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