Listeria monocytogenes - Review After the Outbreak - Microchem
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Listeria monocytogenes – Review After the Outbreak

Listeria Monocytogenes

Listeria monocytogenes – Review After the Outbreak

Listeria monocytogenes is an opportunistic foodborne pathogen that can survive within a wide range of conditions found within the food processing environment. Its most notable factor is the ability to actively grow at refrigeration temperatures.

It is the cause of listeriosis, a potentially fatal infection which targets pregnant women, immunocompromised individuals, children and the elderly. Its main infection route is the ingestion of contaminated food which is typically refrigerated and ready-to-eat. More than 200 people have lost their lives during the 2017-2018 Listeria monocytogenes outbreak in South Africa.

There are several species of Listeria but only L. monocytogenes is typically associated with the human foodborne infection causing listeriosis. All sequence types of Listeria monocytogenes pose a health risk.

Listeria monocytogenes Sequence Type 6 (ST6)

Listeria monocytogenes Sequence Type 6 (ST6) was identified as the strain responsible for the 2017-2018 outbreak linked to the consumption of polony. By using Whole Genome Sequencing (WGS) and Multi Locus Sequence Typing (MLST) it was possible to trace the common strain isolated from patients, foodstuffs and production environments.

It is imperative to note that there are numerous ST strains of L. monocytogenes. All L. monocytogenes sequence type (ST) strains can cause listeriosis and fatalities as with the ST6 strain.

How is Whole Genome Sequencing Used?

More complex information of an isolated L. monocytogenes strain can be obtained using Whole Genome Sequencing (WGS) and Multilocus Sequence Typing (MLST).

WGS is a process that determines the DNA sequence of an entire genome. MLST uses PCR amplification and sequencing of the panel of several targeted genes within the genome. MLST then detects the any differences or changes in these metabolic or housekeeping gene sequences. Sequences produced are then compared to reference databases of the species from previously isolated strain types. In this way the bacterial species can be identified to a strain type level.

What Requirement Should be Used for RTE Foods?

Requirements for RTE Foods

1. Applying Enumeration Testing for Production Samples or Swabs

Detection (absence in 25g) of Listeria monocytogenes applies for ready-to-eat foods in which growth can occur upon further storage.

This includes food which:

  1. Is ordinarily consumed in the same state as that in which it is sold; and
  2. Will not be subject to a listericidal process before consumption; and
  3. Is not one of the following
    1. shelf stable foods
    2. whole raw fruits
    3. whole raw vegetables

Enumeration (count of <100 CFUs/g) of Listeria monocytogenes applies for ready-to-eat foods in which growth will not occur upon further storage (at the end of shelf-life).

This includes food which:

  1. has a pH less than 4.4 regardless of water activity; or
  2. has a water activity less than 0.92 regardless of pH; or
  3. has a pH less than 5.0 in combination with a water activity of less than 0.94; or
  4. has a refrigerated shelf life of no greater than 5 days; or
  5. Are frozen (including foods consumed frozen and those intended to be thawed immediately before consumption)

Enumeration does not apply to ready-to-eat foodstuffs at production. No L. monocytogenes should be detected in 25g for samples after production. This criterion will ensure counts do not reach unacceptable levels at the end of the products’ shelf-life.

Enumeration will not apply in the case of swab testing. The aim with swabbing is to detect areas where cleaning has not been successful and there is a risk for cross-contamination to the final product. Enumerating a count will only lead to a false sense of security as the test is less sensitive and can only detect the presence of L. monocytogenes when levels are >10 CFUs per area swabbed.

2. 2. Thinking Listeria Is Only Found in Drains

If Listeria is present in the production facility, it will most likely be found in the drainage system. Drains act as a collection point for most waste and cleaning run-off. It can then provide nutrients and moisture needed for Listeria growth. Cleaning drains is therefore critical for control as they act as harborage sites and possible sources of contamination.

L. monocytogenes is, however, an environmental contaminant and can be difficult to eradicate in food production facilities. Listeria species can be found in raw foods, soil, sewage, water and the intestinal tract of animals and humans. The bacteria can therefore easily be re-introduced into the production facility after cleaning. Control therefore needs to be directed towards preventing its establishment and growth in the production environment.

How to Control L. monocytogenes

The identification of the main entry and transmission routes of L. monocytogenes are essential to define appropriate risk mitigation strategies in the production environment.

The most effective strategies to control L. monocytogenes in high-risk foods include:

  1. Good manufacturing practices, sanitation standard operating procedures, and hazard analysis critical control point programs to minimize environmental L. monocytogenes contamination. This will prevent cross-contamination in processing plants and at retail;
  2. An intensive environmental sampling program in plants processing high-risk foods and an effective corrective action plan to reduce the likelihood of contamination of high-risk foods;
  3. Time and temperature controls throughout the entire distribution and storage period, including establishing acceptable storage times of foods that support growth of L. monocytogenes to high numbers;
  4. Reformulating foods to prevent or retard the growth of L. monocytogenes; and
  5. Using post-packaging treatments to destroy L. monocytogenes on products.

7 Steps for Effective Cleaning

It is important to note that equipment or the environment needs to be accessible to a cleaning agent or it will not be effective. It is also imperative that cleaning agents be inter-changed at regular intervals to reduce the bacterial resistance.

  1. Dry Clean – Using brushes and scrapers remove heavy soiling or debris from equipment, then floors.
  2. Pre-Rinse – Work top to bottom, scrub and rinse off visible soils left behind on equipment. After that remove any additional debris left on the floors and drains, then rinse off.
  3. Soap and Scour – Apply your foam cleaner to walls then floors and lastly equipment starting at the bottom ending at the top. Clean your drains using a separate set of cleaning tools. Scrub your equipment to remove any residue and prevent the foam cleaner from drying on your equipment.
  4. Post-Rinse – Remove your foam cleaner from walls then floors and lastly equipment in the same order that the foam was applied.
  5. Prepare for Inspection – eliminate any overhead condensation and any pooling water, prepare the area for inspection.
  6. Inspection – see if the equipment has been cleaned properly and correct any mistakes.
  7. Sanitize and Assemble – Sanitize walls, floors, equipment then reassemble the relevant parts to be ready for the start of production.

Additional Information

For additional information on L. monocytogenes see our previous communications on our website:

Raymond HartleyYours sincerely

(Managing Director)