Listeria: Niches and Control Strategies - Microchem
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Listeria: Niches and Control Strategies

Listeria: Niches and Control Strategies

The significance of the foodborne disease listeriosis is well known as it accounts for 0.02% of all foodborne illness and 28% of deaths resulting from foodborne illness.  The current mortality rate of the South African outbreak stands at 40% of cases with known outcomes.

Listeriosis presents as a severe systemic infection that affects individuals with compromised immune systems and most commonly presents as bacteraemia.   The variable incubation period (1-90 days) presents unique challenges to determine the source of contamination, therefore, listeriosis cases that are related as part of a common source outbreak may be widely distributed; in occurrence and by geographic location.

Statistics indicates that recontamination of commercially prepared ready-to-eat (RTE) foods is the primary source of Listeria monocytogenes.  By controlling the establishment and multiplication of L. monocytogenes in these processing environments, it is possible to minimize, and in some cases prevent, the risk of product contamination with sanitation procedures.

The South African outbreak of 2017 – 2018

The South African outbreak consists of several hundred reported cases scattered with regards to time and location.  The outbreak is thought to involve a virulent strain that has established in the food processing environment and has contaminated multiple lots of food over several months of production.  In such a scenario, the outbreak could possibly be attributed to niche within the processing environment where L. monocytogenes has become established.  A niche can be described as a site within the manufacturing environment in which a multispecies biofilm becomes established and proliferates.  These areas are generally difficult to reach and clean by normal procedure, and the area will therefore serve as a reservoir from which the pathogen disperses during processing and contaminates product contact surfaces and final product.

Previously identified niches

Microbiological analysis of the processing environment and the equipment in use is necessary to detect a niche.  Niche sources are often limited to very specific sites of growth that lead to final product contamination (Table 1).

Table 1:  Examples of Identified L. Monocytogenes Niches in Facilities for the Production of RTE Food

(Poultry Products & Meat Products) (Adapted from Tompkin, 2002).

Strategies for controlling Listeria monocytogenes

  1. Prevent the establishment of monocytogenes within niches, that can lead to contamination of the final product, through the implementation of a thorough hygiene plan that incorporates an environmental sampling plan.
  2. Implementation of the above mentioned environmental sampling program that will allow assessment of the exposure environment of the final product.
  3. Implementation of a rapid and effective response procedure with regards to the identification of a positive product.
  4. Identification of the contamination point (niche) by the implementation of a corrective hygiene plan.
  5. Verification of the identification of a contamination point by follow-up sampling.
  6. Implementation of a short term assessment to identify final product contamination trends within the processing facility.


Environmental Sampling Program

Listed below are the two most important factors of the above mentioned Listeria control program which will determine the effectiveness of a Listeria control program.

Design of the Environmental Sampling Program

An environmental sampling plan should be based on historical trends within the processing facility as well as previous experience.  The selection of sampling sites should include areas that have historically been found as good indicators of hygiene control.  The sites sampled should include

  1. equipment surfaces to which the food is exposed,
  2. saturated brine used for chilling cooked products, and
  3. floors near packaging machines and in coolers where exposed product is held for further processing.

The environmental sampling program should be viewed as a routine investigative sampling program that targets selected sites to determine whether loss of control has occurred.  Sampling frequency should depend on the risk to consumers if the food becomes contaminated as well as consideration given to how the food will be handled and prepared before it is eaten by the consumer.  Thus, a successful sampling program will be aggressive in attempting to detect listeriae.


Response to a Positive Finding

The ultimate goal of an environmental sampling program is to maintain control of the processing environment so that all product contact surface samples consistently yield negative results.  If, however, a positive sample is detected the following corrective action is necessary.

  1. The equipment identified as the source of contamination should be dismantled, cleaned, and sanitized.
  2. Smaller equipment with multiple parts require cleaning in a recirculating bath of hot water with detergent and high heat.
  3. Equipment that does not allow dismantling can be covered with a heat resistant tarp with the introduction of steam. However, when steam heating is carried out, the target is to achieve 71oC and to hold this temperature for 20-30 minutes.

It is important to recognise that even with an effective control program, extensive testing will periodically detect a positive result.  Such a finding should be viewed as a “success” because it indicates that the monitoring program has been effective, the problem can be corrected, and consumer protection ensured.  Recrimination against plant management for the presence of this ubiquitous bacterium invariably proves counterproductive in the long term.  For the reasons just mentioned, corporate and regulatory policies should encourage environmental sampling programs and consider positive findings as a success of the monitoring program and less as a failure of control.


Reality: Listeriae Will Continue to be Introduced into the Food-Processing Environment

Despite the extensive efforts, listeriae will continue to be reintroduced into food-processing environments. However, failure to control listeriae on floors increases the likelihood that packaging lines will eventually contaminate and test positive for L. monocytogenes. Therefore, maintaining clean, dry floors can be effective in most situations.  If a dry floor cannot be maintained the following methods can be applied

  1. Scrub the floors with caustic powder, thereafter the floors are to rinsed and sanitized with a high concentration of sanitizer (e.g., 800 to 1,000 ppm quaternary ammonium compound), and dried.
  2. An application of crystalline citric acid to floor to maintain a pH of ≤5 (as indicated by pH paper) can improve control in high risk areas, but it is to be noted that the flooring material must be able to withstand this treatment.
  3. Other methods (e.g. frequent application of sanitizer) may prove necessary in areas where the floor remains wet because of the type of operation. Cleaning and sanitizing procedures should be directed toward Listeria monocytogenes

Contamination is normally limited to a single packaging line, with adjacent lines not being affected. Random contamination from air, people, packaging materials, and so forth, is minor. In a facility with a controlled environment, growth within a niche is the major concern.

It should be clear that an assertion that Listeria contamination is due to poor sanitation is because of a lack of understanding of this difficult issue. Extensive research has shown that bacteria adhering to surfaces in biofilms are more resistant to sanitizers. Of greatest concern are enclosed areas (e.g. within a hollow roller on a conveyor) where food deposits and moisture accumulate and cannot be removed by normal cleaning, scrubbing, and disinfecting. These harbourage sites are not biofilms per se, but rather niches in which a variety of bacteria, Including L. moncytogenes, become established and multiply.


How can Microchem help you?

  • Microchem offers hygiene services that includes a hygiene inspection and hygiene report personalized to your facility.
  • Microchem can assist you with environmental swabbing of your facility to identify niches where Listeria can proliferate from. Areas that are identified as possible sources of contamination can then be targeted with appropriate cleaning techniques. We also supply swabbing kits should you wish to perform the swabbing in-house.
  • We offer qualitative screening for the presence of monocytogenes in final products as well as enumeration of the confirmed L. monocytogenes isolates present.
  • Microchem offers consultation services to assist with hygiene management in your facility.
  • Microchem has close ties with experts in the field, thus staying up to date on the latest trends in research internationally.


Microchem’s Technology

Microchem uses a rapid and sensitive chromogenic test method that is AFNOR certified and validated for the detection of Listeria monocytogenes, the pathogenic strain of the Listeria species.

  1. monocytogenes is isolated using selective enrichment and a specific colorimetric enzyme reaction on chromogenic media. Further confirmatory tests are performed if any presumptive colonies are isolated.

This method detects L. monocytogenes directly, is faster than traditional testing methods, while also being cost-sensitive.


Additional Information

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


Other Services by Microchem

  • Routine Food Microbiology Analyses
  • Food-borne Pathogen Analyses
  • Water Microbiological Analyses
  • Product Shelf-Life Analyses
  • Hygiene Inspections and Reports
  • Microbiological Swabbing
  • Foreign Object Inspections
  • Food Chemistry
  • Nutritional Analyses
  • Trace Elemental and Heavy Metal Analyses
  • Food Colourants (Azo Dyes)
  • Vitamin Analyses
  • Water Chemical Analyses
  • Melamine Analyses
  • Pesticide MRL analysis

Contact Us

Cape Town (Head Office)
1st Floor Fairweather House,
176 Sir Lowry Road
Woodstock, Cape Town, South Africa, 8001
Tel: +27 (21) 465-6996 or 061 394 1913

Sales representative/Consultant: Jadri Groenewald,

Microbiology Lab Manager: Ilse Liedemann,

Operations Manager: Hannes Esterhuizen,

Pesticide Lab:


Gauteng Micro Laboratory

41 Golden Drive,
Morehill, Benoni, South Africa, 1501
Tel: +27 (11) 425-3775 or 061 393 5354

Sales representative/Consultant: Thabisile Sekakatlela,

Microbiology Lab Manager: Hussein Ebrahim,

Pharmaceutical Lab Manager: Brendan Moore,



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