Testing for Listeria Monocytogenes - Microchem
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Bread Meat & Cheese

Testing for Listeria Monocytogenes

Technical overview of Listeria Testing

Listeria species are persistent in the food processing environment. Unlike other bacteria, Listeria can survive and grow at refrigeration temperatures.

The control and monitoring of L. monocytogenes, a major human pathogen, and other Listeria species in the food processing environment is vital to ensuring consumer safety by reacting on possible risks before it contaminates food products.

L. monocytogenes and other Listeria species

Not all Listeria species can cause disease. Pathogenic Listeria species include L. monocytogenes and L. ivanovii, while 13 other non-pathogenic species exist.

L. monocytogenes infects humans and animals, while L. ivanovii has been considered to infect ruminants (cattle, sheep and goats) only.

The majority of human infections caused by L. monocytogenes are foodborne. The occurrence of these bacteria at any level may therefore be of significance in certain refrigerated high risk due to the potential for growth during storage in some of these products.

Listeria species can be used as an indicator to assess the hygienic status of a food product. Listeria species are also environmental contaminants that can survive in both food processing premises and on equipment if inappropriate hygiene measures are used.

Detection vs Enumeration of L. monocytogenes

The test for detection of L. monocytogenes is qualitative and will provide a presence/absence result in 25g of product. Detection requires the sample to be incubated in an enrichment broth overnight in order to multiply the bacteria for further isolation tests. The enrichment process increases the sensitivity of the test. The detection test is far more sensitive and can detect lower amounts of organisms. Detection should be performed when the product specification is for absence of L. monocytogenes in 25g of product or the absence of L. monocytogenes in an area.

The test for a count (enumeration) of L. monocytogenes is quantitative and will provide the amount of L. monocytogenes present in 1g of product. A representative sample suspension is evaluated to determine the amount of bacteria in 1g of sample. This test only detects bacteria present at levels higher than 10 CFUs/g. Enumeration should be performed when the product specification is < 100 CFUs/g of L. monocytogenes.

Two separate test procedures are required to perform each of the tests above.

Regulations and Acceptance Criteria

Countries differ in their regulatory approach to the presence of L. monocytogenes in RTE food.

European Union regulations generally permit a count of up to 100 CFUs/g at the end of shelf life for RTE foods, except those intended for infants and for special medical purposes.

In the USA a ‘zero tolerance policy’ is taken on the presence of L. monocytogenes in any RTE food, and the pathogen should be absent in 25 g of product. However, the US Food and Drug Administration (FDA) published a draft consultation paper proposing to loosen up these controls to allow a maximum limit of 100 CFUs/g in frozen and refrigerated RTE foods that do not support the growth of Listeria.

Specific regulatory guidance on Listeria for food manufacturers is also available in a number of countries.

In high risk food products that may support the growth of L. monocytogenes the absence of the bacteria in 25 g is advised e.g. deli-meats, soft cheeses, hot dogs, pâté. In food products which have a limited potential for the growth of L. monocytogenes, or foods with a shelf-life of <5 days, a limit of <100 CFUs/g is advised.

It should be remembered that for ready-to-eat foods where there is a Listeria control step like cooking, finding any Listeria species will indicate that there has been a control failure or recontamination of the product. This indicates that L. monocytogenes may be present.

Listeria in the Environment

Listeria is common in the environment. It is found in soil and has been isolated from a wide variety of fresh produce. It is also found in marine environments and is often associated with fish and seafood products. Healthy humans can also be carriers.

Kitchen and food processing environments, particularly those that are cold and wet, can be reservoirs for Listeria. The organism can be particularly persistent and difficult to control because of its resistance to unfavourable environmental conditions and ability to grow at low temperatures, especially within biofilms.

The ability of L. monocytogenes to grow in ready-to-eat foods refrigerated for a prolonged time is the main cause of L. monocytogenes foodborne infections in humans.

International trends suggest that if food contact surface samples are found positive at two or more processing steps, end-product testing should be initiated to ensure that finished product is not contaminated with L. monocytogenes. A hygiene criterion of 100cm2 area (10cm x 10cm) is recommended for swabbing.

Our 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.

  • 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.

Microchem can offer alternative rapid detection methods which include:

  • The Polymerase Chain Reaction (PCR): based on the detection of the bacterial DNA of L. monocytogenes from the food enrichment
  • VIDAS technology: based on the detection of the L. monocytogenes bacteria using recombinant phage protein technologies

The rapid alternative methods provide a negative result one day faster than rapid chromogenic testing. The cost of these test methods higher due to the advanced technology used to generate the result faster. Confirmation of positive results requires the isolation of the bacterial culture using culture media.

Control of L. monocytogenes

Cross-contamination of prepared foods can occur from transfer of L. monocytogenes:

  • From the environment and equipment
  • From contact with raw food products
  • From incorrect employee practices

The greatest risk for product contamination occurs when a product contact surface is contaminated.
This risk is highest between the point where a food is cooked or decontaminated and the point where the food is packaged. Control of Listeria must be directed toward preventing the establishment and growth of Listeria in the processing environment. A control program for L. monocytogenes should focus on the more common sources of direct product contamination. Effective control programmes start with environmental monitoring to prevent contamination of final products.

The aim of a Listeria monitoring andcontrol programme is to:

  • Prevent contamination of products with L. monocytogenes
  • Prevent contamination of food contact surfaces
  • Prevent the establishment and growth of the L. monocytogenes in niches in the processing
    environment

Corrective actions implemented after detection of Listeria in the environment must be monitored to
confirm their effectiveness. It is important to continually strive for negative results by responding to each positive sample with appropriate corrective actions in a timely manner.

Best Practice Internationally

Control of the environment is essential to combat the contamination of foodstuffs with L. monocytogenes. Control and monitoring of Listeria in the food processing environment and on food contact surfaces allows risk areas to be targeted before contamination of foodstuffs can occur.

The number of Listeria organisms present in food product is determined with a count per gram of food.
Food is considered unsafe for most consumers if L. monocytogenes is present at unacceptable levels, usually greater than 100 CFUs/g at the end of shelf-life. As Listeria grows at refrigeration temperatures, this number can easily be reached during extended storage periods of food if Listeria is present in the product. It should be remembered that for ready-to-eat foods where there is a Listeria control step like cooking, finding any Listeria species will indicate that there has been a control failure or recontamination of the product.

This indicates that L. monocytogenes may be present.
International trends suggest that if food contact surface samples are found positive at two or more processing steps, end-product testing should be initiated to ensure that finished product is not contaminated with L. monocytogenes.

In high risk food products that may support the growth of L. monocytogenes the absence of the bacteria in 25 g is advised e.g. deli-meats, soft cheeses, hot dogs, pâté. In food products which have a limited potential for the growth of L. monocytogenes a limit of <100 CFUs/g is advised e.g. foods with a shelf-life of <5 days.

Our 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.

L. 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.

Microchem offers alternative rapid detection methods which include:

The Polymerase Chain Reaction (PCR): based on the detection of the bacterial DNA of L. monocytogenes from the food enrichment.
VIDAS technology: based on the detection of the L. monocytogenes bacteria using recombinant phage protein technologies.
The rapid alternative methods provide a negative result one day faster than rapid chromogenic testing. The cost of these test methods higher due to the advanced technology used to generate the result faster. Confirmation of positive results requires the isolation of the bacterial culture using culture media.

Additional Information

For additional information on L. monocytogenes see our previous communication: Testing for Listeria monocytogenes

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