14 May Listeria species and Listeria monocytogenes – Key Point Summary
What is Listeria monocytogenes?
Listeria monocytogenes is a Gram-positive bacterium found widespread in the environment. It is commonly present in sites within the manufacturing environment that are generally difficult to reach and clean by normal procedures. These sites will serve as a reservoir from which the pathogen disperses during processing to contaminate contact surfaces and the final product.
What are Listeria species?
There are many different Listeria species but not all of them can cause disease. Pathogenic Listeria species include L. monocytogenes (considered a human pathogen) and L. ivanovii (considered an animal pathogen), while 13 other non-pathogenic species exist.
Listeria species are also environmental contaminants that can survive in both food processing premises and on equipment if inappropriate hygiene measures are used. Listeria species can therefore be used as an indicator to assess the hygienic status of a food product. Finding any Listeria species will indicate that there has been a control failure or recontamination of the product. This can indicate that L. monocytogenes may be present and further testing for confirmation of L. monocytogenes should be done.
What are Listerosis?
Listeriosis is a severe infection, caused by L. monocytogenes, that affects individuals with compromised immune systems, pregnant women and the elderly. The variable incubation period (1-90 days) makes it difficult to determine the source of contamination. The complications include severe sepsis, meningitis, encephalitis, stillbirths or spontaneous abortions, and preterm birth in pregnant women.
Why is it significant?
Listeriosis accounts for 28% of deaths resulting from foodborne illness. The current mortality rate of the South African outbreak stands at 40% of cases with known outcomes.
How does L. monocytogenes survive in the processing environment?
This organism can be particularly persistent and difficult to control because of its resistance to unfavourable environmental conditions and ability to grow at refrigeration temperatures as low as 0°C, in food with a fairly low moisture content and under salt stress. It therefore easily compromises refrigerated products such as ready-to-eat foods, raw ingredients not cooked before consumption, products that have an extended shelf life under refrigeration and products not subjected to a listericidal process.
What can be done?
It is important to recognize 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.
The implementation of a thorough hygiene plan that incorporates an environmental sampling plan can prevent the establishment of L. monocytogenes within niches in the processing environment which can lead to contamination of the final product.
Implementation of the above mentioned environmental sampling program that will allow a rapid and effective response procedure with regards to the identification of contamination points and possible positive products.
What are the acceptable criteria for L. monocytogenes and Listeria species?
In high risk food products that may support the growth of L. monocytogenes the absence of the bacteria in 25 g at the beginning of shelf-life 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.
Microchem uses rapid and sensitive chromogenic test methods that are internationally certified and validated by AFNOR (Association French Normalization Organization) for the detection of Listeria monocytogenes (the pathogenic strain of the Listeria species) and Listeria species.
These methods detect L. monocytogenes and Listeria species directly and are faster than traditional testing methods, while also being cost-sensitive.
Microchem is accredited for the detection and enumeration of L. monocytogenes. We are currently in process of gaining SANAS accreditation of our Listeria species detection method as well.
For additional information on L. monocytogenes see our previous communications on our website: