Monitoring The Risk Of Listeria Monocytogenes - Microchem
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Monitoring The Risk Of Listeria Monocytogenes

Monitoring The Risk Of Listeria Monocytogenes

In first world countries, outbreaks of food-poisoning are reported and trended. Strict testing requirements are enforced to ensure consumer safety. Food products found to be contaminated with pathogens are subject to a product recall because they put the consumer at risk. Food producers must exercise due diligence in order to safeguard their customers.

Outbreaks of Listeria Contamination

The Centers for Disease Control and Prevention defines an outbreak as when a particular food product is linked to the foodborne illness of more than 2 people by epidemiologic analysis.

Three outbreaks of Listeria were identified in the ‘Top 10 Outbreaks of 2016’ according to Food Safety News in the U.S. Outbreaks were ranked by the number of fatalities and then the number of illnesses reported.


Frozen vegetables were linked to outbreak of Listeria which resulted in 3 deaths. The contamination with Listeria monocytogenes  lead to a product recall of 358 different product types which included organic and traditional frozen vegetable and fruit products.

Packaged salads

Listeria monocytogenes was linked to packaged salads which caused illness in 33 people in the US and Canada. All 33 victims had such severe symptoms they required hospitalization. Four of them died.

Raw milk

A Listeria outbreak was linked to raw milk. One death was confirmed. Raw milk can be used in the manufacture of soft cheeses.

For detailed information on the above consult the following website:

International Product Recalls in 2016-2017

Ice cream and frozen desserts

A premium U.S  ice-cream brand recalled products made in an ice-cream plant where Listeria was found in the manufacturing facility and in the finished product of another ice-cream brand. The production facility has since shut down.

Weight-watchers in Chicago recalled a variant of desserts containing cookie dough found to be contaminated with Listeria monocytogenes .

Deli Meat

A public health alert was issued by the USDA’s Food Safety and Inspection Service for sliced meat consumed at restaurants in 3 different stated in the U.S. The implicated deli meats included sliced roast beef, ham, pastrami and turkey. The products were contaminated with Listeria monocytogenes.

Hummus and Pate

A U.S company is voluntarily recalling hummus because these products have the potential to be contaminated with Listeria monocytogenes. Australian companies have recalled pate due to Listeria contamination. 

Soft Cheeses

Listeria contamination was responsible for a recall of several varieties of cheese from multiple countries. Listeria monocytogenes was found in raw milk cheeses from France which were distributed to Belgium, Denmark, Spain, Ireland, the UK and the U.S.


A New York seafood company stopped production and is recalling 22 different herring products because inspectors found Listeria monocytogenes in the company’s plant.

Smoked salmon and smoked butterfish distributed throughout the U.S were subject to a recall as a result of a routine sampling program by the Florida Department of Agriculture and Consumer Services which revealed the presence of Listeria monocytogenes.


Early in 2017 mushroom products were recalled in Germany due to independent testing confirming the presence of Listeria monocytogenes.

For detailed information consult the following websites:

The risk

Listeria monocytogenes is widespread in the environment. These bacteria can survive in processed, preserved and refrigerated ready-to-eat (RTE) foods such as processed meat and fish, cold meats, dairy products, such as soft cheeses, especially if unpasteurized; and pre-prepared sandwiches and salads.

Eating food contaminated with the Listeria monocytogenes can cause listeriosis, a life-threatening infection. Listeriosis is the third leading cause of death from food poisoning. It hits high-risk groups the hardest, and results in higher rates of hospitalization and death than most other foodborne bacteria.

The disease primarily affects pregnant women and their newborns, people with immune systems weakened by cancer, cancer treatments, or other serious conditions (like diabetes, kidney failure, liver disease, and HIV) and adults older than 65.

Although people can sometimes develop the illness up to 2 months after eating contaminated food, symptoms usually start within several days. A person with listeriosis usually has fever and muscle aches, sometimes preceded by diarrhoea or other gastrointestinal symptoms.

Control of Listeria in the Environment

Ready-to-eat (RTE) foods are in a higher risk category than other foods as there is no additional heat-kill step before consumption.

Listeria monocytogenes can replicate at refrigeration temperatures and is therefore of concern in products with a refrigerated shelf life.

The target goal with Listeria testing is complete absence of Listeria in the post-cooking area. An effective environmental monitoring program will sometimes yield positive Listeria results. These help to identify where corrections should be made to better protect consumers. The monitoring and corrective action process will identify issues and assist the food producer in locating the ‘growth niche’ of Listeria.

Microchem’s Trending of Listeria 2016

Listeria monocytogenes was isolated from 5.8% of surfaces in production facilities and 6.4% of food products tested by Microchem for the presence of the organism. Where products were tested for Listeria at levels higher than 10 CFUs/g, L. monocytogenes was isolated in 0.88% of the products tested.

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.

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.

Additional Information

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

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