Listeria Species 

The genus Listeria belongs to the family Listeriaceae. Listeria are Gram-positive, non-spore-forming rods. There are currently 20 recognized species of Listeria,¹ with Listeria monocytogenes being the most clinically significant.

Unlike many other foodborne pathogens, Listeria can both survive and grow at refrigeration temperatures. This ability, along with the risk of cross-contamination to other foods and surfaces, contributes to its persistence in food processing environments and makes it a significant concern for food safety.

Incidence

Listeriosis is a foodborne illness commonly caused by Listeria monocytogenes. It is a relatively rare but serious infection and can lead to severe symptoms, especially among vulnerable populations such as pregnant women, newborns, the elderly, and individuals with weakened immune systems. Consequently, although not a leading cause of foodborne illness, Listeria monocytogenes is among the leading causes of death from foodborne illness.

Infective Dose

The infective dose of Listeria monocytogenes is undetermined but is believed to vary with the strain and susceptibility of the host. The food source involved may also impact the dose-response relationship. In cases associated with raw or inadequately pasteurized milk, for example, it is likely that fewer than 1,000 cells may cause disease in susceptible individuals.²

Foods Affected

Many foods have been associated with Listeria monocytogenes. Examples include raw milk, inadequately pasteurized milk, chocolate milk, cheeses (particularly soft cheeses), ice cream, raw vegetables, raw poultry and meats (all types), fermented raw-meat sausages, hot dogs and deli meats, and raw and

smoked fish and other seafood. As mentioned previously, Listeria monocytogenes can grow in refrigerated temperatures, which makes this organism a particular problem for the food industry.²  

Potential contamination sources include food worker exposure, airborne transmission, raw materials, and food processing environments. Among these, post-processing contamination on food-contact surfaces poses the greatest threat to product contamination.²

Methods Overview

The following methods described for Listeria species and Listeria monocytogenes include reference and alternative validated and certified methods as follows:

Detection   

1. Reference method: ISO 11290-1:2017 (en): Microbiology of the food chain—Horizontal method for the detection and enumeration of Listeria monocytogenes and of Listeria spp.—Part 1: Detection method. This method is cited in Commission Regulation (EC) No 2073/2005 and other national regulations and guidelines.
2. Reference method: U.S Food and Drug Administration Bacteriological Analytical Manual (BAM) Chapter 10: Detection of Listeria monocytogenes in Foods and Environmental Samples. Cited in the FDA BAM.
3. Reference method: USDA FSIS MLG 8.15: Isolation and identification of Listeria monocytogenes: A culture-media based standard reference method cited in the U.S. Department of Agriculture (USDA) Food Safety and Inspection Service (FSIS) Microbiology Laboratory Guidebook (MLG) Method Number 8.15.
4. Proprietary method, PCR: Thermo Scientific™ SureTect™ Listeria species PCR Assay Workflow.
5. Proprietary method, PCR: Thermo Scientific™ SureTect™ Listeria monocytogenes PCR Assay Workflow.
6. Proprietary method, chromogenic media: Thermo Scientific™ Listeria Precis Detection Methods.

Enumeration

1. Reference method: ISO 11290-2:2017 (en) Microbiology of the food chain—Horizontal method for the detection and enumeration of Listeria monocytogenes and of Listeria spp.—Part 2: Enumeration method.
2. Reference method: FDA BAM Chapter 10: Enumeration of Listeria monocytogenes in Foods.  
3. Proprietary method, chromogenic media: Thermo Scientific™ Listeria Precis Enumeration Methods.

References

1. McSweeney, P.L.H. and J.P. McNamara, Eds. Encyclopedia of Dairy Sciences. 3rd Ed. Elsevier, 2022.

2. U.S. Food and Drug Administration (FDA). The Bad Bug Book. Version 2. 2012.

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