“Listeria” is a family of bacteria. Almost all of the family members get along well with each other and with humans–so much so that people often do not even know the names of these “friendly” bacteria. But one member of the family, Listeria monocytogenes, is a bad actor, a pathogen. If a susceptible person (e.g., immunocompromised) gets infected with Listeria monocytogenes, they could easily get listeriosis. Consequences of this infection include sepsis, meningitis, encephalitis, and death.
Listeria was named in honor of a famous British surgeon, Sir Joseph Lister (from whom the trade name Listerine also comes from), who advocated for the use of antiseptics during surgery. Microbiologically speaking, Listeria species (i.e., “Listeria spp.”) are Gram-positive, facultative anaerobes, which means that they can grow in environments which do not have much oxygen.
Listeria monocytogenes (Lm) is one of the most virulent food-borne pathogens. It can be found in dirt, sewage, rivers and streams, rotting vegetation, and animal feces. Hence, it can easily find its way into a food facility through someone’s shoes, or equipment, or ingredients brought into the plant.
Lm is an extremely strong organism, able to survive pH ranges of 4 to 9, and temperatures from 32F to 113F. It can survive freezing conditions, very dry surroundings, and salty environments. The organism will not necessarily grow under those conditions, but once those conditions revert to more favorable ones, it can begin to grow again. Lm often forms “biofilms” (protective matrices) that protect the organism from being found and make it that much harder to eradicate.
All of these characteristics make Listeria an insidious pathogen.
FDA has published a summary of Lm in “Get the Facts About Listeria.” This summary includes statistics, background, and references. FDA also has a well-vetted set of microbiological procedures to identify Lm in its Bacteriological Analytical Manual.
A thorough microbiology reference book is “Modern Food Microbiology”. Chapter 3 discusses the characteristics of foods which affect pathogen growth, and Chapter 25 covers listeriosis.
Other Listeria species, such as Listeria innocua and Listeria welshimeri, are not pathogenic, and actually compete with Lm. So a microbiological sample taken which is positive for L. innocua might lead one to conclude that Lm is not present when it really is.
Food manufacturers and regulators often judge cooked foods as less risky than ready-to-eat (RTE) foods because cooking kills bacteria. Foods with low pH, preservatives, and/or low water activity (these are sometimes referred to as formulation hurdles) may also be judged as lower risk. However, don’t assume cooked or preserved foods are free of Listeria monocytogenes, as it’s a tough organism. Do your testing and validation work to confirm that Lm is under control.
Editors Note: This article is the first in a series of six brief articles on Listeria. They are intended for food science professionals, especially food safety and quality assurance people. The information presented in these mini-articles will also be of interest to people in all functions.
