Listeria Monocytogenes

definition

In 2010, the genus Listeria contained six species. As of January 2024, there are 28 species which are subdivided into two groups – sensu stricto and sensu lato. The Listeria sensu stricto consists of L. monocytogenes and the nine species that show high similarity to L. monocytogenes, which include: L. innocua, L. ivanovii, L. welshimeri, L. seeligeri along with the more recently described L. marthii, L. cossartiae, L. farberi, L. immobilis, and L. swaminathanii. The Listeria sensu lato are more distantly related to L. monocytogenes to the extent some have argued for re-classification as novel genera. L. grayi was the first species in the Listeria sensu lato group which now includes 18 species (e.g., L. rocourtiae, L. fleischmannii). Among the 28 currently described Listeria spp., L. monocytogenes remains the only species of public health concern as it is the causative agent of listeriosis, a rare but often severe disease. Listeriosis has one of the longest incubation periods compared to other foodborne disease agents (Fig. 1).

Key factors that contribute to the ability of  L. monocytogenes to persist in food and processing environments include the growth at (i) high salt levels (10% (w/v) NaCl), (ii) refrigeration temperatures (2-4˚C), and (iii) a broad pH range (4.7-9.2). Transmission of this pathogen to humans occurs almost exclusively through the consumption of contaminated food. Ready-to-eat (RTE) foods (e.g., deli meat, cheese), stored for extended periods at refrigeration temperatures, have historically been the common food vehicle implicated in outbreaks. More recently, there have been outbreaks associated with frozen foods, fruits, and leafy greens. Given the often high infective dose needed for disease, some countries (e.g., France, Germany, Netherlands) allow for low-levels of L. monocytogenes (e.g., up to 100 CFU/g). However, other countries (US, Austria, Australia, Italy, New Zealand) have enacted a zero-tolerance policy.

Listeria has been isolated from diverse environments around the world (e.g., soil, water, decaying vegetation) as well as from both domestic (e.g., cattle, sheep) and wild animals (e.g., deer, birds). In the food processing environment, the non-monocytogenesListeria sensu stricto serve a key role as indicators of a potential for an L. monocytogenes contamination given (i) their ability to grow under similar conditions, and (ii) historical association data. To date, the Listeria sensu lato have not shown an association to an increased risk for a L. monocytogenes contamination. Notably, several of the Listeria sensu lato lack the hallmark phenotypic characteristics that historically defined this genus (e.g., no growth at 4˚C, non-motile).