New Listeria Species - What do they Mean for Detection Methods

The genus Listeria has considerably expanded since 2009 to now include 27 species with diverse phenotypic and genotypic characteristics. Comparative characterization of all current Listeria species clearly indicates that the current Listeria species represent two distinct groups, including (i) Listeria sensu stricto, which includes Listeria monocytogenes, as well as closely related species that behave similar to L. monocytogenes (such as L. innocua, L. ivanovii, L. seeligeri, L. welshimeri, L. marthii) and (ii) Listeria sensu lato, which includes species that are phenotypically and genotypically distinct from L. monocytogenes (this group includes Listeria grayi as well as the majority (16/21) of the species newly described since 2009, such as L. booriae).

To clearly differentiate Listeria sensu lato from Listeria sensu stricto, reclassification of the Listeria sensu lato species into three different genera (Murraya, Paenilisteria, Mesolisteria) was proposed in 2016 (Orsi et al., 2016); the genus name Murraya had previously been proposed for L. grayi. Importantly, many Listeria sensu lato species show phenotypical characteristics that are different from those typical for Listeria sensu stricto. For example, most Listeria sensu lato species lack flagellar motility, and a distinct subset of Listeria sensu lato species (proposed to represent the new genus Mesolisteria) lacks the ability to grow at temperatures below 7 °C. Hence detection of Listeria sensu lato does not necessarily provide an indication of processing plant conditions that would facilitate L. monocytogenes presence or persistence.

In addition, some of the Listeria sensu lato species (including, but not limited to, L. grayi) show substantially reduced growth in some of commonly used Listeria enrichment media. Consequently, methods used by industry for Listeria spp. detection (“Listeria species assays”) should be validated to detect all members of Listeria sensu stricto. There however is no need for these assays to detect Listeria sensu lato species (including L. grayi, which appears to be a specific “hot button” issue for some, most likely because it is the only Listeria sensu lato species reported before 2009).

It is, however, recommended that laboratories and test kit manufacturers provide information on which Listeria species (sensu lato and sensu stricto) are detected and are not detected with a given kit (or which may be detected with reduced sensitivity, for example because of slow growth in enrichment media). This information is important to allow users to trouble shoot potential inconsistent results, which may, for example, occur if L. grayi or another Listeria sensu lato species (e.g., L. booriae) is detected by a rapid method, but not by a confirmation that uses standard cultural methods. This information is also important as assays that only detect Listeria sensu stricto species would allow end users to narrow the scope of their environmental monitoring program to target the Listeria sensu stricto species, which are the most appropriate indicator (index organism) that indicate that potential for conditions that may increase the risk of L. monocytogenes contamination or persistence.

> DOWNLOAD the table: Performance of bioMérieux Methods for the Detection and Identification of Listeria spp. (sensu stricto, sensu lato)

> Read the general article on Listeria spp. and Listeria monocytogenes

References
- Orsi, R., and M. Wiedmann. 2016. Characteristics and distribution of Listeria spp., including Listeria species newly described since 2009. Applied Microbiology and Biotechnology 100: 5273-5287.
- Carlin, C., S. Roof, and M. Wiedmann, 2022. Assessment of reference method selective broth and plating media with 19 Listeria Species highlights the importance of including diverse species in Listeria method evaluations J. Food Prot. 85:494-510. doi: 10.4315/JFP-21-293.. doi: 10.4315/JFP-21-293
- Carlin, C. R., J. Liao, L. K. Hudson, T. L. Peters, T. G. Denes, R. Orsi, X. Guo, and M. Wiedmann. 2022. Soil collected in the Great Smoky Mountains National Park yielded a novel Listeria sensu stricto species, L. swaminathanii. mSpectrum 10(3):e0044222. doi: 10.1128/spectrum.00442-22

by Martin Wiedman

Gellert Family Professor in Food Safety, Food Science, Cornell University

The overall goal of Martin's academic program is to develop and communicate the scientific knowledge needed to prevent and control foodborne and zoonotic diseases caused by bacteria as well as microbial food spoilage. With his training as a both a veterinarian and food scientist, Martin's programs focus on a comprehensive and interdisciplinary farm-to-table approach to food safety and quality. Martin also serves as co-director of the New York State Integrated Food Safety Center of Excellence.

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