Listeria monocytogenes: Strain Heterogeneity, Methods, and Challenges of Subtyping

Abstract

Listeria monocytogenes is a food-borne bacterial pathogen that is associated with 20% to 30% case fatality rate. L. monocytogenes is a genetically heterogeneous species, with a small fraction of strains (serotypes 1/2a, 1/2b, 4b) implicated in human listeriosis. Monitoring and source tracking of L. monocytogenes involve the use of subtyping methods, with the performance of genetic-based methods found to be superior to phenotypic-based ones. Various methods have been used to subtype L. monocytogenes isolates, with the pulsed-field gel electrophoresis (PFGE) being the gold standard. Although PFGE has had a massive impact on food safety through the establishment of the PulseNet, there is no doubt that whole genome sequence (WGS) typing is accurate, has a discriminatory power superior to any known method, and allows genome-wide differences between strains to be quantified through the comparison of nucleotide sequences. This review focuses on the different techniques that have been used to type L. monocytogenes strains, their performance challenges, and the tremendous impact WGS typing could have on the food safety landscape.

Whole genome sequence-based serogrouping of Listeria monocytogenes isolates

“…The high case-fatality rate of 20% to 30% makes L. monocytogenes a leading cause of food-borne related human mortality (Nyarko & Donnelly 2015).…”

“…For practical and traditional purposes, serotyping is still the first level response subtyping method in public health laboratories, despite its limited value for tracking isolates (Nyarko & Donnelly 2015, Doumith et al, 2004 due to the low discriminatory power of the method, the insufficient reproducibility and antigen sharing between serotypes, respectively (Schönberg et al, 1996). Testing with antisera sets is time-consuming and demanding.…”

Quantitative and ecological aspects of Listeria monocytogenes population heterogeneity

“…Many food products on the other hand, are known to increase heat resistance of microorganisms [45,52]. Validating the current chain model in laboratory medium and to take all the different variability factors into account was already challenging; translating such a model to actual food matrices will be even more demanding.…”

“…Mechanisms of bacterial persister formation are not well understood as they are small in numbers and transient and can change with the type of stress and environment. The switch from normal growth to persistence and vice versa is stochastic and epigenetic in nature [52]. Recent findings have shown that a variety of environmental conditions can induce the formation of persister subpopulations, including starvation, carbon source transitions, DNA damage-induced SOS response, and exposure to antibiotics [53,54].…”

“…Although the phenotypes of ctsR and rpsU variants are very different from the WT, they can be genetically very similar to the WT and more similar than can be discriminated by most of the currently used molecular typing methods. Recent advances and increased accuracy in whole genome sequencing (WGS) will undoubtedly lead to further unravelling of the genetic diversity within L. monocytogenes populations [52]. Deeper sequencing with higher resolution and linking genomic information to phenotypic information will allow for faster screening for potentially stress resistant phenotypes within processing plants [53].…”