Occurrence of Genetic Variants of Listeria monocytogenes Strains

Introduction

S ingle sporadic cases of listeriosis in animals and humans may be caused by two different and unrelated strains (Tham et al., 1999; Tham et al., 2002). However, in the 2000s, there were several reports that isolates of Listeria monocytogenes, saved from outbreaks of listeriosis, cases of sporadic listeriosis, and similar events, did not always belong to a solitary genetic variant. Some of these reports are summarized below.

Orsi et al. (2008) suggest a L. monocytogenes strain persisted at least 12 years in a food-processing plant in Texas. In 1988, the strain caused a sporadic case of listeriosis due to consumption of turkey franks. In 2000, consumption of delicatessen turkey meat, produced at the same plant, was associated with 29 patients with listeriosis. The isolates from 1988 and 2000 differed in one single-nucleotide polymorphism (SNP) in the tRNA Thr-4 prophage and in a large number of SNPs and deletions in the comK prophage. Plasmid loss or gain and phage-mediated genetic changes are suggested by the authors to be the main mechanism in the evolution of L. monocytogenes diversification.

L. monocytogenes isolates from listeriosis outbreaks in the United States in 1998–1999 (hot dogs) and 2002 (turkey delicatessen meats) were subtyped with pulsed-field gel-electrophoresis (PFGE) by Kathariou et al. (2006). Among the L. monocytogenes isolates studied, the Asc I profiles were closely related, and differences only in prophage and internalin-like gene sequences were identified.

In 2008 in Canada, an outbreak of listeriosis occurred due to L. monocytogenes serovar 1/2a in ready-to-eat meat products. PFGE-typing of the epidemic isolates presented two similar, but distinct, Asc I patterns. Sequencing identified the differences as prophage, plasmid elements, SNPs, and indel mutations (Gilmour et al., 2010).

Contaminated cantaloupes caused a listeriosis outbreak in the United States in 2011. L. monocytogenes bacteria with four different PFGE Asc I profiles were isolated from patients, the environment, and fruits. Two of the PFGE profiles were closely related (a difference of two bands) (Laksanalamai et al., 2012; Lomonaco et al., 2013), but were considered variants of the same strain (Laksanalamai et al., 2012): One of the variants may have gained a phage that provided an advantage in surviving.

In a single patient with listeriosis, isolates representing more than one L. monocytogenes genetic variant may appear. Two closely related L. monocytogenes genetic variants (a two-band difference in the DNA profile with PFGE and Asc I) have been identified (Tham et al., 2007) in the same blood sample from a patient with invasive listeriosis. This phenomenon has also occurred in another patient (Tham et al., 2007). In a synovial fluid sample from one individual, three different L. monocytogenes genetic variants were present: The three genetic variants may have evolved from a unique clone (Charlier et al., 2012). Pathogen–host interactions may have different dynamics in different patients, with the pathogen having the opportunity to adapt independently at each site within a specific host. This kind of adaptation will yield genetic variants. The identification of genetic variants requires the study of multiple isolates from a specific outbreak population (Kathariou, 2002).

Also, an expected unique strain can be composed of two genetic variants. In a European PFGE intralaboratory reproducibility test, one laboratory found a difference in one band between duplicate strains (Felix et al., 2012). Even in good-quality profiles, the appearance or disappearance of bands has been observed in a few cases (Felix et al., 2012), although the authors present no explanation.

In Sweden, the most common L. monocytogenes PFGE types (Asc I) from human cases of listeriosis appear in two different but closely related variants (Parihar et al., 2008). When one variant is isolated from one patient, the other closely related variant will eventually be isolated from another patient. As only one isolate from each patient is usually saved, it is unknown whether the two variants are normally present simultaneously in the same patient. However, it is suggested as being possible (Tham et al., 2007; Orsi et al., 2008; Charlier et al., 2012). It is well known that the procedure of picking and subtyping several isolates of L. monocytogenes from platings of a single food item (gravad rainbow trout) can reveal closely related variants (Loncarevic et al., 1996).

Conclusions

In outbreaks—single cases and similar events—the occurrence of more than one L. monocytogenes genetic variant of the same strain may be more common than previously realized. If this is the case, then not only is this important for tracing the routes of infection through molecular typing of isolates, but also even presumably stable unique strains obtained from pure cultures may at times present in more variants.