Multilocus Sequence Typing of Outbreak-Associated Listeria monocytogenes Isolates to Identify Epidemic Clones

Objective: Listeria monocytogenes is a foodborne pathogen found in a wide variety of environments. Subtype characterization of L. monocytogenes isolates from listeriosis outbreaks that have occurred over the last three decades has suggested that a number of outbreaks were caused by a small number of L. monocytogenes epidemic clones (ECs). In this study we compared the prevalence, ecology, and phylogenetic position of outbreak-associated isolates and non-outbreak-associated isolates to probe the evolutionary and ecological characteristics of outbreak-associated L. monocytogenes subtypes, including those representing previously described ECs. Methods: Multilocus sequence typing data for isolates from 15 listeriosis outbreaks in Europe and North America were generated and compared, using a phylogenetic framework, with 180 isolates representing a local sampling of diverse sources, including human sporadic cases. Results: Isolates from 15 listeriosis outbreaks represented eight sequence types (STs). STs corresponding to previously designated ECI (ST1 and ST93) and ECIa (ST29) represented isolates from eight outbreaks. ST17 (corresponding to ECII) was involved in two outbreaks in the United States (1998 and 2002). No other STs were involved in multiple outbreaks. While ST1 was the most common ST among sporadic human cases and non-human listeriosis-related isolates, ST29 was rare among non-human listeriosis-related isolates and was significantly overrepresented among isolates from human listeriosis outbreaks and sporadic cases as compared to isolates from other sources in our local sampling. Conclusions: STs associated with outbreaks (and representing previously designated ECs) appear to differ in their ecology. While association of ECI with multiple human listeriosis outbreaks appears to reflect strain abundance across environments, ECIa seems to represent an L. monocytogenes EC that appears to be overrepresented among outbreaks and sporadic cases and thus may have increased transmission potential.