Compatibility of polymorphic locus sequence typing with Listeria and Salmonella environmental sampling tests

Authors: EDLIND, TOM - Microbitype Llc; Liu, Yanhong

Submitted to: Foodborne Pathogens and Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/5/2021
Publication Date: N/A
Citation: N/A

Interpretive Summary: Listeria monocytogenes and Salmonella species are important foodborne pathogens that cause a large number of cases of human illness. Since these pathogens can cause sporadic or persistent environmental contamination in food processing facilities, multiple environmental sampling tests (ESTs) have been developed to detect contamination with these pathogens. In this paper, strain typing methods were developed for analysis of presumptive-positive ESTs to track down sources of contamination. Results showed that the strain typing methods for L. monocytogenes and Salmonella are reliable, robust, and highly discriminatory, able to accurately distinguish strains of the two pathogens. These methods allow food processors to rapidly and accurately monitor and track down sources of contamination in processing facilities to prevent contaminated food from reaching the consumer.

Technical Abstract: Food processors invest significant resources into environmental sampling to detect contamination with potential pathogens, particularly Listeria monocytogenes and Salmonella enterica. To facilitate these efforts, multiple environmental sampling tests (ESTs) have been developed and commercialized that minimize workload, turnaround time, and cost while providing convenient colorimetric detection. For presumptive-positive ESTs, we hypothesized that a relatively minor additional investment could provide, in addition to pathogen confirmation, valuable strain typing data for tracking pathogen spread through a facility, identifying harborage sites, and distinguishing sporadic from persistent or resident contaminants. This hypothesis is based on the demonstrated compatibility of polymorphic locus sequence typing (PLST) with crude samples including food enrichments. Four ESTs were tested here: broth-based InSite (Hygiena), Path-Chek (Mericon), and Pathfinder (Hardy Diagnostics); and gel-based Petrifilm (3M). ESTs were inoculated with strains representing two common L. monocytogenes serotypes, non-pathogenic Listeria innocua, and six S. enterica serotypes. Following incubation, broths or suspended colonies were heat-treated to inactivate bacteria. Lysates or purified DNAs were prepared and used as templates in PCRs targeting the previously described PLST loci LmiMT1, LisMT2, or SeMT1. Single clear products were obtained from all inoculated ESTs; uninoculated controls were negative. PCR products were subjected to Sanger sequencing, yielding high quality chromatograms. Phylogenetic analysis confirmed identities to previously determined sequences, and revealed relatedness to serotype-matched strains represented in GenBank databases.