Listeria environmental sampling tests are compatible with polymorphic locus sequence typing.

Abstract

Food processors invest significant resources into environmental sampling to detect contamination with potential pathogens, particularly Listeria monocytogenes. 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 species 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. Five Listeria ESTs were tested here: broth-based InSite (Hygiena), Path-Chek (Mericon), and Pathfinder (Hardy Diagnositics); and gel-based Petrifilm (3M) and HardyChrom (Hardy Diagnostics). ESTs were inoculated with strains representing two common L. monocytogenes serotypes and nonpathogenic Listeria innocua. 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 and LisMT2. 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. PRACTICAL APPLICATION: Multiple environmental sampling tests have been commercialized in recent years to facilitate the proactive detection of pathogens, particularly Listeria monocytogenes, within food processing facilities. Coupling a positive detection test with strain typing would enhance its value by providing data that can be used to track pathogen spread through a facility, identify harborage sites, and distinguish sporadic from resident contamination.