Characterization of a Pathogen Strain Collection to Allow for Improved Validation of Sanitizer Efficacy in the Produce Industry

Introduction:  Effective control of foodborne pathogens on produce and in produce associated environments requires science-based validation of interventions and control strategies. It has previously been shown that strains and/or genetic lineages of a pathogen may differ in their ability to survive different stress conditions. We hypothesize however that pregrowth conditions have a significantly greater effect on subsequent stress survival and stress response than genetic diversity within produce relevant pathogens, including Salmonella, Listeria monocytogenes, and shiga toxin-producing Escherichia coli (STEC).

Purpose: To provide a produce associated strain collection for challenge and validation studies. To provide data if multistrain or multigrowth condition best capture the range of phenotypic responses to a given challenge.

Methods:  A proposed strain collection was evaluated by experts for inclusion in the study. All isolates in the final collection were characterized by whole genome sequencing. A subset of five selected strains were grown under eight different stress conditions (e.g., low temperature, high salt, oxidative stress) and exposed for 45 s to 40 ppm of peroxyacetic acid, whereas strains’ ability to survive intervention treatments was determined by reduction of pathogen number.

Results:  The finalized collection is comprised of 23 Salmonella strains, 11 L. monocytogenes strains, 13 STEC strains, and 8 surrogate, indicator and index organisms. With three exceptions, whole genome sequencing data confirmed serotype and lineage classification of the selected isolates. Salmonella strains selected for sanitizer treatment show higher variance of log reduction based on pregrowth conditions (e.g., log reduction of 3.6 to 7) as compared to strain diversity (e.g., log reduction of 3.4 to 5).

Significance:  These data, along with the strain collection, will facilitate selection of appropriate strains and growth conditions for challenge studies. The study will provide a resource to scientifically justify using specific strain sets or cocktails or using strains grown under specific conditions prior to challenge or validation experiments.