NATURE VERSUS NURTURE ASSESSING THE IMPACT OF GENOTYPE VERSUS PHENOTYPE UNDER ENVIRONMENTAL STRESSES IN FOODBORNE PATHOGENS

Abstract

The landscape of food microbial safety has changed dramatically in the genomic era. Modern sequencing technologies are more precise and provide higher resolutions of bacterial genomes than previous molecular subtyping methods. We can identify the virulence, antimicrobial resistance, and other stress related genes that an isolate may possess as well as better establish phylogenetic relationships among isolates. Ideally, a genotype (e.g., gene presence or absence) can predict an organism’s phenotype; however, environmental cues transiently shape pathogens behavior as well. This work takes steps forward on this nature versus nurture dilemma by developing microbial risk assessment methods which balance relevant pre-growth conditions with strain diversity and by associating sanitizer tolerance genes with fitness of persistent isolates. Challenge and validation studies rely on accurate predictions of a target pathogen’s behavior. Key foodborne pathogens including, Salmonella enterica, Escherichia coli and Listeria monocytogenes that were pre-grown under seven different conditions were assessed for their (i) survival of a peroxyacetic acid intervention and (ii) growth and survival on produce. The findings suggest that risk studies need to consider a shift from multi-strain to multi-growth conditions to capture a more accurate range of possible phenotypic responses. In this work, it was also investigated if persistent L. monocytogenes isolates harbor tolerance genes that contribute to their survival in a food processing plant. A longitudinal set of L. monocytogenes isolates collected over 18 years were genotypically characterized. In specific, genes that were identified and phenotypically assessed included sanitizer tolerance genes (bcrABC, qacH), stress tolerance genes against acidity and salt stress (SSI-1), as well as oxidative stress (SSI-2), and heat stress (clpL). Importantly, this work showed that only sanitizer tolerance genes conferred a growth advantage when exposed to low sanitizer concentrations. In contrast, presence of stress tolerance genes did not correlate with superior growth or survival under the evaluated stress conditions.