Salmonella cold adaptation and survival in lettuce
Salmonella enterica is a leading cause of foodborne outbreaks linked to leafy greens, where postharvest conditions strongly influence pathogen persistence. Although refrigeration limits bacterial growth, Salmonella can remain viable for extended periods at low temperatures through metabolic and physiological adaptation strategies. However, the extent to which these cold responses differ among strains remains poorly understood, limiting accurate risk assessment during fresh produce storage. Screening of eleven Salmonella serovars revealed significant variation in their capacity to survive within the lettuce leaf apoplast at 10 deg C. Two isolates with contrasting persistence phenotypes were selected for detailed analysis, and these differences were confirmed at 4 deg C, a representative refrigeration temperature. To explore the genetic basis of this phenotypic variation, we performed RNA-seq transcriptomic profiling under cold stress in nutrient-rich medium. Both isolates grew similarly at 28 deg C but diverged at 4 deg C, with the more persistent strain maintaining higher growth capacity. Transcriptomic responses were strongest shortly after cold shock (0.5 h) and attenuated after prolonged exposure (72 h). Functional enrichment of KEGG pathways and Gene Ontology terms revealed shared repression of central metabolic and energy-producing pathways, coupled with induction of stress-response processes. Notably, the strain exhibiting greater persistence in lettuce showed stronger activation of genes linked to general stress responses and envelope integrity, suggesting isolate-specific physiological strategies that enhance cold tolerance. This study demonstrates that cold adaptation in Salmonella is highly strain-dependent, driven by distinct transcriptional strategies. These findings highlight the importance of intraspecific variation in shaping pathogen survival during postharvest storage and provide insights that can inform more accurate risk assessments for leafy greens.