Enteric pathogens, including Salmonella, are capable of long-term survival after desiccation and resist heat treatments that are lethal to hydrated cells. The mechanisms of dry-heat resistance differ from those of wet-heat resistance. To elucidate the mechanisms of dry-heat resistance in Salmonella, screening of the dry-heat resistance of 108 Salmonella strains, representing 39 serotypes, identified the 22 most resistant and the 8 most sensitive strains for comparative genome analysis. A total of 289 genes of the accessory genome were differently distributed between resistant and sensitive strains. Among these genes, 28 proteins with a putative relationship to stress resistance were selected for to quantify relative gene expression before and after desiccation and expression by solid-state cultures on agar plates relative to cultures growing in liquid culture media. Of these 28 genes, 15 genes were upregulated (P < 0.05) after desiccation or by solid-state cultures on agar plates. These 15 genes were cloned into the low-copy-number vector pRK767 under the control of the lacZ promoter. The expression of 6 of these 15 genes increased (P < 0.05) resistance to dry heat and to treatment with pressure of 500 MPa. Our finding extends the knowledge of mechanisms of stress resistance in desiccated Salmonella to improve control of this bacterium in dry food. IMPORTANCE This study directly targeted an increasing threat to food safety and developed knowledge and targeted strategies that can be used by the food industry to help reduce the risk of foodborne illness in their dry products and thereby reduce the overall burden of foodborne illness. Genomic and physiological analyses have elucidated mechanisms of bacterial resistance to many food preservation technologies, including heat, pressure, disinfection chemicals, and UV light; however, information on bacterial mechanisms of resistance to dry heat is scarce. Mechanisms of tolerance to desiccation likely also contribute to resistance to dry heat, but this assumption has not been verified experimentally. It remains unclear how mechanisms of resistance to wet heat relate to dry-heat resistance. Thus, this study will fill a knowledge gap to improve the safety of dry foods.
Keywords: Salmonella enterica; comparative genomics; desiccation tolerance; dry-heat resistance; high pressure; oxidative stress; pathogen.