Mechanistic insights into Salmonella growth kinetics under solute-induced water activity stresses via transcriptomics...

In food preservation, regulating water activity (a_w) via solutes is a core strategy to inhibit microbial growth. However, its efficacy depends on causal regulation between solute properties, the water status of the matrix, and microbial responses. The differences in regulatory mechanisms among sodium chloride, sucrose, and glycerol require systematic analysis. This study integrates Gompertz modeling, the Norrish equation, transcriptomics, and NMR to reveal how these solutes induce growth differences in S. Enteritidis through a "direct stress-matrix water regulation" synergy that triggers bacterial adaptation. Results show S. Enteritidis' minimum growth a_w differs by solute: 0.96-0.95 (sodium chloride), 0.97-0.96 (sucrose), 0.93-0.92 (glycerol). The Norrish equation confirmed that sucrose has the strongest a_w-lowering ability (k_N = -6.43), while sodium chloride (k_N = -1.36) and glycerol (k_N = -1.01) were less effective. Gompertz kinetics at a_w =0.98 show the sucrose-treated group has a sharply reduced maximum growth rate, prolonged lag phases; the sodium chloride-treated group grows best, and glycerol grows moderately. Transcriptomic and NMR analyses revealed the mechanism: the ionic nature of sodium chloride maintained high free water mobility and distinct three-peak separation (reflecting water status) via ionic hydration. This environment physically supported the activation of osmoprotection and ion efflux pathways (biological response) to maintain efficient metabolism. Sucrose, through strong hydrogen bonding, caused the aggregation of free and bound water peaks and resulted in the highest proportion of semi-bound water (7.1%). This physically forced the inhibition of energy metabolism and oxidative stress pathways, exacerbating growth restriction. Glycerol, through moderate hydrogen bonding, retained a high free water proportion (97.2%) with intermediate mobility, which provided the physiological basis for the activation of metabolic homeostasis pathways to enable adaptive growth. This study clarifies that microbial growth differences stem from solutes' direct stress-matrix water regulation causal regulation, inducing dynamic biological adaptation. These findings provide a theoretical basis for the precise regulation of a_w in food preservation.