The recovery effect and mechanism of sublethally injured Salmonella Typhimurium by lactic acid

LA-induced SI S. Typhimurium could completely repair in beef, milk, and liquid media. • Microstructure and physiological functions of SI cells regained after recovery. • Surface hydrophobicity and membrane composition slightly altered after recovery. • Multiple stress resistance genes were significantly up-regulated after recovery. Foodborne pathogens can survive in a sublethally injured (SI) state during treatment, which can repair themselves under suitable conditions, posing a potential risk to food safety. In this study, the recovery effects of lactic acid-induced SI Salmonella Typhimurium were investigated in beef, milk, and liquid media. The microstructural and physiological function measurements, as well as transcriptomic analysis were performed to elucidate the recovery mechanism of SI bacteria. The results demonstrated that SI bacteria could completely recover in beef, milk, and liquid media. SI S. Typhimurium gradually restored membrane integrity, as evidenced by the recovery of membrane potential, reduced cytoplasmic and outer membrane permeability, normalized morphology, and restored alkaline phosphatase activity. However, surface hydrophobicity remained slightly higher than the control, and FTIR revealed minor but persistent changes in membrane phospholipids and polysaccharides, indicating adaptive membrane remodeling. Metabolic activity and esterase activity progressively recovered, while the elevated intracellular reactive oxygen species were effectively eliminated, accompanied by down-regulation of antioxidant enzyme genes. DNA repair pathways, particularly homologous recombination, remained persistently activated, likely to ensure long-term genomic stability. Notably, recovered bacteria significantly up-regulated a broad range of stress resistance genes, reflecting a transition to a state of potentially enhanced resistance. These findings demonstrated that the recovery of SI S. Typhimurium was an active adaptive process that not only restored culturability on selective media but also conferred potentially enhanced environmental tolerance.