Characterizing variations in resistance of Serratia species to sodium hypochlorite: Cell membrane and oxidative stress responses

Sodium hypochlorite (NaOCl) has widely used in food processing to control microbial contamination. However, the resistant variations among strains leads to incomplete sterilization and poses potential food safety risks, which remains poorly understood. This study investigated the cellular mechanisms underlying the variations in resistance among three Serratia strains (5S7, AP6, and F5) isolated from a fresh chicken production line, which were selected based on MIC determination, time-kill curves evaluation and growth recovery curves analysis. The results demonstrated that the most chlorine-resistant strain (5S7) exhibited the lowest sterilization efficiency and the strongest recovery ability after 50 mg/L NaOCl. Meanwhile, 5S7 showed lower membrane permeability and fluidity, higher membrane hydrophobicity compared with others post-NaOCl treatment. Flow cytometry (FCM) analysis revealed that the F5 (with the weakest resistance) had the highest damage to membrane integrity. Furthermore, based on the measurement of intracellular reactive oxygen species (ROS) levels and the activities of antioxidant enzymes (SOD and CAT) in strains exposed to NaOCl, this study revealed that the enzyme activities in the most resistant strain 5S7 were less impaired compared with those in the less resistant one. Meanwhile, the degree of lipid peroxidation in the 5S7 membrane is even lower. These findings highlight the significant intragenus chlorine-resistant variations, offering key insights into strain-specific resistance mechanisms and guiding optimized disinfection for food safety.