Effects of acid, alkaline, cold, and heat environmental stresses on the antibiotic resistance of the Salmonella enterica serovar Typhimurium

Antibiotic resistance in Salmonella enterica serovar Typhimurium (S. ser. Typhimurium) has become a critical safety hazard in food. Sublethal environmental stresses can influence resistance in Salmonella during food processing. This study simulated environmental stresses in food processing. The antibiotic resistance of three strains of S. ser. Typhimurium (the ATCC 14028 strain and two wild-type isolates from chicken and pork product processing) was evaluated under different pH levels (5.0, 5.5, 6.0, 8.0, and 9.0). Also, dynamic changes in resistance with treatment duration under cold (4 degrees C, -20 degrees C) and heat (55 degrees C) treatment were studied. The results showed that acid and alkaline stresses reduced the resistance of S. ser. Typhimurium to eight antibiotics; meanwhile, the resistance of meropenem (MERO) increased. The minimal inhibitory concentration (MIC) of MERO was increased 16- to 64-fold. With acid or alkaline stress, the extracellular ATP content increased, and the scanning electron microscopy (SEM) result clearly revealed the appearance of wrinkles and holes on the outer membrane of Salmonella. These observations imply changes in membrane permeability, which may decrease the antibiotic resistance of Salmonella. Cold or heat stress increased the resistance of S. ser. Typhimurium to tetracycline, cefotaxime, ceftazidime, nalidixic acid, azithromycin, and ampicillin; the MIC increased 2- to 4-fold. The antibiotic resistance only changed when cold and heat stresses occurred over a certain period of time and remained unchanged when the stress persisted. This study reports on the ability of S. ser. Typhimurium to develop antibiotic resistance after environmental stresses. It can provide valuable information for meat processing to improve interventions and risk management.