Recovery mechanisms of ohmic heating-induced sublethally injured Staphylococcus aureus: Changes in cellular structure and applications in pasteurized milk

This study investigated the alterations in cellular structure of ohmic heating (OH)-induced injured Staphylococcus aureus ( S. aureus ) during repair in nutrient broth, and the potential antibacterial approach of combining OH with food bacteriostatic agents and their application in milk. Specifically, the biological properties (surface hydrophobicity, zeta potential, particle size) and indicators related to cell membrane and cell wall integrity (peptidoglycan content, activities of β-galactosidase, alkaline phosphatase, and lactate dehydrogenase) of injured S. aureus during repair were measured. Additionally, scanning electron microscope and transmission electron microscope analyses were used to observe morphological changes, and protein changes were analyzed using fourier transform infrared spectroscopy and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The results suggested that OH destroyed the biological properties and membrane structure (membrane integrity and permeability increased, cell wall integrity increased, protein and functional group of membrane were damaged) of S. aureus , which could be partially restored after repair. However, the restoration process between cell membrane and cell wall were inconsistent, and morphological changes reinforce these findings. Notably, membrane proteins and functional groups on the bacterial cell surface were difficult to repair. Also, OH combined with low doses of bacteriostatic agents is a potential method to ensure food safety. Carvacrol and low temperatures could inhibit the repair of injured bacteria in pasteurized milk; and reduce precipitate formation and putrid gas production.