Plasma Sterilization for Bacterial Inactivation: Studies on Probable Mechanisms and Biochemical Actions

The underlying mechanisms and biochemical actions responsible for inactivation of pathogenic gram-positive Staphylococcus aureus (S. aureus) and gram-negative Salmonella abony (S. abony) bacteria upon exposure to sub-atmospheric plasma has been investigated. Reduction in colony forming units of the bacteria is established in 60 min and 40 min for S. aureus and S. abony respectively via 6-log reduction curves. The percentage change in reactive oxygen species, such as ^•OH and H₂O₂ formed on bacterial membrane during plasma exposure are analysed using spectroflurometer. S. aureus exhibited a significant increase of 324.23% and 1554.84% in ^•OH and H₂O₂ radicals respectively. Whereas, 98.14% and 54.49% increase in ^•OH and H₂O₂ radicals respectively was observed in S. abony. The oxidation and degradation of DNA is analysed using an ultra violet visible spectrophotometer. The leakage of proteins, lipids, and nucleic acid molecules due to plasma exposure is studied by Attenuated Total Reflectance Fourier-transform infrared spectroscopy (ATR-FTIR). The alteration of secondary protein structure on the cell membrane is observed using Circular Dichroism. Upon exposure to plasma, S. aureus shows a secondary protein structural transition from α-helix (2.4%), β-sheet (78.3%) mixture to modified β-sheet structure (0% α-helix, 79.1% β-sheet). Whereas, S.abony shows a transition from α-helix (1%), β-sheet (64.9%) mixture to modified β-sheet structure (0% α-helix, 74.5% β-sheet). The bacterial morphological study (swelling/shrinking) done using Field Emission Scanning Electron Microscopy (FE-SEM) reveals the deformation of cell membrane. Above findings pave the way for a better understanding of the processes of antimicrobial inactivation strategies when the plasma sterilization process is employed.