The Inactivation of E. coli and B. subtilis Spores Treated Individually, Sequentially and Simultaneously with Humified Air Dielectric Barrier Discharge Plasma and Ultraviolet

The objective of this study was to evaluate the efficacy of humified air dielectric barrier discharge cold plasma (CP) and ultraviolet (UV) at a wavelength of 254 nm in inactivating Escherichia coli (E. coli) and Bacillus subtilis spores (B. subtilis spores). The experimental results showed that simultaneous treatment with CP and UV had the highest antimicrobial activity, followed by sequential and individual treatments. Individual treatment of CP and UV for 10 s decreased E. coli by 2.4 Logs and 1.3 Logs, respectively. After 60 s of CP and UV treatment, B. subtilis spores were decreased by 2.6 Logs and 1.1 Logs, respectively. Simultaneous treatment of CP and UV for 10 s reduced E. coli by 4.6 Logs and B. subtilis spores by 4.4 Logs after 60 s, which was attributed to their synergistic effects. To elucidate the mechanism of protein oxidation in simultaneous treatments, we investigated the chemical stability of simultaneous treatments with CP and UV on 11 amino acids and 4 nucleobases in aqueous solution. Phenylalanine (Phe), methionine (Met), tyrosine (Tyr), tryptophan (Trp), and histidine (His) were oxidized by plasma-generated reactive oxygen and nitrogen species. In addition, guanine (G) and thymine (T) exhibited structural instability; both the five- membered and six-membered rings of guanine can be oxidized, and thymine undergoes oxidative cross-linking by UV-induced formation of thymine dimers. The analysis showed that the chemical instability of amino acids and nucleobases was closely related to the synergistic inactivation effect of CP and UV.