This study investigated the optimum conditions to maximize the inactivation of biofilms using both plasma-treated solutions and post-treatment storage conditions. Chemical properties of plasma-treated solutions were also analyzed to identify their possible biofilm inactivation mechanisms. Escherichia coli O157:H7 biofilms on stainless steel were prepared and immersed in distilled water (DW), 100 ppm of sodium chloride solution (NaCl), or 100 ppm of sodium hypochlorite solution (NaOCl), followed by container-type plasma treatment for 10 min (15 kHz and 250 W). After plasma discharge was switched off, biofilms immersed in the plasma-treated solution were stored for 10 min with the plasma apparatus closed or open. The log reduction of E. coli O157:H7 was NaOCl (3.58) > NaCl (2.06) = DW (1.95) in the closed storage condition. In addition, the bactericidal effect of NaOCl was higher in the closed condition than in the open condition. Concentrations of the hydroxyl radical (OH·) and peroxynitrite (ONOO−) were also the highest for NaOCl in the closed condition. Consequently, plasma treatment using NaOCl solution in the closed condition was found to be the optimum method for inactivation of E. coli O157:H7 biofilms, due to high hydroxyl radical and peroxynitrite concentrations. This method also has the merit of using a lower concentration of NaOCl than that used typically in industry.

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