Bactericidal effects of low-temperature atmospheric-pressure air plasma jets with no damage to plant nutrient solutions

The bactericidal effects of air plasma jets produced with a twisted wire-cylindrical electrode configuration were clarified in terms of plasma-induced damage to plant nutrient solutions. The bacterial suspensions were directly irradiated with air plasma jets using a low gas flow rate, which was shown to significantly inactivate the bacteria suspended in the solutions without reducing the nutrient concentrations. However, the plasma irradiation time required for inactivation depended on the type of bacteria; Escherichia coli (E. coli) and Bacillus subtilis (B. subtilis) were inactivated in 20–30 s, while Staphylococcus aureus (S. aureus) required 7 min. The inactivation of E. coli and B. subtilis decreased with increasing air gas flow rate, whereas the inactivation of S. aureus was independent of the rate. The inactivation could be attributed to a greater number of reactive oxygen and nitrogen species (RONS) from the air plasma jet, including O2 molecules in the feeding gas attaching to the bacterial suspension surface, which do not harm the nutrient components. This can be derived from the results; the air-plasma-jet-activated nutrient solutions (RONS introduced in the solutions) and the N2 plasma jets had only a limited inactivation effect on the bacteria suspended in the solutions.