## 3 Results and Discussion
### Inactivation of E. coli
However, in the case of direct treatment with AALCA, not only long-living RONS could be involved in the bactericidal effect but also short-living RONS (e.g., ONOO\({}^{-}\) and HOO\(\bullet\)), the strong electric field at the stage of corona and repeating spark discharges, and ultraviolet emissions, as it was reported elsewhere [18; 41; 58; 59]. On the other hand, a long-term bactericidal effect produced by long-living RONS in PTW was reported elsewhere, which suggests a strong long-term bactericidal effect of PTW produced by AALCA owing to its high concentration of RONS [31; 32; 43]. To investigate the bactericidal effect of PTW, 5 mL of DI water was treated with AALCA for 1 min at an input power of 28 W. Then, 0.3 mL of the _E. coli_ suspension was mixed with 2.7 mL of the produced PTW and incubated for various times. After incubation, the number of surviving bacteria was estimated using the colony counting method. The estimated values of _E. coli_ after various durations of incubation are presented in Figure 9. It could be clearly observed that the concentration of _E. coli_ gradually decreased as the incubation time increased after suspension of _E. coli_ in DI water treated with AALCA and reached the detection limit of 10 CFU/mL at 60 min, showing a 6-log reduction in the concentration of bacteria. Considering that the concentration of _E. coli_ did not change in the control sample of _E. coli_ suspended in pristine DI water even after 24 h of incubation, it could be concluded that the DI water treated with AALCA had a strong bactericidal effect that originated from the high concentration of RONS and the low pH level [31; 32; 43]. Treatment of liquid using AALCA could be performed irrespective of its electrical conductivity; therefore, further reduction of production costs is possible by using tap or spring water. Tap water was treated with AALCA under the same conditions and the bactericidal effect of plasma-treated tap water was investigated in the same way as for DI water. It can be clearly observed in Figure 9 that tap water treated by AALCA also had a bactericidal effect, and the concentration of bacteria reached the detection limit of 10 CFU/mL after 90 min, showing a 6-log reduction in the concentration of bacteria. Considering that the concentration of bacteria in the control sample of E. coli suspended in pristine tap water did not change after 90 min of incubation and reduced by 1 log only after 24 h of incubation, it can be concluded that, like with DI water, tap water treated with AALCA has a strong bactericidal effect that originates mostly from the high concentration of RONS. The difference in the bactericidal effect can be explained by the complex composition of tap water. Some components of tap water may interfere with chemical reactions in PTW and reduce the amount of species responsible for the bactericidal effect. Additionally, the pH value of the tap water was higher than that of DI water (Figure 4), which may have reduced the bactericidal efficacy of tap water treated with AALCA as compared with DI water [33; 43]. The observed bactericidal effect of both DI and tap water, together with its low production cost, makes PTW produced by AALCA appear to be promising as a low-cost disinfectant which could be used in biomedical applications. However, the composition and properties of tap water can vary strongly depending on the area of use. Therefore, more comprehensive investigations are needed for the practical use of plasma-treated tap water in various locations.