Physicochemical properties of plasma-activated water and associated antimicrobial activity against fungi and bacteria

Abstract

Plasma-activated water (PAW), generated through Surface Dielectric Barrier Discharge, was tested against microbial contaminants. We assessed how the time of exposure to plasma treatment and the gas flow rate impact the chemical composition of PAW, and, in turn, how it influences these factors influence its efficacy against microorganisms. The effectiveness of PAW treatments was evaluated against the fungal phytopathogen Botrytis cinerea and both pathogenic (Xanthomonas campestris pv. vesicatoria) and beneficial (Bacillus amyloliquefaciens) bacteria. The physicochemical properties of PAW were assessed as the concentration of reactive species, namely, NO₃^- and NO₂^- and H₂O₂, pH, and oxidation-reduction potential. Higher levels of reactive species and lower pH during longer treatments were associated with greater antimicrobial efficacy. A correlation study and Principal Component Analysis demonstrated that the RONS content in PAW affected antimicrobial activity, with stronger correlations between NO₂^-, H₂O₂, and fungal inhibition, as well as between NO₂^- and bacterial inhibition. Almost complete inhibition was reached after 1 min of treatment for bacteria (log reduction of 4.5 for B. amyloliquefaciens and 5.1 for X. campestris) and after 3 min of treatment for B. cinerea (approximately 90% inhibition of conidial germination). The obtained results contribute to defining optimized treatment conditions using PAW for antimicrobial decontamination of plant products.

Keywords: Fungal and bacterial pathogens; Low-temperature plasma; PAW; Reactive species; SDBD.

Fig. 1

Typical electric characteristics of the SDBD discharge loaded with water during a voltage burst in terms of (a) applied voltage; (b) plasma current; (c) developed charge; (d) charge-voltage Lissajous figure. In panel a) the positive half cycles 3 (PHC3) and 19 (PHC19) are shown.

Fig. 2

Typical emission spectra of the second positive system SPS(0,0) transition of molecular nitrogen at the beginning [third positive half-cycle (PHC3)] and at the end [nineteenth positive half-cycle (PHC19)] of the voltage burst. The fitting of the spectra is also shown, along with the simulated emissions at different temperatures, in an empty reactor (a) and a reactor filled with 4 mL of water (b).

Fig. 3

The typical concentration of RONS found in PAW produced by 10 s to 5 min air plasma water treatment. The numbers on top of the bars represent the maximum potential plasma doses (D_Max), expressed in J/g_water.

Fig. 4

Mean values of pH and ORP in the PAW obtained after plasma exposure of water ranging from 10 s to 5 min, fed by a 1 slm airflow. The corresponding maximum potential plasma dose for all treatment times is reported in dark yellow.

Fig. 5

Inhibition of germination of B. cinerea conidia caused by PAW obtained after different plasma application times. The correlation coefficient (r) between the two parameters is shown. The level of statistical significance is marked as “***’’ for p < 0.0001, “**” for p < 0.001, “*” for p < 0.01, and as “ns’’ when any statistically significant difference was recorded.

Fig. 6

Inhibition of germination of B cinerea conidia as a function of RONS concentration: (a) hydrogen peroxide; (b) nitrite ions; (c) nitrate ions. In each panel, one reactive species is plotted on the abscissa axes, while the size and colour of the dots represent the other two. It was possible to obtain a sigmoidal fit using a dose response function only in panels (a) and (b) to more clearly visualize possible threshold values of H₂O₂ and NO₂^- for fungal spore inhibition.

Fig. 7

Inhibition of B. amyloliquefaciens and X. campestris pv. vesicatoria caused by PAW obtained after different plasma application times. The correlation coefficient (r) between the two parameters is shown. The level of statistical significance is marked as “***’’ for p < 0.0001, “**” for p < 0.001, “*” for p < 0.01, and “ns’’ when any statistically significant difference was recorded.

Fig. 8

Biplot of the component loadings and scores of the first two principal components (PC1 and PC2) for the analysis performed for (a) the fungus B. cinerea (SAS56); (b) the two bacteria B. amyloliquefaciens (MBI600) and X. campestris pv. vesicatoria (XAN38).

Fig. 9

Visual representation of the SDBD reactor: (a) Three-dimensional illustration of the reactor; (b) SDBD under operating conditions; (c) schematic representation of the experimental setup.