In the last two decades a growing interest has been shown in the investigation of atmospheric pressure plasma jets (APPJs) that operate in contact with liquid samples. In order to form a complete picture about such experimental systems, it is necessary to perform detailed diagnostics of plasma jets, as one step that will enable the adjustment of system properties for applications in different areas. In this work, we conducted a detailed electrical characterisation of a plasma system configuration used for water treatment. A helium plasma jet, with a pin electrode powered by a continuous sine wave at a frequency of 330 kHz, formed a streamer that was in contact with a distilled water sample. An electrical circuit allowed the monitoring of electrical signals supplied to the jet and also to the plasma itself. An electrical characterisation together with power consumption measurements was obtained by using two different methods. The first method was based on the direct measurements of voltage and current signals, while in the second method we used ‘Lissajous figures’. We compared these two methods when used for discharge power estimation and addressed their advantages and limitations. The results showed that both of these methods could be used to successfully determine power consumed by a discharge in contact with water, but only when taking into account power dissipation without plasma.

The scope of the antibacterial effects of plasma-activated water (PAW) is not yet fully comprehended. We investigated the activity of PAW produced by the in-house 3-pin atmospheric pressure plasma jet against carbapenem-resistant Klebsiella pneumoniae and vancomycin-resistant Enterococcus faecalis, with a focus on PAW’s potential to promote susceptibility to conventional antibiotics in these bacteria. Bacterial inactivation was determined by the colony count after 15 and 60 min PAW treatments. Minimum inhibitory concentrations (MICs) measured following repeated exposures to PAW across multiple generations of bacteria enabled the assessment of changes in susceptibility to antibiotics. The PAW’s efficacy was also analyzed through the detection of intracellular reactive oxygen and nitrogen species in treated bacteria. Time-dependent significant inactivation efficiency against K. pneumoniae was observed (log reduction 6.92 ± 0.24 after 60 min exposure), while effects on E. faecalis were limited. PAW demonstrated potential to decrease the MICs of crucial antibiotics. Namely, a 50 to 62.5% decrease in the MICs of colistin against K. pneumoniae and a 25% reduction in the MICs of vancomycin against enterococci were recorded. We found a significant increase in the superoxide anion concentration in K. pneumoniae and E. faecalis cells after PAW treatments. This study indicates that PAW’s inactivating efficacy coupled with the capacity for the potentiation of antibiotic effects is a promising combination against multidrug-resistant bacteria.