Plasma-activated water tests show concentration matters for effective bacterial disinfection

Researchers at EPFL examined how Escherichia coli bacteria behave when exposed to varying concentrations of plasma-activated water (PAW). PAW is created by generating reactive oxygen and nitrogen species through low-temperature plasma exposure, which gives the water antimicrobial properties. The study, published in npj Clean Water, employed two techniques: colony-forming unit (CFU) counting to measure bacterial population reduction, and nanomotion sensing to monitor real-time bacterial activity.

The findings demonstrated a biphasic response in E. coli: at a 62.5% concentration of PAW, the viable bacterial count was approximately twice that of the control group. However, at higher concentrations, the bacteria were inactivated, resulting in an 8-log reduction compared to the initial population. Nanomotion measurements corroborated this pattern, indicating increased bacterial activity at intermediate PAW concentrations before a sharp decline at higher concentrations.

This suggests that the interaction between bacteria and PAW is not a straightforward dose-response relationship. At sublethal concentrations, bacteria exhibited an enhanced physiological response, which could be attributed to a stress-adaptation or hormesis-like effect. However, the researchers emphasize that PAW can also alter the chemical composition of nutrient media, potentially through oxidation of organic compounds or changes in pH and redox conditions.

These modifications could influence bacterial behavior and are an area requiring further investigation.

The study underscores the importance of controlling PAW concentration when utilizing plasma-activated water for antimicrobial applications. In nutrient-rich environments like broth, a concentration that is too low may fail to achieve complete microbial inactivation, instead causing a temporary increase in bacterial activity. The findings also highlight the value of nanomotion sensing as a rapid technique for studying how microorganisms respond to plasma-generated reactive species.

By measuring changes in bacterial physiological activity in real-time, nanomotion sensing could serve as a complementary tool to conventional culture-based measurements when evaluating plasma treatment conditions.