Biphasic bacterial physiological response to plasma-activated water revealed by nanomotion sensing

Abstract Plasma-activated water (PAW) is widely studied for its antimicrobial properties, yet bacterial responses to sub-lethal concentrations remain poorly characterized and understood. Here, we quantitatively investigate bacterial physiological responses to PAW under nutrient-rich conditions using colony-forming unit (CFU) counting and nanomotion sensing. CFU measurements reveal complete bacterial inactivation at high PAW concentrations, confirming its strong antimicrobial activity. However, intermediate concentrations induce increased bacterial proliferation compared to nutrient-matched controls, indicating enhanced physiological activity under sub-lethal exposure. Nanomotion sensing independently confirms this biphasic response, showing increased oscillatory activity at intermediate concentrations and complete signal suppression at bactericidal levels. These measurements provide real-time, single-cell evidence of concentration-dependent physiological modulation. The observed response is consistent with adaptive activation under moderate reactive species exposure and inactivation at higher concentrations. These findings quantitatively demonstrate that bacterial interactions with PAW are not limited to monotonic inhibition but involve a broader physiological response landscape. These results further demonstrate that nanomotion sensing can rapidly quantify antimicrobial thresholds and detect subtle physiological responses to plasma-derived reactive species. This provide new insights into microbial responses to PAW, and may help optimizing plasma-based antimicrobial applications.