Involvement of multiple stressors induced by non-thermal plasma-charged aerosols during inactivation of airborne bacteria.

A lab-scale, tunable, single-filament, point-to-point nonthermal dieletric-barrier discharge (DBD) plasma device was built to study the mechanisms of inactivation of aerosolized bacterial pathogens. The system inactivates airborne antibiotic-resistant pathogens efficiently. Nebulization mediated pre-optimized (4 log and 7 log) bacterial loads were challenged to plasma-charged aerosols, and lethal and sublethal doses determined using colony assay, and cell viability assay; and the loss of membrane potential and cellular respiration were determined using cell membrane potential assay and XTT assay. Using the strategies of Escherichia coli wildtype, over-expression mutant, deletion mutants, and peroxide and heat stress scavenging, we analyzed activation of intracellular reactive oxygen species (ROS) and heat shock protein (hsp) chaperons. Superoxide dismutase deletion mutants (ΔsodA, ΔsodB, ΔsodAΔsodB) and catalase mutants ΔkatG and ΔkatEΔkatG did not show significant difference from wildtype strain, and ΔkatE and ΔahpC was found significantly more susceptible to cell death than wildtype. The oxyR regulon was found to mediate plasma-charged aerosol-induced oxidative stress in bacteria. Hsp deficient E. coli (ΔhtpG, ΔgroEL, ΔclpX, ΔgrpE) showed complete inactivation of cells at ambient temperature, and the treatment at cold temperature (4°C) significantly protected hsp deletion mutants and wildtype cells, and indicate a direct involvement of hsp in plasma-charged aerosol mediated E. coli cell death.