Assessing bacterial recovery and efficacy of cold atmospheric plasma treatments

• Different bacteria response after cold atmospheric plasma (CAP) was observed.
• μmax decreased in all bacteria when the time of CAP treatment increased.
• E. coli dnaK and soxS have shown no recovery after CAP of 90 s.
• N2, N2+, NO, Hα and He were found to be the antimicrobial species generated by CAP.
• Longer CAP treatments resulted in lower bacteria population on lettuce leaves.

Cold atmospheric plasma (CAP) is a non-thermal decontamination technology capable of generating groups of antimicrobial agents including; photons, electrons, positively and negatively charged ions, atoms, free radicals and excited or non-excited molecules. CAP treatments (70 kV, 50 Hz) at different times (i.e. 15, 30, 60, 90 s) were applied to a broth system inoculated either with Escherichia coli NCTC 12900, E. coli ME 9062 or E. coli knockout mutants of soxS, rpoS and dnaK genes. Optical emission spectra were performed and the main antimicrobial reactive substances generated by CAP treatments were found to be N2, N2+, NO, Hα and He. After treatments, reduction levels of the studied bacteria were up to 0.5 log CFU/mL, no showing statistical differences. Moreover, the kinetic growth parameters were calculated by applying the 2-fold dilution method. μmax decreased in all bacteria when the time of CAP treatment increased. E. coli dnaK was the most sensitive mutant to plasma, showing no recovery after CAP of 60 and 90 s. In addition, CAP challenge test treatments were applied on butter head lettuce inoculated with E. coli NCTC 12900 with CAP treatments of 60 and 90 s reducing bacteria levels by more than 1.5 log CFU/cm2.