Aims: This study investigated a range of atmospheric cold plasma (ACP) process parameters for bacterial inactivation with further investigation of selected parameters on cell membrane integrity and DNA damage. The effects of high voltage levels, mode of exposure, gas mixture and treatment time against Escherichia coli and Listeria monocytogenes were examined.
Methods and results: 10(8) CFU ml(-1) E. coli ATCC 25922, E. coli NCTC 12900 and L. monocytogenes NCTC11994 were ACP-treated in 10 ml phosphate-buffered saline (PBS). Working gas mixtures used were air (gas mix 1), 90% N2 + 10% O2 (gas mix 2) and 65% O2 + 30% CO2 + 5% N2 (gas mix 3). Greater reduction of viability was observed for all strains using higher voltage of 70 kVRMS and with working gas mixtures with higher oxygen content in combination with direct exposure. Indirect ACP exposure for 30 s inactivated below detection level both E. coli strains. L. monocytogenes inactivation within 30 s was irrespective of the mode of exposure. Leakage was assessed using A260 absorbance, and DNA damage was monitored using PCR and gel electrophoresis. Membrane integrity was compromised after 5 s, with noticeable DNA damage also dependent on the target cell after 30 s.
Conclusions: Plasma treatment was effective for inactivation of challenge micro-organisms, with a greater sensitivity of L. monocytogenes noted. Different damage patterns were observed for the different bacterial strains attributed to the membrane structure and potential resistance mechanisms.
Significance and impact of the study: Using atmospheric air as working gas resulted in useful inactivation by comparison with high nitrogen or high oxygen mix. The mechanism of inactivation was a function of treatment duration and cell membrane characteristics, thus offering potential for optimized process parameters specific to the microbial challenge.
Keywords: DBD-ACP; DNA damage; Escherichia coli; Listeria monocytogenes; cell integrity; voltage level.
© 2013 The Society for Applied Microbiology.