Pulsed light induced damages in Listeria innocua and Escherichia coli

Pulsed light (PL) is an upcoming nonthermal decontamination technology mainly used for surface sterilization. The objective of this study was to investigate the extent of cellular damage caused by PL treatments of Listeria innocua and Escherichia coli on a polysaccharide surface in order to gain knowledge about the main inactivation pathways.

The impact of PL on the cellular ATP level was investigated as well as the bacterial ability to take up fluorescently labelled glucose (2-NBDG). Furthermore, the extent of DNA damages was assessed by qPCR. The ability of L. innocua and E. coli to photorepair under artificial daylight exposure was quantified. Finally, the induction of reactive oxygen species (ROS) and lipid peroxidation were studied by fluorometric detection of ROS and thiobarbituric acid reactive substances (TBARS). It is shown that intracellular ATP levels and glucose uptake ability do not correlate with the immediate loss of bacterial reproducibility, which indicates that cellular activity and energy may remain on a relatively high level, although growth on tryptic soy agar is not observable. Sequence specific investigation of PL induced DNA damages by qPCR revealed distinct differences between L. innocua and E. coli although the observed inactivation efficacy of PL by the culture based method was similar. Photoreactivation has been observed for both bacteria, a higher recovery rate of up to 2 log was seen in case of E. coli. Intracellular ROS and lipid peroxides were both detectable at relatively high fluencies with E. coli so the contribution of oxidative damage to microbial inactivation of PL cannot be excluded.

Escherichia coli as well as L. innocua cells have proven to maintain residual cellular activity after having been exposed to PL even when they are not able to reproduce any more. High proportions of sublethal damages were also obvious with regard to occurring photoreactivation. The destruction of bacterial DNA seems to be the primary mechanism of inactivation of PL but the involvement of other factors like oxidative stress cannot be excluded.

The observed data underline that bacteria are not immediately inactivated after exposure to PL as different indicators of cellular energy are still detectable even when cells do not reproduce on solid media any more. DNA is the primary target of PL, but as the extent of damage among different bacteria may not reveal their actual sensitivity, other destructive effects should also be considered.