Skip to main content

Gene expression during repair of Salmonella enterica serovar Typhimurium after exposure to cold atmospheric gas plasma: a transciptomic analysis using a DNA microarray approach

Final Report Summary - PLASMARRAY (Gene expression during repair of Salmonella enterica serovar Typhimurium after exposure to cold atmospheric gas plasma: a transciptomic analysis using a DNA microarray approach)

Gene expression during repair of Salmonella enterica serovar Typhimurium after exposure to cold atmospheric gas plasma: a transcriptomic analysis using a DNA microarray approach

Cold atmospheric gas plasma treatment (CAP) emerges as an alternative approach for the decontamination of fresh and minimally processed food. For CAP to be successfully adopted by the food production industry, factors which affect its killing potential must be evaluated. For this reason, PLASMARRAY aimed to study the effect of individual parameters, such as initial cell concentration (105 -105 CFU/filter), growth phase (early and late logarithmic phase, and stationary phase), growth temperature (20, 25, 37 and 45 ºC) and the chemical treatment regime (oxidation, reduction and redox) on the efficacy of cold atmospheric nitrogen plasma inactivation of S. Typhimurium deposited onto membrane filters. A further objective was to evaluate the efficacy of CAP treatment for the inactivation of S. Typhimurium inoculated on the surface of lettuce and strawberry, as well as, in potato tissue. The final aim of PLASMA was to use DNA microarray technology and transcriptomics to determine the generic bacterial response to damage imposed by well characterised cold atmospheric gas plasma as a means of identifying mechanisms of action and improved inactivation protocols.

Effect of microbial loading

The effect of initial microbial concentration on the inactivation of S. Typhimurium by nitrogen CAP was examined. It was found that the rate of inactivation of S. Typhimurium is inversely proportional to initial bacterial concentration, with the D-value (time in minutes for the bacterial survival CFU to be reduced by 1 log cycle) observed at the highest cell concentration assayed (108 CFU/filter) being 14 fold higher than seen at the lowest starting concentration (105 CFU/filter). Addition of increasing concentrations of Pseudomonas fluorescens cells to a Salmonella population of 105 CFU/filter resulted in an exponential decrease in the rate of killing of the Salmonella cells. However, whilst the addition of heat-killed S. Typhimurium cells to 105 CFU/filter live S. Typhimurium cells resulted in a significant decrease in the killing rate, this effect was dose independent. This suggests that although biomass plays a role in the protection against CAP inactivation seen at high cell densities, dead cells and their components released during the heating period are not as effective as viable cells. Fluorescence microscopy showed that the multilayer structure which appears at the highest cells densities could explain the reduced inactivation achieved by the plasma, since the top layer may present a physical barrier that protects underlying cells.

Effect of growth phase

Although is well documented that stationary phase Salmonella bacteria are more resistant to inactivation technologies such as thermal, acidic, high pressure and pulsed electric field treatments compared to exponential phase Salmonella, our data showed that the growth phase (mid-logarithmic phase, late-logarithmic phase and stationary phase) of S. Typhimurium did not significantly affect the CAP inactivation rates.