Inactivation of biofilm-forming bacteria using cold atmospheric plasmas and potential application for decontamination of fresh foods

Cut and pre-packaged fresh vegetable and fruit salad products have traditionally been decontaminated using chlorine. Although this agent is of only limited efficacy as a microbiocide, and is associated with health risks to humans, no satisfactory alternative to it has yet emerged. This work aims at evaluating the prospects for using cold atmospheric plasmas to decontaminate fresh produce. Nonthermal atmospheric plasmas have mainly been studied in the past as methods for killing pathogens on surfaces either in medical or environmental contexts. Most of these studies are based on cell inactivation on surfaces such as glass, plastics and metals which do not usually support cell growth. In food decontamination, there is the complication of interaction occurring between microbial cells and the surface. This interaction can take many different forms: micro-organisms may adsorb to the surfaces of food and simply remain attached to it during processing as is the case with abiotic materials. Alternatively, adherent organisms may exploit nutrients present on the surface of foods and go on to proliferate at the surface. Others may, under certain conditions, exude polysaccharides which serve to protect them from environmental stresses and, incidentally, makes it more difficult to detach them from surfaces. This three-dimensional matrix of polysaccharides and micro-organisms is referred to as a 'biofilm.' In order to facilitate rigorous evaluation of our decontamination methods, we have chosen to conduct our initial experiments using a model system. In future work once we have optimised the operation of our plasma, we intend to use naturally contaminated foods and additionally, to evaluate the effects of cold plasmas on key nutrients in the food. Our model system comprises the biofilm-forming bacterium Pantoea (Enterobacter) agglomerans growing on synthetic membranes to simulate bacterial growth on the surface of plant tissue. As the biofilm develops with time, the production of polysaccharides becomes more extensive and cells contained within the matrix are shielded to an increasing degree. We present data on the kinetics of inactivation of P. agglomerans in biofilms of different ages by cold atmospheric plasmas generated in a helium-oxygen mixture and with a radio-frequency excitation. Data is also presented on the UV treatment of such biofilms in order to permit comparison of both methods of food decontamination.