Significance: Salmonella can establish persistent infection in layer hens which results in the intermittent shedding of bacteria in faeces. This can increase contamination of the farm environment and risk of horizontal contamination of eggs. Thus, controlling Salmonella on-farm is of significance to public health. Chemicals, like quaternary ammonium compounds, are frequently used for decontamination. However, while being effective at reducing Salmonella, they can be detrimental to the environment and there is evidence of emerging resistance. Therefore, alternative decontamination strategies are needed.
Background: Plasma, when discharged in water (plasma activated water, PAW), produces reactive oxygen and nitrogen species, that leads to an increase in conductivity, redox potential, and reduces pH. PAW has been shown to have bactericidal capacities for both Gram-positive and Gram-negative bacterial species.
Aim: Evaluation of the efficacy of five PAW types against Salmonella typhimurium isolates: 3 isolated from different Australian layer hen farms and a control strain, ATCC 14028.
Methods and results: The bactericidal effect of each PAW was evaluated using time-kill curves. PAW types with the highest redox potential and lowest pH had the highest efficacy, eliminating culturable bacteria immediately upon exposure. Dilution of all PAW types to 50% in nutrient broth reduced the bactericidal effectiveness indicating organic material may affect efficacy. The effectiveness of PAW on biofilm formation by Salmonella typhimurium was also assessed by the crystal violet assay. For biofilm growth, each isolate was incubated for 4 days statically at 22°C. All PAW types significantly reduced biofilm biomass between 30-50% compared to regular growth conditions. Bacterial viability within biofilms was also measured using the AlamarBlue assay. Three types of PAW resulted in significant reduction of bacterial cell viability (≥95% reduction, p<0.0001) compared to the growth control.
Conclusion: The bactericidal capacity of these PAW types on Salmonella and the reduction of biofilm biomass highlights their potential use as an alternative control strategy on farm and in the food supply chain.