Impact of Bacterial Growth Phase on Liquid Decontamination Efficiency Using Atmospheric Pressure Plasma

Abstract

Atmospheric pressure plasma processes have been increasingly studied for the microbiological decontamination of liquids. Plasma discharges produce a series of reactive oxygen and nitrogen species (RONS) that diffuse through cell membranes, triggering complex biochemical processes leading to physical and chemical changes in bacteria. The efficiency of plasma liquid decontamination is influenced by the Gram type of bacteria, the bacterial strain, the cell density, and the physiological state of bacteria. The objective of this study was to examine how bacterial growth phases affect the efficiency of plasma liquid decontamination. An atmospheric plasma multijet was used to treat Escherichia coli K12, in the midexponential and stationary phases of growth. The plasma discharge was characterized by optical emission spectroscopy, and the reactive oxygen and nitrogen species (RONS) formed in the treated liquid were quantified by spectrophotometry. The viability assays showed that the growth phase of bacteria influences the effectiveness of the treatment. Cells in the midexponential phase were more susceptible to plasma treatment than stationary-phase cells. After 30 min of plasma treatment, the plasma produced a complete inactivation 7 log reduction in bacteria in the midexponential phase while only a 3 log reduction was observed in stationary phase. Metabolic activity and membrane integrity measurements confirmed the increased sensitivity of exponential phase bacteria compared to stationary cells. In order to study the mechanisms involved in the bactericidal process and cellular defense against plasma-induced oxidative stress, we focused on intracellular ROS levels and genomic DNA damage. The results show a significant difference in intracellular ROS levels between the two phases of growth. However, DNA degradation extent did not reveal any significant differences. These results suggest that a combination of physiological and metabolic responses determine bacterial survival for both growth phases after exposure to plasma treatment.