Inactivation of Bacillus Subtilis in Water by Direct and Indirect Nonthermal Plasma Treatments

In this paper, we present the inactivation study of bacteria suspended in water by atmospheric pressure nonthermal plasma generated by pulsed dielectric barrier discharges (PDBD) in a wire-cylinder reactor. The experiments were carried out with Gram-positive Bacillus subtilis bacteria at an initial concentration of $1 \times 10^{6}\,\,\text {cells}\cdot \text {mL}^{-1}$ . Bacterial inactivation was analyzed comparing two different types of treatments: bacteria in contact with distilled water previously exposed to plasma (indirect plasma treatment) and bacterial suspension in direct contact with plasma (direct plasma treatment). Plate count, microscopy, and spectrophotometry methods were used to study the bactericidal action of the PDBD. The obtained results indicated that both treatments eliminate bacteria, where direct plasma treatment led to a faster biodecontamination with a 4.87 - log10 reduction in 10 min than 2.36 - log10 reduction achieved with indirect plasma treatment. Microscopic observations reveal that treated bacteria were damaged suggesting the disruption of their cell walls during the direct plasma treatment.