A biological diagnostic of atmosphericpressure plasmas

Summary form only given. A spatially-resolved diagnostic of atmospheric-pressure plasmas is demonstrated. This diagnostic makes use of an agar plate, i.e., a Petri dish filled with agar and plated with bacterial suspension. When a flowing atmospheric-pressure plasma impinges on the plate, bacteria are inactivated due to exposure to plasma-generated bactericidal agents such as the radicals O and OH, or possibly other plasma-generated species such as metastable helium. After plasma exposure, the plate is incubated so that a white biofilm grows in spatial regions where there are living bacteria colonies. The absence of this white film is an indication of exposure to bactericidal agents generated by the plasma. Photographing the plate with a camera yields a spatially-resolved image, which corresponds to a cross section of the impinging plasma. In a demonstration of this biological diagnostic, a small diameter plasma jet impinged at normal incidence on an agar plate. The plasma was operated at a very low power, so that there was no significant heating of the biological sample. The plasma device used was a "plasma needle", which generates a weakly-ionized helium glow, with an inert gas flowing past an RF-powered needle electrode at a velocity of about 1 m/s. Atmospheric gases are entrained into the flowing plasma before it reaches the sample surface, located about 3 mm from the nozzle orifice. In this test, the agar was plated with a suspension of Streptococcus mutans (S. mutans) bacteria. The result is spatially-resolved image representing a cross-section of the plasma. We compare this image to side view images of the plasma glow. This comparison is useful in evaluating the spatial regions where the bactericidal agents are generated in the plasma