Physical and Microbiological Characterisation of Staphylococcus epidermidis Inactivation by Dielectric Barrier Discharge Plasma

The inactivation of the Gram-positive bacteria Staphylococcus epidermidis (ATCC 12228) in its vegetative state was studied in vitro after exposure to cold atmospheric pressure plasma generated by direct dielectric barrier discharge (DBD). Compared to UV radiation at 254 nm, plasma UV emission yielded no significant contribution to bacterial inactivation. Analysis of bacterial growth inhibition revealed a pH dependency on growth media. Yet, measurements combined with numerical simulations excluded the pH shift induced by plasma generated reactive species as the main cause of bacterial inactivation. Scanning electron microscopy (SEM) images showed no alteration of cell walls, while fluorescence microscopy revealed lethal damage to cell membranes even after 1 s treatment. When the cell membrane was already severely damaged, also degradation of the bacterial DNA by plasma treatment was found. We conclude that membrane damage due to reactive oxygen species (ROS) and DNA degradation are the main mechanisms of plasma-induced bacterial death that is aggregated by milieu acidification.