Complex responses of microorganisms as a community to a flowing atmospheric plasma

It is well known that microorganisms are highly adaptable to changing microenvironments and to a diversity of external stresses. To investigate this, a vacuum filtration technique was used to deposit Listeria innocua cells in a single monolayer onto a membrane and a pulsed radio-frequency atmospheric plasma plume was used to treat such samples. Unexpectedly however, the resulting inactivation kinetics were biphasic despite the initial homogeneity. This paper reveals complex cascading events in the microbial community, starting with the emergence of discrete isles of cell aggregates, through their evolution into what we refer to as “cell refuges” that protect viable cells from subsequent plasma treatment and finally, the disintegration of the cell refuges leaving only a thin layer of fragmented cell debris. It is shown that plasma-mediated cell aggregates and cell refuges introduce heterogeneity and are key to explaining the biphasic inactivation kinetics. The evolving surface architecture of the bacterial community could lead to the regulation of plasma species that the bacteria come to be exposed to. All of these factors could strongly affect the role played by charged plasma species involved in plasma–cell interactions. The complex and dynamically evolving responses of the bacteria to the gaseous plasma do not conform to the spatial distribution of the plasma, thus representing a process of self-organization. Plasma-mediated self-organization of cell refuges and their selective screening of plasma species are perhaps only early signs of the undoubtedly sophisticated ability of microorganisms as a community to respond to the action of plasmas.