Bochum 2002 – scientific programme
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PV: Plenarvorträge
PV II
PV II: Plenary Talk
Monday, March 18, 2002, 15:30–16:15, HZO 30
Low-temperature sterilization using plasmas: methods and mechanisms — •Michel Moisan — Groupe de physique des plasmas, Université de Montréal, Montréal H3C 3J7, Québec
Plasma sterilization can be achieved under direct contact with the discharge or in its afterglow. The methods and mechanisms of this technique are reviewed using, as a working example, the flowing afterglow of a microwave discharge in N2−O2 mixtures at pressures below 10 Torrs. Spores are utilized as the most resistant form of microorganisms. Sterilizers are characterized by their corresponding survival curve (logarithm of the number of living spores as a function of exposure time). Survival curves of conventional sterilizers (e.g., autoclaves, chemical systems) show a single straight line, whereas plasma sterilizers yield three linear segments (called phases). The analysis of these curves as functions of the operating conditions has enabled us to identify the two principal physico-chemical processes leading to plasma sterilization, namely irradiation of the spore DNA by UV photons and erosion of the spore by free radicals (e.g., O). The UV photons are emitted by atoms or molecules (e.g., NO) excited in the afterglow through collisions. Only a fraction of the photons directed toward the spore actually penetrates into it up to its genetic material to damage it. A spore becomes inactive when the number of lesions caused to its DNA strands is sufficient to prevent it from germinating in a favorable environment. Statistically, one spore on a thousand shows a particular resistance to UV irradiation. This seems to be mostly the case of spores that are partially or totally covered by other spores or by debris of all kinds with, therefore, a thicknesssuperior to that of a single average microorganism. Therefore, the erosion process, induced by oxygen atoms forming volatile compounds with the spore material, is of particular importance.