Inactivation factors of spore-forming bacteria using low-pressure microwave plasmas in an N2 and O2 gas mixture

In this study, we investigated the inactivation characteristics of Geobacillus stearothermophilus spores under different plasma exposure conditions using low-pressure microwave plasma in nitrogen, oxygen and an air-simulated (N2 : O2 = 4 : 1) gas mixture. The microwave-excited surfacewave plasma discharges were produced at low pressure by a large volume device. The directly plasma-exposed spores, up to 106 populations, were successfully inactivated within 15, 10 and 5min of surface-wave plasma treatment using nitrogen, oxygen and an air-simulated gas mixture, respectively, as working gases within the temperature of 75 C. The contribution of different inactivation factors was evaluated by placing different filters (e.g. a LiF plate, a quartz plate and a Tyvek sheet) as indirect exposure of spores to the plasma. It was observed that optical emissions (including vacuumUV (VUV)/UV) play an important role in the inactivation process. To further evaluate the effect of VUV/UV photons, we placed an evacuated isolated chamber, inside which spores were set, into the main plasma chamber. The experimental results show that the inactivation time by VUV/UV photons alone, without working gas in the immediate vicinity of the spores, is longer than that with working gas. This suggests that the VUV/UV emission is responsible not only for direct UV inactivation of spores but also for generation of reactive neutral species by photoexcitation. The scanning electron microscopy images revealed significant changes in the morphology of directly plasma-exposed spores but no change in the spores irradiated by VUV/UV photons only. 1 Author to whom any correspondence should be addressed. New Journal of Physics 11 (2009) 115027 1367-2630/09/115027+15$30.00 © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft