Use of non-thermal atmospheric plasmas to reduce the viability of Bacillus subtilis on spacecraft surfaces

Atmospheric pressure glow-discharge (APGD) plasmas have been proposed for sterilizing spacecraft surfaces prior to launch. The advantages of APGD plasmas for the sterilization of spacecraft surfaces include low temperatures at treatment sites, rapid inactivation kinetics of exposed microbial cells, physical degradation and removal of microbial cells, physical removal of organic biosignature molecules, and short exposure times for the materials. However, few studies have tested APGD plasmas on spacecraft materials for their effectiveness in both sterilizing surfaces and removal of microbial cells or spores. A helium (He)+oxygen (O2) APGD plasma was used to expose six spacecraft materials (aluminum 6061, polytetrafluoroethylene (PTFE), polycarbonate, Saf-T-Vu, Rastex, and Herculite 20) doped with spores of the common spacecraft contaminant, Bacillus subtilis, for periods of time up to 6 min. Results indicated that greater than six orders of magnitude reductions in viability were observed for B. subtilis spores in as short of time as 40 s exposure to the APGD plasmas. Spacecraft materials were not affected by exposures to the APGD plasmas. However, Saf-T-Vu was the only material in which spores of B. subtilis adhered more aggressively to plasma-treated coupons when compared to non-plasma treated coupons; all other materials exhibited no significant differences between plasma and non-plasma treated coupons. In addition, spores of B. subtilis were physically degraded by exposures to the plasmas beginning at the terminal ends of spores, which appeared to be ruptured after only 30 s. After 300 s, most bacteria were removed from aluminium coupons, and only subtle residues of bacterial secretions or biofilms remained. Results support the conclusion that APGD plasmas can be used as a prelaunch cleaning and sterilization treatment on spacecraft materials provided that the biocidal and cleaning times are shorter than those required to alter surface properties of materials. Received 10 July 2007, accepted 11 December 2007