Cleaning to achieve sterility

NASA planetary protection regulations state that a surface may be considered "sterile" if a microbial burden of less than 300 aerobic bacterial spores per square meter can be further treated to achieve a 10
⁴
fold reduction in viable endospores (spores). The results of previous studies have suggested that surfaces might be cleaned to a level that is essentially sterile. Here, we report the results of a comparative analysis of the efficacy/ability of three different cleaning approaches to remove bacterial spores from a series of surrogate spacecraft surfaces. In order to accomplish the most realistic and reproducible spore deposition, an aerosol chamber capable of nebulizing innocuous Bacillus atrophaeus (ATCC, 9372 [formerly B. subtilis var niger]) spores was developed and used. This enabled the relatively uniform inoculation of spores as individual entities at a concentration in excess of 10
⁵
spores per 2.2 cm
²
surrogate surface coupon. Coupons prepared in this fashion were subsequently delivered to three cleaning facilities: vendor A for cleaning via standard aerospace precision cleaning protocols, vendor B for cleaning via ultra-pure water, and vendor C for cleaning via liquid boundary layer disruption. Variations in the chemistry of the cleaning solutions were also explored. Preliminary results suggest that "sterility," as defined by NASA, may indeed be achieved by various cleaning procedures, despite the fact that none of these methods were originally designed, nor are they currently conducted, with such a goal in mind