Modeling of recovery efficiency of sampling devices used in planetary protection bioburden estimation

Abstract

ABSTRACT Planetary protection is a discipline dedicated to preventing biological contamination between celestial bodies and Earth during space exploration to ensure the integrity of scientific research. National space agencies such as the National Aeronautics and Space Administration and the European Space Agency set biological cleanliness requirements for their missions based on the guidelines provided in The Committee on Space Research’s Planetary Protection Policy. Compliance with these requirements is often demonstrated by surface sampling of spacecraft hardware and associated surfaces to enumerate the number of microorganisms present and establish that they are below the required level. The discipline has employed a variety of tools to perform sampling; the recovery efficiency of which is a key parameter used to generate spacecraft level cleanliness estimates. In this study, we investigated how recovery efficiency differs by inoculum amount and species. This was analyzed across different sampling devices using a set of microorganisms applied to stainless steel surfaces (representative of spacecraft surfaces) and two processing methods. Data were analyzed by developing a probabilistic model of the experimental process, from stainless steel coupon inoculation through recovery of spores observed in the form of colony forming units. The model quantifies the probability that an individual spore is recovered, a key metric for predicting bioburden and statistically assessing bioburden requirements. A cost function was developed to identify those assay methods that provided optimal bioburden estimation capability. Results show the nylon-flocked swab and the TX3211 wipe yield the highest recovery efficiency of those tested. IMPORTANCE Planetary protection at the National Aeronautics and Space Administration (NASA) requires bioburden on certain spacecraft to be estimated via sampling in order to comply with biological cleanliness requirements. To achieve this, the recovery efficiency of devices used to sample the spacecraft pre-launch must be understood and their uncertainty quantified in order to produce the most reasonable estimates of bioburden. This study brings together experiments performed by NASA and the European Space Agency with approved swab and wipe sampling devices, inoculating steel coupons with laboratory strains of Bacillus spp. spores commonly recovered from spacecraft assembly clean rooms ( B. atrophaeus , B. megaterium , B. safensis and B. thuringiensis ), with a mathematical model of the assay process to assess recovery efficiency. The statistical treatment developed in this study allows comparison of bioburden estimates made from different devices processed by different methods. This study also gives stakeholders and practitioners a statistically rigorous approach to predict bioburden that can be folded into future modeling efforts.