Microbial Protocols for Spacecraft: 3. Spore Monolayer Preparation Methods for Ultraviolet Irradiation Exposures

Area of Science:

• Astrobiology
• Microbial Physiology
• Spacecraft Engineering

Background:

• Accurate microbial inactivation kinetics are essential for developing robust survival models for spacecraft.
• Standardized protocols for preparing and testing microbial bioburdens on simulated spacecraft materials are needed.
• The liquid droplet protocol is evaluated for applying Bacillus subtilis spores to aluminum coupons.

Purpose of the Study:

• To determine optimal conditions for creating uniform microbial spore monolayers on spacecraft materials.
• To assess the impact of spore density and application methods on UV irradiation inactivation assays.
• To establish standardized protocols for microbial inactivation studies in planetary protection.

Main Methods:

• Application of Bacillus subtilis spores to aluminum coupons using the liquid droplet protocol.
• Varied spore densities (2-5 x 10^6 spores/cm^2) to create low-density monolayers.
• Utilized sterile deionized water (SDIW) and SDIW with Triton X-100 surfactant.
• Assessed spore uniformity and UV irradiation sensitivity on coated and uncoated aluminum coupons.

Main Results:

• Optimal spore monolayers were achieved at densities between 2 and 5 x 10^6 spores/cm^2.
• Sterile deionized water (SDIW) on uncoated aluminum, or SDIW + Triton X-100 on coated/uncoated aluminum, yielded the best monolayers.
• Triton X-100 improved uniformity without altering UV sensitivity.
• Higher spore densities (2 x 10^7 or 2 x 10^8 spores/cm^2) resulted in multilayer aggregation, reducing assay precision.

Conclusions:

• Precise microbial inactivation kinetics require uniform spore monolayers, free from aggregates and multilayers.
• The liquid droplet protocol, with optimized spore density and surfactant use, provides reliable methods for preparing test surfaces.
• Standardized protocols are proposed to enhance comparability of results across different laboratories for spacecraft processing and planetary protection.