Applied testing of antibacterial surfaces for spaceflight and confined habitats

Abstract

Microorganisms are present in any built environment, whether it is a space mission where the crew lives in an indoor environment with harsh outdoor conditions, or a clinical setting where the inhabitants are particularly susceptible to their environment. The indoor microbiome of enclosed habitats such as the e.g. International Space Station or the Concordia station in Antarctica is significantly shaped by the people who inhabit them. Consideration and assessment of microbial spread, growth and adaptation during long-term missions is essential to prevent contamination of the enclosed environment. One promising approach is the use of innovative antimicrobial surfaces, which have shown the potential to prevent the spread of pathogens. In the “Touching Surfaces” project, novel copper-based antimicrobial surfaces were tested in direct application. To do so, nine different antimicrobial surfaces were implemented in each piece of hardware, called 'Touch Arrays'. Three different metals were included: stainless steel as an inert metal, copper as an antimicrobial surface and brass (CuZn37) as a moderately antimicrobial surface. For each metal, three different surface topographies or patterns are tested, one of which is a smooth reference surface. In addition, direct laser interference patterning (DLIP) is used to create two different laser structures in the different metals. One pattern is about the size of many bacteria (3 µm) and the other is smaller than the average bacterial cell (800 nm). Touch Arrays have been installed in a variety of settings, including schools in Germany, the ISS, and in other projects to also test surfaces on the Concordia station in Antarctica. During their exposure time in the different environments, Touch Arrays were touched frequently over a defined period of time. On return to our laboratories, the surfaces of the Touch Arrays from Touching Surfaces were analyzed using electron microscopy which showed that despite organic contamination, most of the surfaces remained intact. However, on some structures organic contamination formed patterns over the structures. Using 16S rRNA sequencing we found that most of the bacteriadetected on the surfaces were human-associated. But we did not find a statistically significant difference in the microbial communities between the different surface-types, i.e. structured vs unstructured, copper-based vs. steel. Isolation of microorganisms from the surfaces suggests an increased antibacterial efficacy for the copper-based nanometer-structured (800 nm) surfaces as we were not able to cultivate isolates from them. To evaluate the antibacterial efficacy after frequent touching, we performed wet contact killing assays using an environmental methicillin-resistant Staphylococcus aureus (MRSA) strain. Additionally, we measured the copper ion release during the wet contact killing assays. This showed that touching the surfaces reduced copper ion release and overall antibacterial efficacy against MRSA. In conclusion we were able to assess the antibacterial efficacy of the surfaces after direct in-field application in the project Touching Surfaces. Hence, this setup is suitable and easily adaptable for testing antimicrobial surfaces in other settings and for other surfaces.