![]() īoth ISS environment and cleanrooms on Earth are maintained by high-efficiency particulate arrestance (HEPA) filters. In addition, understanding of the ISS microbiome could facilitate the necessary maintenance of this closed habitat and thereby assist in preventing degradation of its components by some microorganisms. As long-duration human missions are planned in the future, detection of human pathogens and possible mitigation practices must be developed. However, it is important to monitor the presence of any opportunistic pathogenic microorganisms. Due to the technically rigorous methods required for culturing many microorganisms, characterization of human-associated microbial populations in the ISS environment remains a significant challenge. The PMA-based assay thus allows for a more accurate approximation of the viable microbial community in terms of richness as well as abundance. The use of the reagent propidium monoazide (PMA) before DNA extraction eliminates cells with a compromised membrane. However, differentiating viable and yet-to-be-cultivable microbial populations requires an appropriate sample processing technology. “Deep” sequencing of ISS samples can answer questions on abundance and diversity of the microorganisms. Although next-generation sequencing (NGS) analyses are now broadly implemented in many microbiology-related scientific fields, especially in microbial ecology and human microbiome projects, use of these techniques for closed habitats has just begun and warrants more research. Previous studies show that permanent changes have occurred within the microbial species during experiments aboard the ISS. Moreover, the National Research Council (NRC) specifically recommended that NASA study changes in microbial populations in response to selective pressure associated with microgravity, which characterizes life aboard the ISS. Since built environments are known to have specific microbiomes, it is of the highest interest to the National Aeronautics and Space Administration (NASA) scientific community to explore the environmental microbiome of the ISS as a closed environment. The microbial characterization of the International Space Station (ISS) has been mostly limited to traditional culture-based microbiology and selective molecular biology methods, such as Sanger sequencing, for supporting tasks such as water remediation, food safety, and crewmember health. Finally, the results will allow comparisons with other built sites and facilitate future improvements on the ISS that will ensure astronaut health. This information can be used to identify sites that can be targeted for more stringent cleaning. The present results also demonstrate the value of measuring viable cell diversity and population size at any sampling site. The results obtained will facilitate future studies to determine how stable the ISS environment is over time. For example, Corynebacterium and Propionibacterium (Actinobacteria) but not Staphylococcus (Firmicutes) species are dominant on the ISS in terms of viable and total bacterial community composition. The results of this study provide strong evidence that specific human skin-associated microorganisms make a substantial contribution to the ISS microbiome, which is not the case in Earth-based cleanrooms. However, the treatment did not appear to have an effect on the bacterial composition (diversity) associated with each sampling site. The viable bacterial populations seen by PMA treatment were greatly decreased. Actinobacteria were predominant in the ISS samples whereas Proteobacteria, least abundant in the ISS, dominated in the cleanroom samples. Statistical analyses showed that members of the phyla Actinobacteria, Firmicutes, and Proteobacteria were dominant in the samples examined but varied in abundance. The 16S rRNA gene Illumina iTag sequencing was used to elucidate microbial diversity and explore differences between ISS and cleanroom microbiomes. Samples collected from the ISS and two cleanrooms at the Jet Propulsion Laboratory (JPL, Pasadena, CA) were analyzed by traditional cultivation, adenosine triphosphate (ATP), and propidium monoazide–quantitative polymerase chain reaction (PMA-qPCR) assays to estimate viable microbial populations. A second objective was to determine if the built environments of Earth-based cleanrooms associated with space exploration are an appropriate model of the ISS environment. The primary goal of this study was to characterize the viable microbiome of the ISS-built environment. Understanding the composition of the ISS microbial community will facilitate further development of safety and maintenance practices. The International Space Station (ISS) is a unique built environment due to the effects of microgravity, space radiation, elevated carbon dioxide levels, and especially continuous human habitation. ![]()
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