Community matters: stress tolerance and survival of Bacillus subtilis and Staphylococcus capitis in a synthetic space habitat consortium.

Microorganisms impact every ecosystem they inhabit, ranging from environments on our planet to extraterrestrial habitats such as the International Space Station (ISS) in low Earth orbit (LEO). Regardless of the extent of cleaning, some microorganisms will inevitably stay in the respective environment. Even cleanrooms, which are meant to reduce contamination, have their own distinct microbiome (). This is also true for the ISS, whose microbiome is dominated by human-associated microorganisms and “adaptation specialists” (). While the ISS microbiome is shaped by its environment (), the microbiome also affects the wellbeing of their inhabitants (; ). The advent of next-generation sequencing has enabled a more comprehensive understanding of the extraordinary richness and diversity of microbial life. Microorganisms typically exist within microbial consortia, which are groups of diverse microorganisms, including bacteria, archaea, fungi, and viruses, that coexist and interact within specific niches or environments (). These consortia exhibit various interactions, such as mutualism, competition, and commensalism (). Microbial consortia regulate a wide range of complex processes, including wastewater treatment for chemical contaminant reduction, and the microbial communities within the human gut are essential for digestion and gut health (; ). However, most experimental investigations to date have focused on individual microbial species, and elucidating the complexity of microbial communities continues to present significant challenges. Even relatively simple natural communities often consist of more than 10 distinct members (). Due to the complexity of naturally found consortia, experimental designs use synthetic microbial consortia as surrogates, which can be artificially engineered to analyze key features, such as interactions and survival of natural communities. Synthetic microbial consortia can be designed in a bottom-up approach, meaning the consortium is built by combining species with known characteristics, or in a top-down approach where the consortium is a simplified version of a respective natural community (). Synthetic microbial consortia can aid in studying microbial interactions in a simplified, predictable environment, or can reduce variability compared to more complex natural microbial communities. With the help of synthetic microbial consortia, microbial interactions such as competition, cooperation, or signaling can be investigated, which can affect stability, metabolic efficiency, and resistance (; ,). Resistance and survival of microorganisms are strongly influenced by the ecological context in which they exist (). One very special ecological niche is the indoor environment of space stations. Conditions in space stations imposed on microorganisms include microgravity, higher ionizing radiation levels, exposure to desiccation and cleaning agents. Some studies have shown that under spaceflight conditions virulence and resistance toward antibiotics was increased in (; ), biofilm formation was enhanced under spaceflight conditions in (), and microorganisms from the ISS might have adapted to their environment in space and showed enhanced biofilm formation (; ). This is particularly relevant due to the unique conditions in space, which have a negative effect on the astronauts' health by reducing their immune functions (; ). However, other studies have found that while prevalence of antibiotic resistance genes and virulence factors were not increased on the ISS, present microorganisms showed adaptation to the unique environmental conditions (, ). Sporulation constitutes a critical microbial survival strategy in spacecraft-associated environments. Endospore-forming Firmicutes, particularly species, are frequently identified in spacecraft assembly cleanrooms and on the ISS due to their remarkable resistance to desiccation, radiation, oxidative stress, and nutrient limitation (; ; ). spores endure extended exposure to extreme environmental conditions through mechanisms such as reduced metabolic activity, specialized spore structures, low water content, and robust DNA protection and repair systems (; , ; ). Owing to their persistence and significance for forward contamination, spore-forming microorganisms are central to planetary protection and space microbiology research (; ).