Growth of 48 Built Environment Bacterial Isolates on Board the International Space Station (ISS)
Background: While significant attention has been paid to the potential risk of pathogenic microbes aboard crewed spacecraft, much less has focused on the non-pathogenic microbes in these habitats. Preliminary work has demonstrated that the interior of the International Space Station (ISS) has a microbial community resembling those of built environments on earth. Here we report results of sending 48 bacterial strains, collected from built environments on earth, for a growth experiment on the ISS. This project was a component of Project MERCCURI (Microbial Ecology Research Combining Citizen and University Researchers on ISS).
Results: Of the 48 strains sent to the ISS, 45 of them showed similar growth in space and on earth. The vast majority of species tested in this experiment have also been found in culture-independent surveys of the ISS. Only one bacterial strain that avoided contamination showed significantly different growth in space. Bacillus safensis JPL-MERTA-8-2 grew 60% better in space than on earth.
Conclusions: The majority of bacteria tested were not affected by conditions aboard the ISS in this experiment (e.g., microgravity, cosmic radiation). Further work on Bacillus safensis could lead to interesting insights on why this bacteria grew so much better in space.
From 2012-2014, we conducted a nationwide citizen science project, Project MERCCURI http://spacemicrobes.org/, aimed at raising public awareness of microbiology and research on board the International Space Station (ISS). Project MERCCURI (Microbial Ecology Research Combining Citizen and University Researchers on the ISS) was a collaborative effort involving the "microbiology of the Built Environment network" (microBEnet) group, Science Cheerleader, NanoRacks, Space Florida, and SciStarter. One of the goals of Project MERCCURI was to examine how a number of non-pathogenic bacteria associated with the built environment would grow on board the ISS compared to on earth.
Most previous work growing bacteria in space has focused on species known to contain pathogenic strains (e.g. Escherichia coli (Klaus 1997) (Brown 2002) and Pseudomonas (Crabbé 2011) (Kim 2013)), and much less attention has been paid to the "normal" microbes that surround us (i.e., species not known to be pathogenic). An understandable bias towards pathogens and pathogenic pathways is highlighted by work on topics such as biofilm formation ((Kim 2013a), (McLean 2001)), antibiotic resistance/production ((Benoit 2006), (Juergensmeyer 1999), (Lam 2002) reviewed in (Klaus 2006)), and virulence ((Nickerson 2000), (Hammond 2013)).
While concern about pathogens in spacecraft is certainly warranted, it should be emphasized that the ability of a pathogen to survive outside a host and the ability to infect a host are both, at least in part, dependent on the existing community of non-pathogenic microbes in those locations. For example, mechanically ventilated hospital air shows a much higher abundance of potential pathogens (Kembel 2012), presumably due to the lack of inflow or competition from other microbes. Similarly, the infectivity of some pathogens has been shown to be very dependent on the host microbiome (e.g. (Schuijt 2015), (Ichinohe 2011), (van 2015) (Reeves 2011)). Therefore, it is important to understand the entire microbial ecosystem of spacecraft. Indeed, in recent years, several culture-independent studies have examined the microbiome of the ISS ((Castro 2004), (Venkateswaran 2014), (Moissl 2007)), including another part of Project MERCCURI (Lang Submitted). These studies have shown, not surprisingly, that the microbiome of the ISS bears a strong resemblance to the microbiome of human-associated built environments on earth. Therefore it is of interest to see how microbes from human-associated environments behave in space.
For this study, samples from human-associated surfaces (e.g. toilets, doorknobs, railings, floors, etc.) were collected at a variety of locations around the United States, usually in collaboration with the public. A wide variety of bacteria were cultured from these samples, and 48 non-pathogenic strains were selected for a growth assay comparing growth in microgravity on the ISS and on earth.
Materials and Methods
Samples were collected from built environment surfaces throughout the United States on cotton swabs (Puritan 25-806 2PC) and mailed (usually overnight) to the University of California Davis where they were transferred to lysogeny broth (LB) plates. Colonies were chosen for further examination based on maximizing morphological variation. Each chosen colony was double-dilution streaked and then the identity determined by direct PCR and Sanger sequencing using the 27F (5'-AGAGTTTGATCMTGGCTCAG-3') and 1391R (5'-GACGGGCGGTGTGTRCA-3') primers (see (Dunitz 2015) for details). Sanger sequences were trimmed and aligned using Geneious (Kearse 2012). The resulting consensus sequence was identified through a combination of BLAST (Altschul 1990) and building phylogenetic trees using the Ribosomal Database Project (RDP) website (Cole 2014). The 48 candidates for spaceflight were chosen on the basis of biosa