A novel bacterial strain, designated as 1F8SW-P5T, was isolated from the wall of the crew quarters on the International Space Station. Cells were Gram-staining-positive, strictly aerobic, non-spore-forming, chemoheterotrophic, and mesophilic rods exhibiting catalase-positive and oxidase-negative reactivity. Strain 1F8SW-P5T shared the highest 16S rRNA gene similarity with Microbacterium proteolyticum CECT 8356T (99.34%) and the highest gyrB gene similarity with Microbacterium algihabitans KSW2-21T (91.34%). Its strongest matches via average nucleotide identity and DNA-DNA hybridization were to Microbacterium hydrothermale CGMCC_1.12512T (84.36% and 25.80%, respectively). 1F8SW-P5T formed a distinct lineage during phylogenetic and phylogenomic analysis. The biochemical, phenotypic, chemotaxonomic, and phylogenomic features substantiated the affiliation to 1F8SW-P5T as a new species of Microbacterium, for which we propose the name Microbacterium mcarthurae, with the type strain 1F8SW-P5T (=DSM 115934T =NRRL B-65667T). Based on metagenomic data collected during the Microbial Tracking mission series, M. mcarthurae was identified from all surfaces (n = 8) over an 8-year period, with an increase in relative abundance over time. This is of potential concern, as we observed resistance to all tested fluoroquinolone antibiotics (n = 6), two β-lactam antibiotics, and one macrolide antibiotic, which was not predicted based on isolate or plasmid genotype alone. Furthermore, we found an increase in virulence, compared to Escherichia coli, when tested within a Caenorhabditis elegans model. This pathogenic profile highlights the importance of continued characterization of spacecraft-associated microbes, the characterization of previously unidentified antimicrobial resistance and virulence genes, and the implementation of targeted mitigation strategies during spaceflight.
Importance: Crew members are at an increased risk for exposure to and infection by pathogenic microbes during spaceflight. Therefore, it is imperative to characterize the species that are able to colonize and persist on spacecraft, how those organisms change in abundance and distribution over time, and their genotypic potential for and phenotypic expression of pathogenic traits (i.e., whether they encode for or exhibit traits associated with antibiotic resistance and/or virulence). Here, we describe a novel species of Microbacterium collected from the crew quarters on the International Space Station (ISS), 1F8SW-P5T, for which we propose the name Microbacterium mcarthurae. M. mcarthurae was found to be distributed throughout the ISS with an increase in relative abundance over time. Additionally, this bacterium exhibits a unique antibiotic resistance phenotype that was not predicted from whole-genome sequencing, as well as increased virulence, suggesting the need for the identification of previously undescribed antimicrobial resistance genes and monitoring/mitigation during spaceflight.
Keywords: Actinomycetota; Caenorhabditis elegans; International Space Station (ISS); Microbacterium; antibiotic resistance; space biology; virulence.