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Abstract
Astronauts have been previously shown to exhibit decreased salivary lysozyme and increased dental calculus and gingival inflammation in response to space flight, host factors that could contribute to oral diseases such as caries and periodontitis. However, the specific physiological response of caries-causing bacteria such as Streptococcus mutans to space flight and/or ground-based simulated microgravity has not been extensively investigated. In this study, high aspect ratio vessel S. mutans simulated microgravity and normal gravity cultures were assessed for changes in metabolite and transcriptome profiles, H2O2 resistance, and competence in sucrose-containing biofilm media. Stationary phase S. mutans simulated microgravity cultures displayed increased killing by H2O2 compared to normal gravity control cultures, but competence was not affected. RNA-seq analysis revealed that expression of 153 genes was up-regulated ≥2-fold and 94 genes down-regulated ≥2-fold during simulated microgravity high aspect ratio vessel growth. These included a number of genes located on extrachromosomal elements, as well as genes involved in carbohydrate metabolism, translation, and stress responses. Collectively, these results suggest that growth under microgravity analog conditions promotes changes in S. mutans gene expression and physiology that may translate to an altered cariogenic potential of this organism during space flight missions.
Highlights
Streptococcus mutans is a primary causative agent of dental caries, demonstrated by its isolation from carious tooth lesions in humans,[1] its ability to initiate caries in germ-free rodent models of infection,[2] and the established link between high levels of this bacterium in the oral cavity and active caries.[3]
Total number of up-regulated genes = 94, total number of down-regulated genes = 153 microgravity HARV cultures (Supplemental Table S2). This analysis revealed the presence of four significant functional annotation clusters in the down-regulated normal gravity genes, which included genes involved in carbohydrate metabolism and transport, transcriptional regulators, and carbohydrate transporters/other membrane proteins
The transformation efficiency was not affected by HARV growth itself, as the HARV transformation efficiencies were similar to those measured in a parallel non-rotating culture of S. mutans (Fig. 6). The effects of both spaceflight and microgravity analog growth on virulence-related phenotypes of bacterial pathogens such as Salmonella enterica serovar Typhimurium, Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus have been well-described,[28,29,30,31,32,33,34,35,36,37,38] comparatively little research has been dedicated to studying the specific effects of space flight and simulated microgravity on virulence attributes of oral bacteria such as S. mutans
Summary
Streptococcus mutans is a primary causative agent of dental caries, demonstrated by its isolation from carious tooth lesions in humans,[1] its ability to initiate caries in germ-free rodent models of infection,[2] and the established link between high levels of this bacterium in the oral cavity and active caries.[3]. The approach to maintaining astronaut oral health during space flight missions has been routed in preventative dentistry, whereby astronauts are subjected to rigorous preflight dental screening and treatment.[15] in the face of future long-term space flight missions, research and discovery of new methods for maintaining oral hygiene and dental health under a microgravity environment are required.[15] Previous studies on the effects of spaceflight on the oral microbiota have primarily focused on culture-dependent methods of quantifying the numbers of oral bacteria in human subjects before and after spaceflight.[16,17,18] astronauts have been shown to exhibit decreased salivary lysozyme and increased dental calculus and gingival inflammation in response to space flight.[16] assessment of the physiology and virulence potential of oral pathogens such as S. mutans under controlled microgravity analog conditions has been relatively lacking In this respect, rotating wall vessel bioreactors are a common technology used to grow bacteria under microgravity analog conditions in ground-based studies. Both the simulated microgravity and normal gravity cultures underwent the same pattern of growth when monitored with this end-point approach: Log phase growth occurred between 0 and 6 h, and both cultures were in early stationary phase at 8 h growth, followed by entry into late stationary phase/death phase between 24 and 48 h growth
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