Comparison of Streptococcus mutans growth-phase-dependent transcriptome changes in two different simulated microgravity models

Streptococcus mutans is a cariogenic member of human dental plaque biofilm. Our previous study of S. mutans early stationary-phase rotary cell culture system (RCCS) cultures revealed significant transcriptome changes under low-shear modelled microgravity (LSMMG) compared to normal gravity (NG), as well as an altered cell clumping phenotype. In this current study, S. mutans mid-exponential phase and late-exponential phase cultures were compared using the RCCS (LSMMG and NG orientations) as well as simulated microgravity (‘0 g’) using a random positioning machine (0 g RPM). The previously described stationary-phase LSMMG-altered cell clumping phenotype was also observed at earlier growth phases in LSMMG and 0 g RPM. Significant fold-change differences in gene expression (late-exponential/mid-exponential phase) were observed in LSMMG, NG and 0 g RPM, with 24 genes shared between LSMMG and 0 g RPM cultures (including upregulated pyrimidine biosynthesis genes and decreased genes involved in purine biosynthesis, iron transport and malolactic fermentation). Additional significant functional enrichments included TCA cycle (LSMMG and NG), starch and sucrose metabolism (LSMMG and 0 g RPM) and folate biosynthesis (LSMMG, 0 g RPM and NG). Overall, these data provide insight into S. mutans physiology in response to simulated microgravity. Further investigation of transcriptome changes shared between LSMMG and 0 g RPM may allow us to better predict how this bacterium responds to spaceflight conditions.