Species-Dependent Aggregation of Oral Bacteria Under Simulated Microgravity

Objective: Simulated microgravity (SMG) alters fluid dynamics and may influence microbial aggregation. However, its effects on oral bacteria remain unclear. This study investigated the effects of SMG on the growth and aggregation of representative oral bacteria. Methods: Four facultative oral bacteria representing distinct ecological and adhesive properties—Staphylococcus aureus(opportunistic pathogen), Streptococcus mutans (glucan-producing cariogenic bacterium), Streptococcus salivarius (commensal fructan-producing bacterium), and Streptococcus sanguinis (early colonizer)—together with the periodontal pathogen Porphyromonas gingivalis were cultured under SMG (10−3G) or normal gravity (NG). Streptococci were grown with or without sucrose to evaluate the role of extracellular polysaccharides. Growth was assessed by optical density, and aggregation was quantified by image analysis. Results: SMG induced aggregation in a species-dependent manner. S. mutans and S. salivarius formed spherical aggregates under SMG, whereas little or no aggregate formation was observed in S. aureus, S. sanguinis, or P. gingivalis. Aggregation of S. mutans was markedly reduced without sucrose, indicating dependence on glucan-mediated adhesion. S. salivarius also showed reduced aggregation under sucrose-free conditions, although limited aggregation persisted. Growth kinetics were virtually unaffected by SMG. Conclusions: SMG promotes aggregation of selected oral bacteria through species-specific mechanisms. These findings suggest that microgravity may alter the architecture of oral microbial communities by enhancing particular intercellular adhesion pathways.