Recolonization of Streptococcus mutans on the Tooth Surface in Microgravity

Objectives: Attachments of Streptococcus mutans , an etiological agent to dental caries, to enamel surfaces are influenced by production or flow of saliva in oral cavity. Unlike earth gravity, microgravity environment may change production or flow of, for example, parotid gland saliva. As a result, development of dental caries may be revealed to either up-regulation or down-regulation in the microgravity. We aimed to investigate whether the microgravity affect recolonization of S. mutans on the tooth surface in the animal model. Methods: E2F-1 knockout mice that have abnormal features such as production loss of saliva were used for recolonization of S. mutans in flight experiments (parabolic and circular flights). The mice inoculated orally by 7 x 10⁹ S. mutans / in 250 m l PBS were placed in 12 times of a parabolic (20 seconds, microgravity and 20 seconds, 2G gravity) or a circular flights (20 seconds, only 2G gravity) by a plane. Quantitative analyses of S. mutans were performed by culture technique in swab samples on the tooth surfaces before and after test flights and control flight (normal flight without parabolic and circular flights). Results: In the parabolic flight experiments, the recolonization of S. mutans increased in compared with control. While, in the circular flight experiments, the recolonization decreased in compared with control. In analyses using these results, it was demonstrated that the microgravity tended to be more recolonized S. mutans on the tooth surface than earth gravity. Conclusion: The microgravity may affect flow of saliva and biofilm formation on the tooth surface in the oral cavity. This study suggest that attention to oral hygiene including special care may be required to diminish the risk of oral diseases in long time life for continued health of the pilots in the space station.