| Background and Objective: During the orbital flight of astronauts,microgravity influence factors are always present,and microgravity can affect the body’s physiological functions.It has been found that the risk of oral diseases such as periodontal disease and caries increases in weightlessness or simulated microgravity environment,and the development of caries and periodontitis is closely related to microbes in the oral cavity.As a complex,flowing biological fluid in the oral cavity,dysfunction of the active ingredient,flow rate and other secretion functions of saliva can lead to oral tissue disorders.With the advent of high-throughput sequencing technology,alterations in salivary microbes are increasingly being used for monitoring and diagnosis of oral and even systemic diseases.However,the effects of microgravity on the physicochemical properties of saliva in the oral cavity are rarely studied,and the effects of microgravity on oral microbes are still insufficient.Therefore,this study simulates microgravity in space using a ground-based-6° head-down bed rest(HDBR)test to observe the effects of microgravity on oral salivary secretion and the salivary microbial community to provide a scientific basis for the prevention and treatment of oral diseases in astronauts.Methods: Sixteen healthy young male volunteers were recruited for a twenty-eight-day experiment,which included a seven-day adaptation period,a fifteen-day HDBR period,and a six-day recovery period.Non-stimulated whole saliva was collected on day 1(AS group),on days 5(BS group),10(CS group),and 15(DS group)of HDBR,and day 6(ES group)of recovery.Salivary p H and salivary flow rate were measured,and the V3–V4 region was sequenced and analyzed in 80 saliva samples using 16 S r RNA gene sequencing.Results: 1.There were no significant differences in salivary p H and salivary flow rate between any two groups(P > 0.05).However,salivary p H tended to decrease and salivary flow rate tended to increase in the BS,CS,and DS groups compared with the AS group,and they all returned to pre-HDBR levels in the ES group.2.A total of 4,451,246 high-quality sequences were obtained from 80 saliva samples;5,216 OTUs were obtained by clustering with 97% consistency,of which 3,544(67.94%)OTUs were annotated to the database;a total of 25 phyla,36 classes,75 orders,137 families,298 genera and 381 species were annotated.3.Alpha diversity showed that there was no significant difference in Alpha diversity between any two groups(P>0.05).4.Beta diversity showed that there were significant differences(P<0.05)between the AS group and the other four groups.5.Species significant difference analysis showed that the relative abundance of the dominant genera of Actinomyces,Parvimonas,Peptostreptococcus,Porphyromonas,Oribacterium,and Capnocytophaga increased significantly,and the relative abundance of Neisseria and Haemophilus decreased significantly after HDBR.However,the relative abundance of Oribacterium and Capnocytophaga did not recover to the preHDBR level in the ES group.6.Network analysis showed that the number of correlations between genera decreased after HDBR and did not reach the original level in the ES group.7.PICRUSt analysis showed that the relative abundance of genes for replication and repair,translation and nucleotide metabolism functions was significantly higher and the relative abundance of genes for poorly characterized and cellular processes and signaling functions was lower after HDBR.Conclusion: No significant changes in salivary p H and salivary flow rate were observed in young male subjects in 15 days-6° HDBR,while the structure of the salivary microbial community changed significantly,with an increase in the dominant microbial taxa associated with oral diseases and a decrease in bacteria associated with oral health,and changes in the correlations between genera,which may have an impact on the homeostasis of the salivary microbial community. |
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