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[Objective]To figure out factors contributing to the rampant caries of a young man,and to explore his oral microbial structure.[Methods]Clinical samples including blood,saliva,supragingival dental plaque,tissues in the caries cavity,and mucosal swabs were collected with the patient,s consent.Blood were sent to the West China Hospital of Stomatology for immune examination and routine test.Microbes of saliva,supragingival dental plaque,tissues in the caries cavity,and mucosal swabs were investigated using gram-staining,culture,VITEK automatic microorganism identification machine,PCR-DGGE and sequencing.Moreover,the main cariogenic bacterium,i.e.,Streptococcus mutans,was quantified with qPCR.[Results]The patient has a high level of IgE (218.7 IU/ml) and segmented neutrophil (7.56×109/L) in his blood.Streptococci with extremely long chains were found in the saliva samples under microscope.By culturing,we found that bacterial concentration was highest in saliva (7.6× 107 CFU/ml) while C.albicans was highest in the dental plaque (2.1× 103 CFU/ml).In addition,relative abundance of hemolytic bacterium was detected to be highest in tissues from caries cavity (43.5%) and species belonging to streptococci and lactobacilli were recovered by MS and Rogosa culture medium from all clinical microbial samples.In total,38 bacterial species were identified by VITEK automatic microorganism identification system,of which streptococci and Leptotrichia wade were isolated from dental plaque sample,and Streptococcus mutans,Fusobacterium nucleatum,and Streptococcus tigurinus were isolated in the tissue from caries cavity.S.mutans was found to be rich in mucosal swabs,saliva and tissue of caries cavity via qPCR.Compared to samples collected from a healthy individual and another two rampant caries patients,decreased bacterial diversity (number of bands) and increased abundance (band brightness) were observed in the volunteer via the PCR-DGGE profile,and multiple Leptotrichia sp.were detected by gel sequencing.[Conclusion]The outgrowth of pathogenic microorganisms such as S.mutans and Leptotrichia sp.,and dysbiosis of oral microbial community might contribute to the pathogenesis of the rampant caries occurring.