Abstract: Objective:To study the co?culture of Leptotrichia buccalisstrain (L.buccalis) and its supernatant with Streptococcus mutans (S.m) to observe the acid production, adhesion changes and ldh, gtfb, gtfc, gtfd gene expression changes. Methods: L.buccalis were isolated from the supragingival plaque and identified by biochemical identification and sequencing, L.buccalis culture group, S.mculture group, L.buccalis and S.m co?culture group, and S.m were added to the L.buccalis supernatant culture group and cultured for 9 hours, and the acid production capacity of each group was compared every hour. Then the L.buccalis culture group, the S.m culture group, the S.m co?culture with 1, 2 and 3 mL of L.buccalis culture group and add 1,2 and 3mL L.buccalis supernatant into S.m culture group to cultivate for 6 hours, and use real?timeRT?PCR method to determine and compare the gene expression of ldh, gtfb, gtfc and gtfd; Add 1,2,3 mL of L.buccalis supernatant to S.m andc o?culture for 9h, and the pH was measured every 1 hour. Compare the changes in acid production capacity; add 50,100 and 150 μL of L.buccalis supernatants to S.m and culture for 24h, and observe the changes in the adhesion number of S.m under a laser confocal microscope. Add 1,2,3mL of L.buccalis supernatant to S.m and culture for 6h,and use real?time RT?PCR to determine the gene expression of ldh, gtfb, gtfc and gtfd. Results: This experiment successfully isolated L.buccalis; After adding L.buccalis or L.buccalis supernatant to S.m, the pH value decreased faster, and the number of bacterial adhesions increased, and the volume of the L.buccalis or L.buccalis increased ldh, gtfb, gtfc and gtfd gene expression increased. Conclusions: L.buccalis and its supernatant can regulate the acid production and adhesion ability of S.m.

Key words: caries, dental plaque biofilm, L.buccalis supernatant, Streptococcus mutans, acid production