Abstract: To explore the changes of bacteria in crevicular fluid during orthodontic treatment. Method: The crevicular fluids were cultured in anaerobic condition. Results: The gingivitis index was increased obviously and the percentages of Pg, Aa, Fn were increased correspondingly in the first two months of orthodontic treatment, whereas they were remarkbly reduced in the following treatment, which was close related with the improvement of oral hygiene. Conclusion: The three species of bacteria may be the main pathogens causing periodontal disease in patients with orthodontic treatment.

Key words: Orthodontic appliances, Crevicular fluid, Bacteria