Effects of antibiotics on biofilms of P. gingivalis strains 381, HW24D1, 6/26, and W83, determined by using the flow cell model. Biofilms of P. gingivalis strain 381 were prepared by using perfused culture medium and an MRD for 14 days, and then each antibiotic was added for 3 days and ATP bioluminescence assay was performed. The ATP volume of the control group was set to 100%, and these figures indicate the percentage amounts of ATP when antibiotics were added. AZM inhibited biofilms at sub-MIC levels. ERY decreased ATP levels of the biofilm cells at levels over its MIC in strains 381 and W83. (A) AZM for strain 381; (B) ERY for strain 381; (C) AZM for strain HW24D1; (D) ERY for strain HW24D1; (E) AZM for strain 6/26; (F) ERY for strain 6/26; (G) AZM for strain W83; (H) ERY for strain W83. Ten biofilm samples were examined at each concentration. *, P < 0.005 (Student's t test).