Objective To compare the susceptibility of Porphyromonas gingivalis to metronidazole at different planktonic cell densities and in biofilm, and to evaluate the role of cell density in antibiotic drug resistance in Porphyromonas gingivalis biofilm. Methods The minimum inhibitory concentration（MIC） and the minimum bactericidal concentration（ MBC） of metronidazole against Porphyromonas gingivalis were detected by a broth dilution method under a final inocula of 106 CFU·mL-1 and 109 CFU·mL-1（cell number equal to biofilm）. After the initial biofilm formed in the microtiter plate wells, the MIC and MBC of metronidazole to the intact and succedent resuspended biofilm were determined. Results The MIC and MBC of metronidazole against 106 CFU·mL-1 planktonic Porphyromonas gingivalis were 0.063, 0.125 mg·L-1 respectively. However, those against 109 CFU·mL-1 planktonic Porphyromonas gingivalis were 25, 50mg·L-1. Against intact Porphyromonas gingivalis biofilm, the MIC was 25mg·L-1 and MBC was higher than 125mg·L-1, those against resuspended biofilm was 25, 125 mg·L-1 respectively. Conclusion The resistance of Porphyromonas gingivalis to metronidazole increases along with the augment of the bacterial density. Cell density plays an important role in the resistance of biofilm. However, extracellular matrix and the integrity of biofilm may be the other influence factors for the biofilm resistance.
