Tenacity of biofilms formed by P. gingivalis wild tstrain and mutants. Standardized cultures of P. gingivalis were inoculated into dTSB in saliva-coated 12-well polystyrene plate and incubated in a static manner at 37°C for 60 hours, with the resulting biofilms sonicated for 1 second. Immediately after sonication, supernatants containing floating cells were removed by aspiration and the biofilm remains were gently washed with PBS. P. gingivalis genomic DNA was isolated from the biofilms and the numbers of P. gingivalis cells were determined using real-time PCR. Relative amounts of bacterial cell numbers were calculated based on the number of wild-type cells without sonication considered to be 1.0. Percentages shown indicate the amount of remaining biofilm after sonic disruption. The experiment was repeated independently three times with each strain in duplicate. Standard error bars are shown. Statistical analysis was performed using a Scheffe test. *p < 0.05 and **p < 0.01 in comparison to the wild-type strain.