Effects of various antibiotics on Porphyromonas gingivalis biofilms

Objectives: Antibiotic resistance of biofilm-forming bacteria contributes to chronic infections such as marginal and periapical periodontitis, which are strongly associated with Porphyromonas gingivalis. Parts of macrolides inhibit the biofilm formation to affect the quorum sensing system in Pseudomonas aeruginosa. We evaluated the effects of various antibiotics including macrolides on P. gingivalis biofilm. Methods: Minimum inhibitory concentrations (MICs) of azithromycin, erythromycin, ofloxacin, ampicillin, gentamicin for planktonic P. gingivalis strain 381 were determined by a microdilution method. P. gingivalis 381 was incubated and formed biofilms on 96-well flat-bottom plates under anaerobic conditions. After the supernatants were removed, the mediums containing each antibiotic of several concentrations (0.6-640 µg/ml) were added, and the plates were incubated for 72 hours. P. gingivalis biofilms were stained with 1% crystal-violet, rinsed, and supernatants were extracted by ethanol. The optical density (OD) was measured at 595 nm using a spectrophotometer. Results: MICs of azithromycin, erythromycin, ofloxacin, ampicillin, and gentamicin for planktonic P. gingivalis 381 were 0.5, 0.5, 0.5, 0.06, and 512 µg/ml, respectively. The numbers of biofilm-forming P. gingivalis cells were statistically reduced at azithromycin and erythromycin concentrations of ³ 0.6 µg/ml, compared with the controls (p<0.01). The OD values of the biofilm-forming cells were significantly decreased at ofloxacin and ampicillin concentrations of ³2.5, ³20µg/ml, respectively, compared with the control groups. Gentamicin showed no significant efficacy against P. gingivalis biofilms. Conclusions: The significant reductions of P. gingivalis biofilm by azithromycin and erythromycin were equal to their MICs for these 2 antibiotics, but those by ofloxacin and ampicillin were equal to 5 and more than 300 times of each MICs, respectively. Macrolides examined in this study were effective to P. gingivalis biofilms as well as their planktonic cells. This study was supported by a Grant-in-Aid for Scientific Research (#18390505, #19592198) from the Japan Society for the Promotion of Science.