Numerous methods have been described for antimicrobial susceptibility testing. Objectives: To evaluate reference strains of human oral microorganisms against metronidazole using a fast microdilution broth procedure. Methods: Twenty-nine strains of bacteria: Actinobacillus actinomycetemcomitans, Actinomyces gerencseriae, israelli, odontolyticus, Campylobacter gracilis, rectus, Capnocytophaga gingivalis, sputigena, Eikenella corrodens, Fusobacterium nucleatum ss nucleatum, nucleatum ss polymorphum, periodonticum, Gemella morbillorum, Neisseria mucosa, Peptostreptococcus micros, Prevotella melanogenica, intermedia, Propionibacterium acnes, Selenomonas noxia, Streptococcus anginosus, gordonii, salivarius, mitis, oralis, sanguis, intermedius and Tannerella forsythensis were tested for their capacity to grow with metronidazole. Bacteria were initially grown on agar plates optimized for growth of the specific species. Plates were incubated for 7 days at 35ºC, in 80% N₂, 10% CO₂ and 10% H₂. Aliquots of these samples were used to inoculate eppendorf tubes containing 2% mycoplasma broth (BBL) and placed into a 96-well microtiter plate. For the antibacterial assays, metronidazole was dissolved in distiled water to a final concentration ranging (0, 1, 2, 4, 8, 16, 32 and 64 µg/ml). The adequacy of growth conditions and the sterility of the medium were checked in columns of control wells. The microplates were seeded with 0.1ml of bacterial inoculum and the samples were then incubated and growth evaluated by microplate reader at 0, 27, and 68 h under anaerobic conditions by the increase 630 nm absorbance. Results:The results were expressed as MIC, i.e. the minimum concentration which completely inhibited bacterial growth. Metronidazole displayed greater activity towards the F. nucleatum ss polymorphum; periodonticum; P.intermedia; melanogenica and C.sputigena(MIC £ 1 µg/ml) and A. actinomycetemcomitans serotype a (MIC ³ 32 µg/ml). Metronidazole was inactive against streptococci and other bacteria.Conclusions: These data indicate that the microdilution broth method using microplate reader can be used to monitoring of antibiotic (metronidazole) activity against oral bacteria under anaerobic conditions.

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