Objectives: The in vitro susceptibility of various putative periodontopathic species to azithromycin were assessed using the E-test method. Methods: Inocula prepared from pure cell suspensions of each test organism were adjusted to a 1.0 McFarland turbidity standard and applied to Oxyrase plates containing enriched Brucella blood agar. After drying, azithromycin E-test strips were placed, and minimal inhibitory concentrations (MIC) read after 72 hours of aerobic incubation where elliptical zones of bacterial inhibition intersected the E-test strip MIC scales. Results: Porphyromonas gingivalis, Porphyromonas endodontalis, Prevotella intermedia, Bacteroides forsythus, Dialister pneumosintes, Actinobacillus actinomycetemcomitans (serotypes a and b), Fusobacterium nucleatum, Campylobacter rectus, Selenomonas noxia, Centipeda periodontii, Streptococcus constellatus, Streptococcus intermedius, and Staphylococcus aureus were sensitive in vitro to azithromycin at MIC values ranging from < 0.016 to 1.0 mg/ml. Enterococcus faecalis, Eschericheria coli, and Pseudomonas aeruginosa demonstrated in vitro resistance to azithromycin (MIC values=12-64 mg/ml). Peptostreptococcus micros (ATCC 33270 and 44 subgingival isolates from severe periodontitis patients) exhibited considerable in vitro variability to azithromycin (MIC range < 0.016 to > 256 mg/ml; MIC₅₀=0.094 mg/ml; MIC₉₀=> 256 mg/ml). Conclusions: The in vitro sensitivity of many putative periodontal pathogens, with the exceptions of E. faecalis, enteric rods/peudomonads, and many clinical strains of P. micros, to therapeutic levels of azithromycin indicates the drug's potential usefulness in treatment of human periodontitis.

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