E-test: a new technique for antimicrobial susceptibility testing for periodontal microorganisms.

The purpose of the study was to validate the Epsilometer test (E-test) method for antimicrobial susceptibility testing of selected periodontopathic microorganisms using the agar dilution method as a standard. The E-test has been developed to provide a direct quantification of antimicrobial susceptibility of microorganisms. The device consists of a predefined, continuous, and exponential gradient of antibiotic concentrations immobilized along a rectangular plastic test strip. After 48 hours incubation a drop-shaped inhibition zone intersects the graded test strip at the inhibitory concentration (IC) of the antibiotic. Twenty-two subgingival plaque samples from periodontitis sites were plated on trypticase soy agar supplemented with 5% rabbit blood or 5% sheep blood and trypticase soy agar supplemented with vancomycin and bacitracin. A total of 60 strains of key periodontal pathogens (Prevotella intermedia, Porphyromonas gingivalis, Actinobacillus actinomycetemcomitans, Eikenella corrodens, Campylobacter rectus, and Capnocytophaga) isolated from 22 sites of 12 patients were studied. The antibiotics tested were clindamycin, metronidazole, tetracycline, ciprofloxacin, cefoxitin, and ampicillin at concentrations above and below the achieved blood or gingival crevicular fluid levels. As a standard reference the minimal inhibitory concentrations (MICs) were determined using the agar dilution method. MICs were compared with ICs determined using the E-test method. The results showed an agreement ranging from 67% to 100%; sensitivity ranging from 75% to 100%; predictability ranging from 56% to 100% and specificity ranging from 33% to 96%. The E-test ICs for ampicillin, cefoxitin, and metronidazole against the Gram-negative capnophilic and microaerophilic rods and the black-pigmented anaerobic rods ICs for ampicillin, clindamycin, metronidazole, and tetracycline showed a high percentage of agreement with the agar dilution MICS.(ABSTRACT TRUNCATED AT 250 WORDS)