Minimum requirements for a rapid and reliable routine identification and antibiogram of Fusobacterium necrophorum

Three hundred fifty-seven isolates of Fusobacterium necrophorum from human infections in Denmark were consecutively collected over a 3 year period for the purpose of establishing the minimum requirements for rapid and reliable routine identification of Fusobacterium necrophorum using phenotypic characters. The first 40 isolates were fully characterized by the most common phenotypic tests mentioned in the literature, while the last 317 where identified solely by the established minimum requirements for rapid and reliable routine identification of Fusobacterium necrophorum. All but one isolate were identical in all phenotypic tests. The outlying strain differed in morphology and the ability to agglutinate erythrocytes. On the basis of our findings it should be possible within 3-4 days to identify and differentiate F. necrophorum from other species including other Fusobacterium spp. by the unique but subspecies specific colony morphology, susceptibility to kanamycin and metronidazole, the smell of butyric acid, chartreuse colour fluorescence, and beta-haemolysis on horse blood agar. Three-hundred fifty-six isolates were identified as F. necrophorum subsp. funduliforme while one strain was F. necrophorum subsp. necrophorum. The species and subspecies level was confirmed for the first 40 isolates by real-time PCR. MIC in mg/l was determined for the 40 isolates. MIC(90) was 0.047 for penicillin, 0.047 for clindamycin, 0.25 for metronidazole, 0.38 for cefuroxime, >32 for imipenem, 0.012 for meropenem, and 2 for erythromycin. All 357 isolates were susceptible to penicillin and metronidazole indicating that these antibiotics are still the drugs of choice in antibiotic therapy of F. necrophorum infections, but therapy with clindamycin may be an alternative. Erythromycin should be avoided.