The deoxyribonucleic acids (DNA) of Fusobacterium polymorphum ATCC 10953 and F . fusiforme ATCC 23726 were submitted to DNA-DNA hybridization procedures. A polynucleotide homology of 78% was demonstrated, indicating that the recent grouping of all oral Fusobacterium strains into one species, F . nucleatum, should be reevaluated. The polynucleotide homology between Leptotrichia buccalis ATCC 19616 and the two strains of Fusobacterium tested was less than 30%. The guanine plus cytosine (G+C) values of the DNA ofFusobacterium and Leptotrichia strains tested were similar and unusually low (24.5 to 25.6 mol%). Since bacteria of the family Bacteroidaceae reportedly have G+C values of 41 to 42 mol%t, the placing of the genera Fusobacterium and Leptotrichia into this family is questionable. The similar unusually low G+C values of the DNA of Fusobacterium and Leptotrichia may indicate a close phylogenetic relationship for these genera.
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