Phenotypic characteristics and DNA relatedness in Prevotella intermedia and similar organisms

We have recently isolated from the gingival pockets, periapical lesions and saliva some anaerobic gram-negative, black-pigmented rods. Many of these isolates exhibited phenotypic characteristics similar to Prevotella intermedia (Bacteroides intermedius). However, several of these isolates, although resembling P. intermedia in most of the phenotypic expressions, were capable of fermenting lactose, a biochemical characteristic atypical of P. intermedia. These atypical clinical isolates (strains capable of fermenting lactose) and isolates exhibiting more typical phenotypic characteristics (i.e., lactose nonfermenting) were definitively identified as P. intermedia by DNA-DNA hybridization using a photoprobe biotin method. Quantitative hybridization of clinical isolates with labeled DNA of P. intermedia-type strains (ATCC 25611 and ATCC 33563) showed that almost all the clinical strains isolated from disease sites of adults belonged to the ATCC 25611 group, whereas strains isolated from the saliva of children belonged to the ATCC 33563 group. These data, together with the phenotypic characterization of the isolates, suggested that P. intermedia is a heterogeneous species both phenotypically and genetically.