Isolates previously thought to be Prevotella intermedia have been shown to be a closely related species now known as Prevotella nigrescens. The purpose of this study was to determine the efficacy of sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and polymerase chain reaction (PCR) to differentiate endodontic isolates of P. nigrescens from P. intermedia. Fifty-six strains of black-pigmented bacteria isolated from endodontic infections and conventionally identified as P. intermedia were used in this study. Using SDS-PAGE, novel polypeptide bands were used to differentiate P. nigrescens from P. intermedia. PCR was accomplished with specific primers for the 16S ribosomal RNA gene of both strains. Of 56 endodontic isolates, 41 (73%) strains were identified by SDS-PAGE as P. nigrescens and 15 (27%) strains as P. intermedia. Of the 41 strains of P. nigrescens identified by SDS-PAGE, PCR identified 37 strains as P. nigrescens. Restriction endonuclease digestion of amplified 16S ribosomal RNA genes indicated that the remaining four strains originally identified by SDS-PAGE as P. nigrescens were actually strains of Prevotella distinct from P. nigrescens and P. intermedia. Of 15 strains of P. intermedia identified by SDS-PAGE, PCR identified 14 strains as P. intermedia; but, one strain was identified as P. nigrescens. The results indicated that PCR was a more precise method than SDS-PAGE to differentiate P. intermedia from P. nigrescens. This study confirms that P. nigrescens is more commonly isolated in pure culture from endodontic infections than P. intermedia.
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