Diagnostic use of negative staining of bacteria?

A definitive diagnosis requires demonstration or isolation of the organisms responsible for the infection. Direct Gram stain may be helpful because of the frequently unique morphology of Gram-negative anaerobic bacilli. In general, these organisms are pale staining and they may stain erratically. Fusobacterium cells may exhibit classic tapered ends and filamentous forms, with or without swollen areas and large round bodies. Direct gas-liquid chromatography of clinical specimens occasionally provides important clues to the presence of certain Gram-negative anaerobic bacilli. A large amount of butyric acid in the absence of isobutyric or isovaleric acid indicates the presence of Fusobacterium. The presence of succinic acid and only Gram-negative rods seen on Gram stain, or of both succinic and isobutyric acid in the specimen, indicates that Bacteroides is present. Both direct and indirect fluorescent antibody techniques may be useful for rapid detection of Bacteroides, Prevotella, and Fusobacterium in clinical material.