Other than Gram stain of patient specimens, there are no specific procedures for the direct detection of these organisms in clinical material. Gram stain of Arcanobacterium spp. shows delicate, curved, gram-positive rods with pointed ends and occasional rudimentary branching. This branching is more pronounced after these organisms have been cultured anaerobically. Arcanobacterium spp. stain unevenly after 48 hours of growth on solid media and also exhibit coccal forms.
Lactobacillus is highly pleomorphic, occurring in long chaining rods, as well as in coccobacilli and spiral forms (Figure 20-1),
E. rhusiopathiae stains as both short rods and long filaments. These morphologies correspond to two colonial types, that is, (1) rough colonies that contain slender, filamentous gram-positive rods with a tendency to overdecolorize and become gram-negative and (2) smooth colonies that contain small slender rods. This variability in staining and colonial morphology may be mistaken for a polymicrobial infection both on direct examination and culture.
Gardnerella are small, pleomorphic gram-variable or gram-negative coccobacilli and short rods. The wet mount and Gram stain of vaginal secretions are key tests for diagnosing bacterial vaginosis caused by G. vaginalis. A wet mount prepared in saline reveals the characteristic "clue cells," which are large, squamous epithelial cells with numerous attached small rods. A Gram-stained smear of the discharge shows the attached organisms to be gram-variable coccobacilli. Clue cells typically are present, and large numbers of gram-
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