• The microorganism shown is Actinomyces, a gram-positive, branched, filamentous bacterium that typically aggregates into granules, called sulfur granules because of their grossly recognizable yellow color.
• The microorganisms stain positively with PAS, Gomori methenamine silver (GMS), and Gram but are not acid-fast. Nonetheless, an acid-fast stain is useful to distinguish Actinomyces from Nocardia, which is partially acid-fast.
• The inflammatory reaction consists of abscesses, often interconnected via sinus tracts that may drain spontaneously to body surfaces; foamy macrophages; ill-defined granulomas; and fibrosis. The granules are often surrounded by refractile, intensely eosinophilic, clublike projections (Splendore-Hoeppli phenomenon).
• Precipitates of antigen-antibody complexes are not unique to Actinomyces and can be seen around fungi, helminthic eggs and adult worms, and certain bacterial colonies.
• Clinically, the disease is classified into cervicofacial, thoracic, abdominal, and pelvic actinomycosis. Because of its mass effect, actinomycosis is often mistaken for a neoplasm.
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