Morphological Characteristics of Actinomycosis
Histopathology
Actinomycosis is a chronic granulomatous infection of the skin caused by anaerobic, filamentous, Gram-positive bacteria. On histological examination, actinomycosis lesions are characterized by granulomas composed of histiocytes, lymphocytes, and fibroblasts, as well as areas of necrosis and microabscesses. The presence of sulfur granules, which are yellow-brown, spherical, basophilic microcolonies of bacteria, is a common feature of actinomycosis.
Microscopy
Under light microscopy, Gram-staining of the bacteria reveals Gram-positive rods, often in aggregates known as “sulfur granules”. These rods may be found in the abscess cavities of the granulomas, and they typically measure between 0.5-2.0 microns by 4-10 microns. Ultrastructural examination of the bacteria reveals that they possess a thick peptidoglycan layer, as well as a capsule composed of polysaccharides and proteins.
Culture
Actinomycosis is typically diagnosed by culturing a tissue sample or aspirate from the infected area. The bacteria can be cultured on agar or in liquid media, and they will form branching filaments. The colonies of the bacteria can be identified by their characteristic “fried egg” appearance, which is caused by the release of the enzyme lipase into the medium. The bacteria can also be identified by their biochemical patterns, such as their positive reaction to Voges-Proskauer, urease, and nitrate tests.