The grains are yellow in color and hard in consistency. During surgery it may be difficult to distinguish the grains from fat which makes radical excision of the lesion difficult. This is especially so since the actinomycetoma lesion is not encapsulated.
In sections, the grain is rounded to oval, dense and homogenous. Characteristically marks of the microtome knife are seen in the grain in the form of parallel cracks. The grain stains a light purple or pink color in haematoxylin and eosin stained sections. The grain varies in size from 30 to 200 um. Hyphal elements embedded in cement can be visualized by Gram stain.
The grain is usually surrounded by an intense neutrophil polymorphonuclear leukocyte infiltrate (Type 1). Outside this zone, there is a vascular layer containing macrophage, lymphocytes, plasma cells and giant cells. The giant cells usually contain fragments of the grain. Some macrophages have a foamy cytoplasm. It looks as though the fragmentation of the grain induced by neutrophils is less severe than in M. mycetomatis. This may be due to the more compact and hard grains of S. somaliensis. Small grains surrounded by macrophages and giant cells (Type 11) are occasionally seen but pure epithelioid granuloma (Type 111) apparently does not occur. Giant cells containing viable actinomycetes are believed to aid the spread of the organism in the tissue and to the regional lymph nodes. Despite the invasive nature of S. somaliensis and other actinomycetes, tendons and nerves are resistant to invasion.
Ultra-structurally, the grain consists of heterogeneous and amorphous matrix arranged in an irregular and reticulate structure surrounding electron lucent areas between 1 and 5 um. In some of these spaces bacterial filament are found. The organisms are usually unicellular and coccoid and the cell wall is electron dense.