A sulfur granule is depicted in an abscess in a case of thoracic actinomycosis. The granules are composed of branched, gram-positive filaments of Actinomyces israelii. Though these organisms resemble fungi morphologically, they are bacteria of the Order Actinomycetes; they are anaerobic. They lack mycolic acid in cell walls and thus are not acid-fast.

Infection is characterized by multiple abscesses with sinus tracts which interconnect and contain sulfur granules. The thorax is involved in about 15% of cases; other cases are cervicofacial, abdominal, pelvic, and/or cerebral.

From the slide collection of the late Dr. Charles Kuhn