Question
Laboratory diagnosis of actinomycosis
Answer
Actinomycosis is a chronic, slowly progressive suppurative and granulomatous infection caused most commonly by Actinomyces israelii, a Gram-positive, filamentous, branching, anaerobic/microaerophilic bacterium that is part of normal oral/gastrointestinal flora, becoming pathogenic when it invades tissue following mucosal breach (dental extraction, trauma, or surgery).
Clinical presentation: classically presents as a chronic, indurated swelling with multiple draining sinus tracts, most often in the cervicofacial region (“lumpy jaw”), but also thoracic, abdominal, or pelvic forms; the discharge characteristically contains visible “sulphur granules” (yellow, macroscopic granules representing colonies of the organism).
Laboratory diagnosis:
- Direct examination of pus/sulphur granules — crushed between two slides and examined; Gram stain shows Gram-positive branching filaments; the granules, when sectioned, show a characteristic radiating, “ray fungus” appearance under low power — filaments radiating outward with eosinophilic clubbed ends (a Splendore-Hoeppli reaction), historically leading to the mistaken belief the organism was a fungus (hence “-mycosis” in the name, despite it being a true bacterium).
- Culture — technically demanding, requiring strict anaerobic/microaerophilic conditions and prolonged incubation (up to 2–3 weeks); grows as characteristic “molar tooth” colonies on enriched media (e.g., brain-heart infusion agar).
- Histopathology — biopsy of the lesion shows the characteristic sulphur granules with surrounding neutrophilic infiltrate and granulation tissue, confirming the diagnosis.
- Molecular methods (PCR, 16S rRNA sequencing) — increasingly used where culture is difficult or inconclusive.
Treatment: prolonged high-dose penicillin therapy (often for several weeks to months), sometimes combined with surgical drainage/debridement of sinus tracts and abscesses, since the organism’s typically deep-seated, fibrotic lesions are relatively difficult to eradicate with antibiotics alone.