Actinomyces

Actinomyces is a filamentous, gram-positive rod that grows anaerobically to microaerophilically and forms branching hyphae resembling fungi, though it is a true bacterium (no chitin/ergosterol in cell wall). It is part of normal flora of the mouth, GI tract, and female genital tract. Pathogenesis involves disruption of mucosal barriers (e.g., dental extraction, trauma, aspiration, IUD placement), allowing the organism to invade deeper tissues and form chronic, indurated abscesses with draining sinus tracts. A hallmark finding is 'sulfur granules'—yellow, cohesive colonies of organisms with a filamentous, radiating (Splendore-Hoeppli phenomenon) appearance seen in pus. Common presentations include: cervicofacial actinomycosis ('lumpy jaw'), the most common form, following dental work; thoracic actinomycosis, from aspiration, mimicking malignancy or TB; abdominal/pelvic actinomycosis, associated with prolonged IUD use, appendicitis, or bowel surgery. Diagnosis relies on anaerobic culture (slow growing), Gram stain showing branching gram-positive filaments, and histology demonstrating sulfur granules. Treatment is prolonged high-dose penicillin (often IV followed by oral for weeks to months) due to the organism's tendency to form dense fibrotic abscesses with poor antibiotic penetration; surgical drainage may be needed. Contrast with Nocardia: Nocardia is aerobic, partially acid-fast (weakly acid-fast due to mycolic acids), and typically causes pulmonary or CNS infection in immunocompromised patients, treated with sulfonamides (e.g., TMP-SMX), whereas Actinomyces is anaerobic, not acid-fast, and occurs in immunocompetent hosts with mucosal disruption, treated with penicillin. This classic exam distinction ("SNAP": Sulfa for Nocardia, Actinomyces = Penicillin) is high-yield for USMLE.