STEPTRACKERMICROBIOLOGY | GRAM-POSITIVE BRANCHING RODS | HIGH-YIELD REVIEW

Actinomyces & Nocardia

Two filamentous, branching gram-positive rods that look like fungi — one anaerobic and oral, the other aerobic, acid-fast and opportunistic.

FOCUS ON
Actinomyces israelii: anaerobe, not acid-fast, normal oral/GI/female tract flora → face and neck abscesses after dental trauma, draining sinus tracts with yellow sulfur granules; IUD-associated pelvic disease → penicillin. Nocardia: aerobe, weakly acid-fast, soil → pulmonary infection in immunocompromised (mimics TB), brain abscess, cutaneous after trauma → TMP-SMX. Pairing: Nocardia → sulfonamide, Actinomyces → penicillin.
FEATURE
ACTINOMYCES
NOCARDIA
CLUE
Oxygen
Anaerobic
Aerobic
—
Acid-fast
No
Weakly (modified Ziehl-Neelsen)
Both are gram ⁺ branching filaments
Habitat
Normal flora of mouth, GI tract, female genital tract
Soil, dust
Not transmitted person to person
Host
Immunocompetent (after tissue injury)
Immunocompromised (steroids, transplant, AIDS), also some healthy
—
Classic disease
Cervicofacial "lumpy jaw" abscess after dental extraction or trauma
Pulmonary nodules/cavities (TB-like), then brain abscess
Nocardia: always image the brain
Other sites
Thoracic, abdominal (after surgery), pelvic actinomycosis with long-term IUD
Cutaneous (gardening thorn injury), mycetoma
—
Signature finding
Sinus tracts draining yellow sulfur granules that cross tissue planes
Branching, beaded, weakly acid-fast rods
Granules are bacterial colonies, not sulfur
Catalase / urease
Catalase −
Catalase ⁺, urease ⁺
Nocardia is a CGD pathogen
Treatment
High-dose penicillin for weeks–months; drainage
TMP-SMX (± imipenem, amikacin for CNS)
Nocardia–sulfa · Actinomyces–penicillin
Look-alikes
Fungal mycetoma, TB scrofula
TB (acid-fast, lung), Aspergillus (septate hyphae)
Aspergillus = septate hyphae with acute (45°) branching

Mechanism — how each gets in

ActinomycesAlready in the mouth
→
Dental trauma / surgeryBreaches mucosa
→
Chronic abscessBurrows across tissue planes
NocardiaInhaled from soil
→
Weak cell-mediated immunity
→
Lung → blood → brain

Branching organisms

ORGANISM
TYPE
CLUE
Actinomyces
Anaerobic bacterium
Sulfur granules, penicillin
Nocardia
Aerobic bacterium, acid-fast
Brain abscess, TMP-SMX
Aspergillus
Mold, septate hyphae, 45°
Voriconazole
Mucor/Rhizopus
Mold, nonseptate, 90°
DKA, amphotericin

How it's tested

COMMON CONFUSIONNocardia lung disease looks like reactivation TB (cavities, weight loss) — weakly acid-fast branching rods on modified stain point to Nocardia.
THE TRAPA transplant patient on TMP-SMX prophylaxis rarely gets Nocardia; one who stopped it may.
YOUR APPROACHBranching gram ⁺ rods → oxygen and acid-fast status → host and site → sulfa or penicillin.

If you see… think…

Jaw abscess with draining yellow granules after tooth extraction → Actinomyces
Pelvic mass in a woman with an IUD for years → Actinomyces
Lung cavity then brain abscess in a transplant patient → Nocardia
Weakly acid-fast branching filaments → Nocardia
Treatment of Actinomyces → penicillin
Treatment of Nocardia → TMP-SMX
Anaerobic, not acid-fast branching rods → Actinomyces
Skin nodules spreading up the arm after gardening, bacterium → Nocardia (fungus: Sporothrix)
Catalase ⁺ organism causing infection in CGD → Nocardia
Infection crossing fascial planes, ignoring anatomy → actinomycosis