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Gram-positiveBacilliBranching filaments

Actinomyces israelii

Filamentous, branching Gram-positive anaerobic bacterium that causes actinomycosis, characterized by chronic suppurative infections with sulfur granules typically affecting the cervicofacial, thoracic, and abdominal regions.

Organism Card

DomainMust know
Identity
  • Gram-positive, filamentous, branching rods (beaded appearance)
  • Anaerobic to microaerophilic; NOT acid-fast (distinguishes from Nocardia)
  • Normal commensal of oral cavity, GI tract, and female genital tract
  • Classified as a bacterium (not a fungus), despite fungus-like morphology
Lab discriminator
  • "Sulfur granules" (yellow granules) in pus/tissue = pathognomonic [1]
  • Slow-growing on anaerobic culture (may need 2–3 weeks); colonies described as "molar tooth" morphology
  • NOT acid-fast on modified Ziehl-Neelsen (vs Nocardia which IS partially acid-fast)
  • Gram stain: filamentous branching G+ve rods
Reservoir / transmission
  • Endogenous infection from patient's own oral, GI, or genital flora
  • NOT transmitted person-to-person
  • Requires break in mucosal barrier (dental procedures, trauma, IUD, surgery, aspiration)
  • Cervicofacial form classically follows dental extraction or jaw trauma [2]
Key virulence
  • Low intrinsic virulence; opportunistic when mucosal integrity lost
  • Forms chronic granulomatous abscesses with sulfur granules
  • Tissue invasion crosses anatomical planes and ignores tissue boundaries ("crosses tissue planes")
  • Dense biofilm-like colonies protect from host immunity
  • Often polymicrobial (co-infecting bacteria e.g. Aggregatibacter, Eikenella act as "companion organisms" reducing local O₂)
Clinical syndromes
  • Cervicofacial actinomycosis (most common, ~55%): lumpy jaw; chronic draining sinus tracts on jaw/neck; follows dental work [2]
  • Thoracic actinomycosis (~15%): mimics lung cancer or TB; chest wall sinus tracts; follows aspiration
  • Abdominal/pelvic actinomycosis (~20%): post-appendicectomy, perforated viscus; classic association with IUD use (pelvic actinomycosis)
  • CNS: rare; brain abscess or chronic meningitis [3]
  • Key clue: chronic infection crossing tissue planes + draining sinuses + sulfur granules
Diagnosis
  • Specimen: pus/tissue from sinus tract or abscess drainage
  • Histopathology: sulfur granules with "ray fungus" (club-shaped peripheral filaments on H&E)
  • Anaerobic culture: slow-growing, "molar tooth" colonies; may take ≥ 14 days
  • Gram stain of granule: branching filamentous G+ve rods
  • Pitfall: culture often negative if anaerobic conditions not maintained or if short incubation [1]
Treatment
  • First-line: prolonged high-dose IV penicillin G (4–6 weeks), then oral amoxicillin for 6–12 months total
  • Alternative: doxycycline, erythromycin, or clindamycin if penicillin allergy
  • Surgical drainage of abscesses / excision of sinus tracts often needed
  • Remove IUD in pelvic actinomycosis
  • Resistance to penicillin is extremely rare
Prevention
  • Good oral hygiene and dental care
  • Appropriate IUD surveillance (routine Pap smear may detect Actinomyces-like organisms)
  • No vaccine available; no isolation required; not notifiable
Classic traps
  • Actinomyces vs Nocardia: both are branching filamentous G+ve rods, but Nocardia is aerobic, partially acid-fast, and causes pulmonary disease in immunocompromised; Actinomyces is anaerobic, NOT acid-fast, and causes cervicofacial disease in normal hosts
  • Do NOT confuse with fungal infection despite "mycosis" in name — it is a bacterium
  • Lumpy jaw + sulfur granules + sinus tracts = Actinomyces (not Nocardia, not fungus)
  • Pelvic mass in woman with IUD → think Actinomyces (not only malignancy)

Exam Intelligence

  1. GC 101 (Diagnosis of Infections) emphasises that prolonged anaerobic culture is essential — standard short-incubation aerobic cultures will miss Actinomyces [1]. This is a common exam pitfall: a "culture-negative" chronic abscess with sulfur granules should prompt request for extended anaerobic incubation.

  2. GC 105 (Medically Important Microbes) lists Actinomyces israelii among important anaerobic bacteria every doctor should know [4]. Key teaching: it is a bacterium, NOT a fungus — despite the "-mycosis" suffix.

  3. GC 219 (Infections and Tumours in Pharynx and Oral Cavity): cervicofacial actinomycosis is the classic oral/pharyngeal infection that presents as a chronic draining jaw lesion after dental procedures [5]. Examiners may present a post-dental-extraction jaw mass with draining sinuses — the answer is Actinomyces israelii.

  4. Ryan Ho Neurology notes include Actinomyces as a cause of chronic meningitis and brain abscess [3] — this is a less common but examinable presentation, especially in the differential of chronic/subacute CNS infections alongside TB and Cryptococcus.

  5. GC 071 (It is Red and Painful) — in the context of chronic soft-tissue infection with sinus formation, Actinomyces should be considered alongside TB and atypical mycobacteria [6]. The distinguishing feature is sulfur granules.

  6. AOS Microbiology: Actinomyces is classically tested as "the anaerobic branching G+ve rod that is NOT acid-fast" — the single most important discriminator from Nocardia [7].

FeatureActinomyces israeliiNocardia asteroides
Oxygen requirementAnaerobic/microaerophilicStrict aerobe
Acid-fast stainNegativePartially acid-fast (modified ZN +)
Classic hostImmunocompetent (after mucosal breach)Immunocompromised (steroids, transplant)
Classic siteCervicofacial (lumpy jaw)Pulmonary → brain abscess
Sulfur granulesPresent (pathognomonic)Absent (granules rare)
TreatmentPenicillin (prolonged)TMP-SMX
  • Vignette clue "sulfur granules" → always Actinomyces, never Nocardia.
  • "Crosses tissue planes" is a classic phrase examiners use to hint at actinomycosis.
  • IUD + pelvic abscess in a woman → think Actinomyces before thinking malignancy.
  • Chronic lung mass mimicking carcinoma with chest wall sinus → thoracic actinomycosis; examiners may present imaging that looks like bronchogenic CA.

No directly relevant past paper questions specifically examining Actinomyces israelii were identified in the indexed past paper context (2016–2025 Fourth Summative papers). However, note the following related stems:

  • 2023 Fourth Summative Minicase Q9 [8]: "Name three microorganisms that can commonly cause infective spondylitis" — the expected answers include S. aureus, M. tuberculosis, and Brucella/Salmonella. Actinomyces is NOT a standard answer here but may appear as a distractor for chronic bone infections.

  • The concept of chronic meningitis aetiology (where Actinomyces appears in Ryan Ho's list [3]) is fair game for SAQ questions asking for differential diagnoses of chronic/subacute meningitis — alongside TB, Cryptococcus, Brucella, and malignant meningitis.

[1] Lecture slides: GC 101. Diagnosis of infections [Handouts].pdf [2] Lecture slides: GC 219. Infections and tumours in pharynx and oral cavity.pdf [3] Senior notes: Ryan Ho Neurology.pdf [4] Lecture slides: GC 105. Medically important microbes what every doctor should know.pdf [5] Lecture slides: GC 219. Infections and tumours in pharynx and oral cavity.pdf [6] Lecture slides: GC 071. It is red and painful.pdf [7] AOS material: AOS - Microbio.pdf [8] Past papers: 2023 Fourth Summative Minicase.pdf

Corynebacterium diphtheriae

Gram-positive, non-spore-forming, club-shaped bacillus that produces a potent exotoxin causing diphtheria, characterized by a pharyngeal pseudomembrane and systemic toxin-mediated myocarditis and neuropathy.

Nocardia asteroides

Partially acid-fast, branching filamentous Gram-positive aerobic bacterium causing pulmonary nocardiosis and disseminated infections, particularly in immunocompromised hosts.

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