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Actinomyces israelii


Gram-positive bacilli —————————–
>Actinomyces israelii led
>Arcanobacterium haemolyticum (formerly known as Corynebacterium haemolyticum)
>Bacillus species:
     1] Bacillus anthracis
     2] Bacillus cereus
     3] Bacillus subtilis
>Clostridium species:
     1] Clostridium difficile
     2] Clostridium perfringens
     3] Clostridium tetani
>Corynebacterium species:
     1] Corynebacterium diphtheria
     2] Corynebacterium jeikeium
     3] Corynebacterium urealyticum
>Erysipelothrix rhusiopathiae
>Listeria monocytogenes
>Lactobacillus species (several e.g.  L. acidophilus,L. brevis, L. buchneri, L. casei,
      L. fermentum, L. gallinarum , L. gasseri)
>Nocardia species:
     1] Nocardia asteroides
     2] Nocardia brasiliensis
>Propionibacterium acnes
>Rhodococcus equi  (formerly Prescottia equi, Corynebacterium equi)


  • Genus of the actinobacteria class of bacteria.
  • Gram-positive, rod-shaped, non-spore-forming, non-acid-fast, facultative anaerobe.
  • Actinomyces are facultatively anaerobic (except A. meyeri, a strict anaerobe).
  • Actinomyces species do not form endospores
  • Actinomyces colonies form fungus-like branched networks of hyphae.
  • Actinomyces are known for causing disease in humans and are normally present in the gums and are the most common cause of infection in dental procedures and oral abscesses.
  • Many Actinomyces species are opportunistic pathogens particularly in the oral cavity.

Actinomyces israelii:

  • Gram-positive rod-shaped bacteria within the Actinomyces.
  • Known to live commensally on and within humans (A. israelii is a normal colonizer of the vagina, colon, and mouth).
  • Opportunistic pathogen and a cause of actinomycosis. Infection is established first by a breach of the mucosal barrier during various procedures (dental, GI), aspiration, or pathologies such as diverticulitis.
  • Many physiologically diverse strains of the species are known to exist, though all are strict anaerobes.
  • Manifestations of disease: Oral-cervicofacial disease is the most common form of actinomycosis.  Another form of actinomycosis is thoracic disease, which is often misdiagnosed as a neoplasm, as it forms a mass that extends to the chest wall.  Abdominal disease is another manifestation of actinomycosis. 
  • Actinomycosis may be considered when a patient has chronic progression of disease across tissue planes that is mass-like at times, sinus tract development that may heal and recur, and refractory infection after a typical course of antibiotics.

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Important considerations:  The choice of an agent should be based on local antimicrobial sensitivities, site of infection, cost, and comorbid conditions.   Generally, the most common agents/regimens are listed first.    Listed dosages may need to be adjusted for renal dysfunction.

  1. Amoxicillin 500mg orally three times daily or Ampicillin 2 grams IVPB every 4-6 hours
  2. Penicillin G 2-4 million units IV q4-6h or Penicillin VK 500mg orally every 6 hours
  3. Clindamycin  600mg IV every 6 or 8 hours or 300mg orally four times daily 
  4. Doxycycline 100mg orally twice daily
  5. Erythromycin 500mg orally four times daily 
  6. Ceftriaxone 1-2 grams IV q24h
  7. Others
Actinomyces israelii