• The actinomyces are true bacteria, although they have in the past been considered to resemble fungi because they form branching filaments.
  • They are related to the corynebacteria and Mycobacteria in the chemical structure of their cell walls and some are acid-fast.
  • It is important to differentiate them from fungi because infections with actinomyces should respond to antibacterial agents whereas similar clinical presentations caused by fungi are resistant to antibacterials.
  • This genus contains many species, some of which are important to man as producers of antimicrobials agents.
  • A few are pathogenic to man and animals; A. israelii causes actinomycosis.

Actinomyces israelii:

  • Characteristics:
    • Gram-positive anaerobic filamentous branching rods. Non-sporing, non acid-fast.
  • Transmission:
    • A. israelii is part of normal flora in mouth, gut and vagina. Infection is endogenous. There is no person-to-person spread.
  • Diseases:
    • Actinomycosis follows local trauma and invasion from normal flora.
    • Hard non-tender swellings develop which drain pus through sinus tracts.
    • Cervicofacial lesions are most common, but abdominal lesions after surgery and infection related to intrauterine contraceptive devices also occur.
  • Laboratory Identification:
    • Forms ‘sulphur granules’ composed of a mass of bacterial filaments in pus.
    • These can be identified by washing pus, squashing granules and observing in stained microscopic preparations.
    • Gram-positive branching rods also visible in stained pus.
    • Forms characteristic ‘breadcrumb’ or ‘molar tooth’ colonies on blood agar after 3-7 days anaerobic incubation at 35ºC.