Actinomyces: Properties, Disease, Lab Diagnosis

Actinomyces comprised a heterogeneous group of gram-positive, pleomorphic bacilli. Other properties are:

  • facultatively anaerobic or microaerophilic,
  • non-spore-forming,
  • non-acid-fast (this is a contrast to another bacteria, Nocardia which is acid-fast)
  • Non-motile

Mode of infection

Infection of Actinomyces is mostly endogenous and may result from local trauma such as a broken jaw or dental extraction.


Many Actinomyces species are part of the oral microflora of humans. Some species of Actinomyces are found in supragingival and subgingival plaques of adult patients suffering from periodontitis and gingivitis.
These same organisms also play a significant role in the development of root-surface caries. Actinomyces also cause classical actinomycosis, wound infections, abscesses, and genital tract infections.

Actinomycosis is a chronic suppurative and granulomatous infection characterized by multiple abscess formation of sinuses, discharge containing granules, and on later stage; fibrosis and tissue destruction. The typical lesion of actinomycosis appears as a hard, non-tender swelling that develops slowly and eventually drains pus through sinus tracts. Hard, yellow granules (sulfur granules) composed of a mass of filaments are formed in pus.

Actinomyces israelii and Arachnia species are the most common causes of actinomycosis in humans. Actinomycosis can affect any organ and characteristically manifests as:

  1. Cervicofacial actinomycosis: It is associated with poor dentition, dental manipulations, and other head-and-neck infections (e.g., otitis, mastoiditis).
  2. CNS actinomycosis: It results after hematogenous dissemination or by direct extension of actinomycotic infections in the head/neck areas. Patients usually present with chronic meningitis or meningoencephalitis complicated by the development of subdural empyema, epidural and spinal abscesses, and cranial osteomyelitis. In 75% of cases, CNS actinomycosis presents with single or multiple brain abscesses, usually in the temporal or frontal lobes.
  3. Thoracic actinomycosis: It involves the chest and mediastinal region.
  4. Abdominal actinomycosis: Actinomycosis of the Ileocecal region.
  5. Pelvic actinomycosis: Pelvic actinomycosis can occur in women who have retained an intrauterine device for a long period of time.

Laboratory Diagnosis


Based on the affected site, the specimens collected include discharge from the sinuses or fistula, rarely bronchoalveolar lavage, sputum or tissue sections.

Diagnosis in the laboratory is made by

  1. seeing gram-positive branching rods, especially in the presence of sulfur granules, and
  2. seeing growth when pus or tissue specimens are cultured under anaerobic conditions. Organisms can be identified by immunofluorescence.

Direct Microscopy

Pus discharge is thoroughly washed in saline in a test tube and the sediment is collected that contains gritty, white or yellowish sulfur granules, of <5 mm in size. Granules are crushed between two slides and smears are made.

Gram-staining (Brown-Brenn modification)

It shows a central mass of gram-positive filamentous bacilli, radiating peripherally with hyaline, club-shaped ends. Clubs are composed of complexes formed due to interaction of bacteria derived polysaccharide and protein with host cell salts and polypeptides.

Granules of actinomycosis are hard and not emulsifiable which differentiates them from granules produced other conditions. Actinomyces species can also be detected directly from the sample by methods such as:

  1. Fluorescent antibody techniques using fluorescent-tagged species-specific monoclonal antibodies.
  2. Fluorescent in situ hybridization (FISH) using species-specific probes.

Note that in contrast to N.asteroides, Actinomyces is not acid-fast.

Actinomyces in culture and Gram-stain

Histopathological staining such as hematoxylin-eosin and Gomori’s stained tissue sections may reveal granules composed of eosinophilic clubs surrounding basophilic filaments and foamy macrophages (sun-rays appearance).


Pus containing sulfur granules are washed and cultured anaerobically at 37°C on media such as:

  1. Thioglycollate broth: Growth of A. israelii resembles fluffy balls at the bottom of the tube, this can be differentiated from other species (A. bovis produces uniform turbidity).
  2. Brain heart infusion (BHI) agar: It forms small spidery colonies at 48 hours which become enlarged and heaped up in 10 days.

Species identification

It is done when the culture isolate is subjected to:

  1. Biochemical reactions
  2. Gas-liquid chromatography (GLC) for detection of the products of glucose metabolism.
  3. Molecular methods, such as PCR-RFLP are also available for speciation.

Treatment and prevention

Treatment consists of prolonged administration of penicillin G, coupled with surgical drainage. There is no significant resistance to penicillin G. No vaccine or prophylactic drug is available.

References and Further Readings

  1. Procop, G. W., & Koneman, E. W. (2016). Koneman’s Color Atlas and Textbook of Diagnostic Microbiology (Seventh, International edition). Lippincott Williams and Wilkins.
  2. Tille, P. (2017). Bailey & Scott’s Diagnostic Microbiology (14 edition). Mosby.

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. As an asst. professor, I am teaching microbiology and immunology to medical and nursing students at PAHS, Nepal. I have been working as a microbiologist at Patan hospital for more than 10 years.

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