Anaeróbios

Actinomyces israelii

Filamentous, branching Gram-positive bacilli

Identification & Growth

Identification

Anaerobic or microaerophilic, not acid-fast (this separates it from Nocardia)
'Molar tooth' colonies after 7–14 days

Growth Conditions

Anaerobic blood agar, 35–37 °C, prolonged incubation of 7 to 14 days

Clinical Significance

Cervicofacial, thoracic and pelvic actinomycosis (IUD-associated), with sulphur granules and fistulae

Intrinsic Resistance

Metronidazole, aminoglycosides and aztreonam have no activity

Bench Alerts

Negative cultures are common: warn the laboratory to prolong incubation
Prolonged penicillin therapy is the standard treatment

Clinical Notes

Clinical presentations

Cervicofacial actinomycosis: indolent, slow-growing mass, often following dental trauma or dental procedures, with sinus tracts draining sulfur granules.
Pelvic actinomycosis associated with long-term intrauterine device (IUD) use, which can mimic gynecologic malignancy.
Thoracic and abdominal forms, typically from aspiration or contiguous spread, forming masses that invade adjacent tissue disregarding normal anatomical planes.

Specimens and collection

Purulent material or biopsy tissue, collected avoiding mucosal flora contamination when possible, for sulfur granule examination and anaerobic culture.
Prolonged culture (up to 2–3 weeks) under strict anaerobic conditions, as this is a slow-growing, fastidious organism.
Histopathology showing sulfur granules (basophilic aggregates with eosinophilic fringes) supports the diagnosis when culture is negative.

Epidemiology and at-risk populations

Member of normal oral, gastrointestinal and genital microbiota; disease arises from breach of a mucosal barrier (dental trauma, surgery, IUD).
Low intrinsic virulence; typically requires co-inoculation with companion bacteria (polymicrobial infection) to establish disease.
Poor oral hygiene and dentition are important predisposing factors for the cervicofacial form.

Resistance and therapeutic implications

Universally susceptible to penicillin G, which remains the treatment of choice, given as a prolonged course (weeks to months) due to the granulomatous, fibrosing nature of the disease.
Doxycycline, clindamycin or ceftriaxone are alternatives in penicillin-allergic patients.
Infection is often polymicrobial; empirical coverage should consider companion organisms (e.g. Actinobacillus actinomycetemcomitans in the cervicofacial form).

Bench and reporting notes

Anaerobic or microaerophilic, branching, filamentous Gram-positive rod that can be mistaken for a fungus on microscopy.
Diagnosis is often delayed due to low clinical suspicion; consider in slow-growing masses unresponsive to short-course antibiotic therapy.
Communicate to the clinician the need for prolonged anaerobic culture, as standard 48-hour results may be falsely negative.

Sources

Educational decision-support content. It does not replace laboratory validation, current guidelines or review by the responsible professional.

FAQ: Frequently Asked Questions

How to identify Actinomyces israelii in the lab?

Actinomyces israelii is identified through anaerobic or microaerophilic, not acid-fast (this separates it from nocardia), 'molar tooth' colonies after 7–14 days.

What are the intrinsic resistances of Actinomyces israelii?

This organism is naturally resistant to metronidazole, aminoglycosides and aztreonam have no activity. These drugs should not be reported as susceptible.

Where is Actinomyces israelii commonly found?

It is typically associated with cervicofacial, thoracic and pelvic actinomycosis (iud-associated), with sulphur granules and fistulae.

Antibiogram InterpreterCheck BrCAST 2026 breakpoints Rugai InterpreterBiochemical identification tools