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Bacterium

Actinomyces israelii

Common name: A. israelii

Mixed Oral Oral Gut Urogenital
Mixed
Effect
Oral
Impact
Oral, Gut, Urogenital
Location
Common
Prevalence
Last reviewed: April 4, 2025

Oral commensal, causative agent of actinomycosis

Prevalence: Part of normal oral flora; actinomycosis incidence ~1 per 300,000 annually

Interacts with: Oral microbiome, Dental plaque bacteria, Fusobacterium nucleatum, Streptococcus species, Biofilm former

Overview

Scientifically accurate microscopy-style illustration of Actinomyces israelii showing its characteristic gram-positive filamentous branching bacterium forming characteristic sulfur granules

Actinomyces israelii is a Gram-positive, filamentous, anaerobic to microaerophilic bacterium that is a natural inhabitant of the human oral cavity, gastrointestinal tract, and urogenital tract. It is the most prevalent Actinomyces species isolated in human infections, responsible for approximately 70% of orocervicofacial actinomycosis cases.[1]

Key Characteristics

A. israelii is characterized by distinctive features:[2]

  • Morphology: Branching, filamentous bacteria with a "molar tooth" colony appearance on agar
  • Metabolism: Obligate or facultative anaerobe; slow-growing (5-14 days for colonies)
  • Sulfur granules: Forms pathognomonic yellowish granules (40-400 μm) composed of tangled bacterial filaments
  • Catalase-negative: Distinguishing biochemical characteristic
  • Non-spore-forming, non-motile: Lacks flagella

Role in the Human Microbiome

As a commensal organism, A. israelii colonizes multiple body sites:[3]

Oral Cavity

  • Dental plaque and biofilms (supragingival and subgingival)
  • Gingival crevices and periodontal pockets
  • Tonsillar crypts
  • Tongue surface (associated with oral malodor)

Gastrointestinal Tract

  • Part of the alimentary tract microbiota
  • Can cause abdominal actinomycosis, particularly involving the ileocecal region

Urogenital Tract

  • Normal inhabitant of female genital tract
  • Colonization promoted by intrauterine device (IUD) use

Biofilm Formation and Bacterial Interactions

A. israelii plays a central role in oral biofilm ecology:[4]

Early Colonization

  • Acts as a primary colonizer of tooth surfaces
  • Provides attachment sites for secondary colonizers through coaggregation

Coaggregation Partners

  • Streptococcus oralis and S. gordonii: Type 2 fimbriae recognize streptococcal receptor polysaccharides (RPS)
  • Fusobacterium nucleatum: Acts as a bridging organism connecting early and late colonizers
  • Prevotella loescheii: Serves as coaggregation bridge between A. israelii and other species
  • Veillonella and Campylobacter gracilis: Significant coaggregation partners in root caries

IUD Biofilm Formation

In vitro studies demonstrate A. israelii can form spider-like colonies and porous biofilm structures on copper IUD surfaces, attaching via extracellular polymer production.[5]

Pathogenesis of Actinomycosis

Actinomycosis occurs when mucosal barriers are breached:[2]

Predisposing Factors

  • Trauma (dental procedures, surgery, injury)
  • Poor oral hygiene
  • Foreign bodies (IUDs, dental implants)
  • Immunosuppression
  • Chronic inflammation
  • Tissue devitalization

Disease Mechanism

  1. Bacteria gain access to deeper tissues through mucosal disruption
  2. Form dense masses of interlinked branched filaments that inhibit phagocytic clearance
  3. Produce proteolytic enzymes enabling tissue invasion
  4. Create abscesses with characteristic sulfur granules
  5. Spread contiguously across tissue planes (not lymphatically)

Clinical Forms

Form Frequency Key Features
Cervicofacial 50-65% "Lumpy jaw"; mandible, cheeks, chin involvement
Thoracic 15-30% Following aspiration; pulmonary involvement
Abdominopelvic 20-32% IUD-associated; ileocecal predilection
CNS 2-4% Brain abscesses most common

IUD-Associated Pelvic Actinomycosis

A well-documented association exists between IUD use and pelvic actinomycosis:[6]

  • Colonization rate: 7% of IUD users have Actinomyces-like organisms on Pap smear (range 1.6-44%)
  • Risk factors: Prolonged IUD use (>5 years); chronic endometrial inflammation
  • Presentation: Pelvic abscesses, tubo-ovarian involvement, peritonitis
  • Prevention: IUD replacement every 5 years recommended

Diagnosis

Gold Standards

  • Culture: Slow-growing; requires anaerobic conditions; negative in 76% of cases
  • Histopathology: Sulfur granules with Gram-positive branching filaments
  • Molecular: 16S rRNA gene sequencing for species identification

Emerging Methods

  • MALDI-TOF MS (limited accuracy for A. israelii)
  • PCR-based detection
  • ARDRA (Amplified 16S rDNA Restriction Analysis)

Treatment

A. israelii infections require prolonged antibiotic therapy:[2]

First-Line Treatment

  • Intravenous: Penicillin G 12-24 million units/day for 2-6 weeks
  • Oral maintenance: Penicillin V or amoxicillin for 6-12 months

Alternative Agents (Penicillin Allergy)

  • Ceftriaxone
  • Doxycycline
  • Clindamycin
  • Erythromycin

Key Considerations

  • Intrinsically resistant to metronidazole
  • Surgical debridement often required
  • Combined medical-surgical cure rate >90%
  • Recent evidence suggests some patients cured with <6 months therapy

Documented Strains

ATCC 12102 (Type Strain)

Actinomyces israelii ATCC 12102

Moderate research
ATCC 12102 DSM 43320
Type strain reference for actinomycosis researchSelective medium developmentPCR identification benchmarking

Key Findings

–

Actinomycosis reference

Type strain for all A. israelii pathogenesis and diagnostic studies

Isolated from a human brain abscess; the only ATCC/DSM-deposited type strain for A. israelii and the reference for cervicofacial actinomycosis research; Gram-positive filamentous anaerobe causing the classic sulfur granule pathology

Associated Conditions

→ Periodontal disease

Related Organisms

C
Candidatus Saccharibacteria Oral
F
Fusobacterium nucleatum Oral
P
Porphyromonas gingivalis Oral
P
Prevotella melaninogenica Oral
S
Streptococcus mitis Oral
S
Streptococcus mutans Oral

Frequently Asked Questions

What is Actinomyces israelii?

Actinomyces israelii is a bacterium found in the human microbiome.

Where is Actinomyces israelii found in the body?

Actinomyces israelii is primarily found in the Oral, Gut, Urogenital.

What are the health impacts of Actinomyces israelii?

Actinomyces israelii primarily impacts Oral and is context-dependent for human health.

Research References

  1. Könönen E, Wade WG. Actinomyces and Related Organisms in Human Infections. Clinical Microbiology Reviews. 2015. doi:10.1128/CMR.00100-14 ↩
  2. Valour F, Sénéchal A, Dupieux C, et al.. Actinomycosis: etiology, clinical features, diagnosis, treatment, and management. Infection and Drug Resistance. 2014. doi:10.2147/IDR.S39601 ↩
  3. Li J, Li Y, Zhou Y, et al.. Actinomyces and Alimentary Tract Diseases: A Review of Its Biological Functions and Pathology. BioMed Research International. 2018. doi:10.1155/2018/3820215 ↩
  4. Kolenbrander PE, Andersen RN, Blehert DS, et al.. Communication among Oral Bacteria. Microbiology and Molecular Biology Reviews. 2002. doi:10.1128/mmbr.66.3.486-505.2002 ↩
  5. Carrillo M, Valdez B, Vargas L, et al.. In vitro Actinomyces israelii biofilm development on IUD copper surfaces. Contraception. 2010. doi:10.1016/j.contraception.2009.09.008 ↩
  6. Gajdács M, Urban E. The Pathogenic Role of Actinomyces spp. and Related Organisms in Genitourinary Infections. Antibiotics. 2020. doi:10.3390/antibiotics9080524 ↩