Bacterium
Actinomyces israelii
Sulfur-granule filamentous anaerobe
ak-tin-oh-MY-seez iz-RAY-lee-eye
High-yield clue
Yellow 'sulfur granules' draining from sinus tracts and cervicofacial 'lumpy jaw' after dental trauma are the core clues; unlike Nocardia it is anaerobic and not acid-fast.
Overview
A branching, filamentous Gram-positive anaerobe of the oral cavity that causes chronic draining sinus-tract infection. It is the classic non-acid-fast counterpart to Nocardia in coursework.
Classification
- Gram-positive
- Branching filamentous rod
- Anaerobe
- Not acid-fast
Lab & identification clues
- Sulfur granules (yellow filament colonies)
- Branching filaments on Gram stain
- Anaerobic growth
- Not acid-fast (contrast with Nocardia)
Associations
- Cervicofacial 'lumpy jaw'
- Dental trauma / poor oral hygiene
- IUD-associated pelvic study association
Commonly confused with
- Nocardia species
- Fungal hyphae
Frequently asked questions
What is Actinomyces israelii?
A branching, filamentous Gram-positive anaerobe of the oral cavity that causes chronic draining sinus-tract infection. It is the classic non-acid-fast counterpart to Nocardia in coursework.
What is the Gram stain of Actinomyces israelii?
Actinomyces israelii is Gram positive.
What are the lab clues for Actinomyces israelii?
Lab clues for Actinomyces israelii include Sulfur granules (yellow filament colonies), Branching filaments on Gram stain, Anaerobic growth, and Not acid-fast (contrast with Nocardia).
What is Actinomyces israelii commonly confused with?
Actinomyces israelii is commonly confused with Nocardia species and Fungal hyphae.
Original student-study summary. Sources checked: OpenStax Microbiology 2e, NCBI Bookshelf Medical Microbiology, and CDC topic pages where applicable; reviewed 2026-06. Educational only; no diagnosis, treatment, dosing, or specimen-handling guidance.