Actinomyces israelii is best described as which type of organism?

• A. Gram-positive cocci that are acid-fast
• B. Non-acid-fast gram-positive filamentous anaerobe
• C. Gram-negative rod
• D. Spore-forming Gram-positive rod

Answer: (B)

Actinomyces israelii is best described as a non-acid-fast gram-positive filamentous anaerobe. Its Gram-positive nature comes from a thick peptidoglycan cell wall that retains crystal violet. It forms branching, filamentous structures rather than round cocci or simple rods, giving a characteristic filamentous appearance under the microscope. It is an anaerobe, growing best in low-oxygen environments, which helps explain its tendency to cause chronic infections in anaerobic settings and to form draining sinus tracts with sulfur granules. It does not form spores, distinguishing it from spore-forming Gram-positive rods. It is not acid-fast, setting it apart from Nocardia, which is weakly acid-fast and usually aerobic.