Actinomyces spp

Gram Stain

• Gram positive non-spore forming irregular/branching bacilli (facultative anaerobic)

Clinical Significance

These organisms are part of the normal flora of the gastrointestinal, genitourinary, and upper respiratory tracts. 

They are implicated in polymicrobial infections, especially chronic wounds/abscesses (including brain, lung and breast abscesses) and draining sinus tracts.  They may cause bacteremia, endocarditis/septicemia, bone/joint and medical device related infections, and pelvic infections related to intrauterine contraceptive devices.

Actinomyces spp have recently been implicated in osteoradionecrosis or bisphosphonate related osteonecrosis of the jaw bones.

Actinomycosis (cervicofacial, thoracic, or abdominal) may be caused by several Actinomyces spp. including most commonly A. israelii, A. naeslundii, A. odontolyticus, A. meyeri, and A. viscosus.

Several new genera of Actinomyces have been associated with genitourinary infections and skin related abscesses. 

 A. turicensis - associated with balanitis, urethritis, penile abscess, and prostatitis in males and pelvic inflammatory disease, endometritis, cervicitis, vaginitis, vulval and perineal abscesses, premature rupture of membranes in females, urinary tract infections, appendicitis, cholecystitis, and bacteremias.

 A. urogenitalis - associated with bone and soft tissue infections as well as genital abscesses.

 A. turicensis, A. radingae, A. europeus, and A. neuii - associated with abscesses, including hidradenitis, pilonidal, and umbilical abscesses as well as chronic ulcers often in association with other anaerobes.

Usual Susceptibility Pattern

These organisms are typically susceptible to penicillin, carbapenems, vancomycin, and linezolid.  

Susceptibility to clindamycin, erythromycin, ceftriaxone, and tetracyclines is variable.

Daptomycin is less active than vancomycin and linezolid.

A. urogenitalis is typically resistant to clindamycin (but susceptible to erythromycin). 

Actinomyces spp are resistant to metronidazole. 

Quinolones do not have reliable activity.