Actinobacillus spp

• A. equuli
• A. hominis
• A. lignieresii
• A. suis
• A. ureae

Gram Stain

•  Gram negative medium sized bacilli/cocci (often bipolar staining) singly/pairs/short chains (facultative anaerobic)

Clinical Significance

These organisms are part of the normal flora of the upper respiratory and genitourinary tracts in humans and animals.

A. equulis / A. suis – cause a variety of illnesses in horses and pigs. Human infections are rare and usually associated with horse or pig bites.

A. hominis - associated with bacteremia in patients with severe liver disease and from the respiratory tract of patients with chronic lung disease.

A. lignieresii - causes actinobacillosis in cattle and sheep which is similar to actinomycosis with formation of sulfur granules. It may cause soft tissue infections in humans, associated with a bite or other contact with a cow or sheep.

A. ureae - associated with bacteremia, endocarditis, meningitis, bone marrow infection, atrophic rhinitis, pneumonia, conjunctivitis, otitis media, and peritonitis.  Predisposing factors include immunosuppression, diabetes, surgery, or trauma.

Usual Susceptibility Pattern

Actinobacillus spp. are resistant to clindamycin and vancomycin. 

The activity of penicillin, macrolides, and aminoglycosides is variable.

These organisms are usually susceptible to cephalosporins, TMP/SMX, quinolones, rifampin, and tetracycline.