Aggregatibacter spp

• A. actinomycetemcomitans
• A. aphrophilus
• A. segnis

Gram Stain

• Gram negative bacilli/cocci occasional filamentous forms (facultative anaerobic)

Clinical Significance

These organisms are part of the commensal flora of the upper respiratory tract.

A. actinomycetemcomitans – associated with endocarditis, brain abscesses, meningitis, parotitis, septic arthritis, osteomyelitis, disciitis, paraspinal abscesses, pneumonia, pericarditis, and soft tissue infections.  In soft tissue infections, this organism is often co-isolated with Actinomyces spp. Patterns of infections may resemble Actinomyces infection (i.e. cervicofacial, thoracic, abdominal).  This organism is also associated with periodontal disease (along with Porphyromonas gingivalis).

A. aphrophilus – associated with endocarditis, meningitis, brain abscesses, pneumonia, sinusitis, septic arthritis, osteomyelitis, and wound infections. 

A. segnis - associated with endocarditis, pancreatic abscesses, empyema, and  pyelonephritis.

Usual Susceptibility Pattern

A. actinomycetemcomitans is resistant to clindamycin and vancomycin. The activity of macrolides and aminoglycosides is variable. This organism is usually susceptible to cephalosporins, TMP/SMX, quinolones, rifampin, and tetracyclines. Susceptibility to penicillin and ampicillin is variable (altered penicillin binding protein, permeability mutations, and rarely beta-lactamase production).

A. aphrophilus and A. segnis are usually susceptible to ampicillin, 2nd/3rd generation cephalosporins, TMP/SMX, quinolones, and tetracyclines.  One case of third generation cephalosporin resistance has been reported for A. aphrophilus.