Actinobacillus bacteria. Coloured scanning electron micrograph (SEM) of the bacteria Actinobacillus (or Actinobacter) actinomycetemcomitans (Aa), a cause of periodontitis and subacute endocarditis. Aa are gram-negative bacillus (rod-shaped) bacteria that are normal inhabitants of the mouth and throat. They can however cause inflammation in the tissues supporting the teeth (periodontitis) and in the internal lining of the heart (endocarditis). Aa are often found with Actinomyces sp., the causal agent of actinomycosis. Treatment is with antibiotics. Magnification: x7,900 at 6x7cm size. Magnification: x27,000 at 8x10 inch size.