Actinomyces viscosus

Actinomyces viscosus is a human and animal pathogen/pathobiont which colonises the mouths of 70% of adult humans. "A. viscosus" has a low level of virulence and is often mistaken with other actinomycetes.
Morphology.
"A. viscosus" is Gram-positive, facultatively anaerobic, rod-shaped, and filamentous. It grows slowly on nonselective media, forming gray and white colonies.
Pathogenesis.
"A. viscosus" causes periodontal disease in animals and has been isolated from human dental calculus and root surface caries, as well as the oral cavity of hamsters and actinomycotic lesions in swine, cats, and dogs. 
Furthermore, it has been shown to cause endocarditis in humans. "A. viscosus" has also been known to cause lung infections, but only in very few cases. Infections are treatable with penicillin for three-week therapies.
Diagnosis.
"A. viscosus" infection symptoms are indistinguishable from Actinomyces israelii infection symptoms or Actinomyces bovis infection symptoms. "A. israelii" and "A. bovis" infections usually cause actinomycotic infections, but sometimes and very rarely will the pathogen be "A. viscosus". "A. viscosus" colonies test positive for catalase and negative for indole.
Treatment.
Multiple-week antibiotic therapies have cured actinomycotic infections caused by "A. viscosus" in every recorded case. Therapies include treatment with penicillin, sulfadimethoxine, flucloxacillin, clindamycin, tetracycline, and ticarcillin. "A. viscosus" is usually resistant to vancomycin, metronidazole, cefalexin, and dicloxacillin. Treatments last at least three weeks, with some exceptions. Although "A. viscosus" is difficult to distinguish from other closely related actinomycetes, the general determination of being an actinomycete is sufficient for treatment of infections.