A patient complains to his dentist about a draining lesion in his mouth. A Gram stain of the pus shows a few gram-positive cocci, leukocytes, and many-branched gram-positive rods. Branched yellow sulfur granules are observed by a microscope. Which of the following is the most likely cause of the disease?

The patient presented with typical symptoms of actinomycosis. Actinomyces israelii is normal flora in the mouth. However, it causes a chronic draining infection, often around the maxilla or the mandible, with osteomyelitic changes. Treatment is high-dose penicillin for 4 to 6 weeks. The diagnosis of actinomycosis is often complicated by the failure of A. israelii to grow from the clinical specimen. It is an obligate anaerobe. FA reagents are available for direct staining of A. israelii. A rapid diagnosis can be made from the pus. FA conjugates are also available for A. viscosus and A. odontolyticus, anaerobic actinomycetes that are rarely involved in actinomycotic abscesses. Corynebacterium diphtheriae is a gram-positive rod, as is P. acnes, and S. aureus would be a large gram-positive cocci.