Fusobacterium bacteria, computer illustration. These bacteria are Gram-negative, anaerobic, non-motile rod-shaped prokaryotes (characterized by a long, slender shape and pointed ends). They are part of the human microbiota, where they are found in the mouth and upper respiratory tract. They occasionally cause abscesses and blood infections and possibly play a role in periodontal disease and respiratory infections. The most common species recovered from patients are F. nucleatum and F. necrophorum. They are susceptible to penicillin.