Actinomycosis is caused by an infection with the bacteria, Actinomyces israelii. Actinomyces bacteria are normally present in the mouth, nose and throat. If the lining to the nose, throat or mouth is injured, the Actinomyces bacteria invade the tissue beneath the surface and start to grow. Sometimes, the bacteria enter through inflamed lung tissue. The bacteria may spread to other locations in the body through the bloodstream.