Actinomycosis. Scanning electron micrograph (SEM) from a patient with pelvic actinomycosis. Actinomycosis is a rare type of bacterial infection caused by a group of bacteria called actinomycetaceae. Most bacterial infections are confined to one part of the body because the bacteria are unable to penetrate the body's tissue. However, actinomycosis is unusual in that the infection is able to move slowly but steadily through body tissue. Pelvic actinomycosis is where the infection develops inside the pelvis. It usually only occurs in women because most cases are caused when the actinomyces bacteria are spread from the female genitals into the pelvis. Most cases of pelvic actinomycosis are thought to be associated with the long-term use of an intrauterine device (IUD). This type of contraceptive is often known as the coil. Magnification x 7500 when printed at 10cm wide.