Final answer:
A tubo-ovarian abscess is frequently caused by the sexually transmitted bacteria Chlamydia trachomatis and Neisseria gonorrhoeae, which can lead to pelvic inflammatory disease.
Step-by-step explanation:
The usual cause of a tubo-ovarian abscess is infection by sexually transmitted bacteria, specifically Chlamydia trachomatis and Neisseria gonorrhoeae. These infections can lead to pelvic inflammatory disease (PID), which is a serious infection of the female reproductive organs that can include the ovaries, fallopian tubes, and uterus. Complications of PID can result in tubo-ovarian abscesses, which are pocket-like collections of pus. While several bacteria can cause infections leading to tubo-ovarian abscesses, the two most common STIs, gonorrhea (N. gonorrhoeae) and chlamydia (C. trachomatis), are frequently to blame, especially when PID is involved.