Syphilis bacterium (Treponema pallidum). Coloured transmission electron micrograph (TEM) showing a Treponema pallidum bacterium (green) in penile skin (brown). Syphilis is usually sexually transmitted but may also pass from an infected pregnant woman to her unborn child (congenital syphilis). The last of three stages of syphilis is the most severe and can cause damage to the heart, blood vessels, brain and spinal cord. Treatment with penicillin drugs is effective if administered during the early stages. Magnification: x6000 when printed 10cm wide.