Treponema pallidum. Coloured Transmission Electron Micrograph (TEM) showing a Treponema pallidum bacterium. This spirochaete bacterium is spirally twisted in form, elongated and appearing thread- like (yellow). It causes syphilis in humans. Treponema pallidum is sexually transmitted, and may also be passed from an infected pregnant woman to her unborn child (congenital syphilis). The disease has 3 stages: the last stage (tertiary syphilis) may cause severe damage to the heart, blood vessels, brain and spinal cord. Treatment with penicillin drugs is effective if administered in the early stages. Magnification: x4000 at 6x4.5cm size.