No, the syphilis-causing bacterium, Treponema pallidum subspecies pallidum is not eradicated without treatment.
Although the chancre (skin lesion) associated with primary syphilis will only last for three to six weeks and heal even if treatment is not received, the bacteria still remains and the infection progresses to the secondary stage.
The secondary stage is characterized by skin rashes and other symptoms are also possible (e.g. lesions, fever, sore throat, hair and weight loss, fatigue). Like the primary symptoms, secondary symptoms will also disappear whether or not treatment is received. Without treatment, syphilis progresses to the latent stage.
There are no visible signs or symptoms of syphilis during the latent (hidden) stage. However, syphilis-causing bacteria (T. pallidum) are still present. The latent stage can last for many years, with 15-40% of untreated individuals developing tertiary syphilis.
Tertiary syphilis is potentially fatal and can occur 10-30 years or more after acquiring a syphilis infection. Multiple different organ systems can be affected including the brain, nerves, eyes, heart, liver, bones, and joints.
Do not take the risk and get tested if you have been exposed and/or are displaying symptoms. Treatment is effective and simple, with only a single intramuscular injection required in the early stages, or three doses for the late latent stage of infection.