The breakthrough could pave the way for a preventive vaccine and better diagnostic tests and treatments.

Syphilis once caused widespread epidemics and is still a major cause of illness and death around the world. People with syphilis lesions are also at higher risk of HIV infection.

A vaccine could be on the way

Syphilis can be treated with injections of penicillin or other antibiotics - but the early symptoms can be mild, absent or mimic those of other diseases, so that many people do not seek treatment when they first become infected.

In addition, the disease is difficult to diagnose and often repeated blood tests are needed.

Until now scientists have been unable to keep Treponema pallidum, the bacterium which causes syphilis, alive in the laboratory long enough to work on a vaccine or better treatments.

Anthony Fauci, director of the US National Institute of Allergy and Infectious Diseases, said: "Completion of this project is an extraordinary boost for efforts to develop a protective vaccine."

Scientists can use the genetic blueprint for T. pallidum to devise diagnostic tests that are more specific, more accurate and easier to use.

Easy-to-administer treatments are needed, especially in developing countries.

Dr George Weinstock, who led the gene mapping study, said: "The genome sequence represents an encyclopedia of information on this elusive bacterium.