Syphilis, caused by Treponema pallidum (T. pallidum), has re-emerged in the UK and globally. There are 11 million new cases annually. The WHO stated the urgent need for single-dose oral treatments for syphilis to replace penicillin injections. Azithromycin showed initial promise, but macrolide resistance-associated mutations are emerging. Response to treatment is monitored by serological assays that can take months to indicate treatment success, thus a new test for identifying treatment failure rapidly in future clinical trials is required. Molecular studies are key in syphilis research, as T. pallidum cannot be sustained in culture. The work presented in this thesis aimed to design and validate both a qPCR and a RT-qPCR to quantify T. pallidum in clinical samples and use these assays to characterise treatment responses to standard therapy by determining the rate of T. pallidum clearance from blood and ulcer exudates. Finally, using samples from three cross-sectional studies, it aimed to establish the prevalence of T. pallidum strains, including those with macrolide resistance in London and Colombo, Sri Lanka.
The sensitivity of T. pallidum detection in ulcers was significantly higher than in blood samples, the likely result of higher bacterial loads in ulcers. RNA detection during primary and latent disease was more sensitive than DNA and higher RNA quantities were detected at all stages. Bacteraemic patients most often had secondary disease and HIV-1 infected patients had higher bacterial loads in primary chancres. Treatment kinetics following benzathine penicillin injection were assessed in four men. The mean half-life for both blood and ulcer T. pallidum nucleic acid clearance was found to be short. All patients had serology consistent with cure at one month. Two T. pallidum strain types were found in Colombo, neither harbouring macrolide resistance. In London, several strain types were identified, the majority of which contained genetic determinants of macrolide resistance.