An improved method for recovering clinical isolates and assessing pathogenicity of blood from secondary syphilis patients.

BACKGROUND

Recovering clinical isolates of Treponema pallidum (Tp) can expedite the research on molecular epidemiology and vaccine development of syphilis. Nevertheless, obtaining numerous isolates through rabbit infectivity testing remains unsatisfactory, particularly in secondary syphilis cases. This study re-evaluates the reliability of orchitis as a phenotypic indicator for assessing Tp clinical strains during rabbit infectivity testing.

METHODS

Treponemal and nontreponemal tests were used to analyze the correlation between serological titers and Tp burden. An optimal titer threshold was identified and applied to verify the maximum achievable Tp burden across different initial inoculation doses. A modified recovery protocol was developed for isolating Tp clinical strains from chancre exudates or whole blood, and its success rate was subsequently evaluated.

RESULTS

We demonstrated that a Tp particle agglutination titer of 1:1280 reliably indicated the peak burden of Tp Nichols strain and clinical isolates during rabbit infectivity testing. To achieve stable recovery, the initial inoculum should be at least 105 organisms. Using this optimized protocol, we successfully recovered and maintained Tp clinical isolates from 82.69% (43/52) of syphilis patients overall. Notably, the success rate from whole blood samples of secondary syphilis cases increased markedly to 82.50% (33/40) despite many of these specimens testing negative for Tp DNA by quantitative polymerase chain reaction.

CONCLUSIONS

This study shows that the pathogenicity of secondary syphilis blood is greater than previously recognized. Thus, monitoring Tp particle agglutination titers enables the recovery of Tp clinical isolates, even from specimens with extremely low bacterial burdens.