The serum resistance-associated gene as a diagnostic tool for the detection of Trypanosoma brucei rhodesiense

In the search for new diagnostic methods that would distinguish Trypanosoma brucei rhodesiense from T. b. brucei and T b. gambiense, we have developed two polymerase chain reaction (PCR) primer sets. The first primer set was derived from the serum resistance-associated (SRA) gene of T. b. rhodesiense that confers resistance to lysis by normal human serum (NHS). The specificity of the SRA-based PCR was tested on 97 different trypanosome populations originating from various taxonomic groups, host species, and geographic regions. Only one of 25 T b. rhodesiense samples was negative in this PCR. and none of 72 other samples were positive in this assay. Interestingly, a reference T. brucei strain (TREU927/4) currently used for genome sequencing was negative for the SRA gene; however, this strain was resistant to lysis by NHS. The second primer set was derived from a specific variant surface glycoprotein (VSG) expression site where the SRA gene is expressed (R-ES). This primer set identified the strain as T. b. rhodesiense in 17 of 17 SRA gene-positive strains in which it was tested. These data strongly suggest that expression of the SRA gene is generally involved in resistance to lysis by NHS in T. b. rhodesiense strains.