Assessing the Contribution of an HtrA Family Serine Protease During Mammalian Infection.

Tick-borne relapsing fever (TBRF), characterized by recurring febrile episodes, is globally distributed and among the most common bacterial infections in some African countries. Despite the public health concern that this disease represents, little is known regarding the virulence determinants required by TBRF during infection. Because the chromosomes of TBRF show extensive colinearity with those of Lyme disease (LD) , the exceptions represent unique genes encoding proteins that are potentially essential to the disparate enzootic cycles of these two groups of spirochetes. One such exception is a gene encoding an HtrA family protease, BtpA, that is present in TBRF , but not in LD spirochetes. Previous work suggested that orthologs may be important for resistance to stresses faced during mammalian infection. Herein, proteomic analyses of the TBRF spirochete, , demonstrated that BtpA, as well as proteins encoded by adjacent genes in the genome, were produced in response to culture at mammalian body temperature, suggesting a role in mammalian infection. Further, transcriptional analyses revealed that was expressed with the genes immediately upstream and downstream as part of an operon. To directly assess if is involved in resistance to environmental stresses, deletion mutants were generated. mutants demonstrated no growth defect in response to heat shock, but were more sensitive to oxidative stress produced by butyl peroxide compared to wild-type . Finally, mutants were fully infectious in a murine relapsing fever (RF) infection model. These results indicate that BtpA is either not required for mammalian infection, or that compensatory mechanisms exist in TBRF spirochetes to combat environmental stresses encountered during mammalian infection in the absence of BtpA.