ABSTRACTBorrelia burgdorferi, the agent of Lyme disease, undergoes rapid adaptive gene expression in response to signals unique to its arthropod vector or vertebrate hosts. Among the upregulated genes under vertebrate host conditions is one of the five annotated homologs of oligopeptide permease A (OppA5, BBA34). A mutant lackingoppA5was constructed in an lp25-deficient isolate ofB. burgdorferistrain B31, and the minimal regions of infectivity were restored via a shuttle vector pBBE22 with or without an intact copy ofbba34. Immunoblot analysis of thebba34mutant revealed a reduction in the levels of RpoS, BosR, and CsrABbwith a concomitant reduction in the levels of OspC, DbpA, BBK32, and BBA64. There were no changes in the levels of OspA, NapA, P66, and three other OppA orthologs. Quantitative transcriptional analysis correlated with the changes in the protein levels. However, thebba34mutant displayed comparable infectivities in the C3H/HeN mice and the wild-type strain, despite the reduction in several pathogenesis-related proteins. Supplementation of the growth medium with increased levels of select components, notably sodium acetate and sodium bicarbonate, restored the levels of several proteins in thebba34mutant to wild-type levels. We speculate that the transport of acetate appears to contribute to the accumulation of key metabolites, like acetyl phosphate, that facilitate the adaptation ofB. burgdorferito the vertebrate host by the activation of the Rrp2-RpoN-RpoS pathway. These studies underscore the importance of solute transport to host-specific adaptation ofB. burgdorferi.