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The RpoS transcription factor of Borrelia burgdorferi is a ‘gatekeeper’ because it activates genes required for spirochaetes to transition from tick to vertebrate hosts. However, it remains unknown how RpoS becomes repressed to allow the spirochaetes to transition back from the vertebrate host to the tick vector. Here we show that a putative carbohydrate‐responsive regulatory protein, designated BadR (Borrelia host adaptation Regulator), is a transcriptional repressor of rpoS. BadR levels are elevated in B. burgdorferi cultures grown under in vitro conditions mimicking unfed‐ticks and badR‐deficient strains are defective for growth under these same conditions. Microarray and immunoblot analyses of badR‐deficient strains showed upregulation of rpoS and other factors important for virulence in vertebrate hosts, as well as downregulation of putative tick‐specific determinants (e.g. linear plasmid 28‐4 genes). DNA …