Borrelia burgdorferi undergoes differential gene expression during transmission from its tick vector to a vertebrate host. The addition of blood to a spirochete culture at 35°C for 48 h had a dramatic effect on gene expression of this organism. Utilizing B. burgdorferi whole genome DNA arrays, we compared the transcriptomes of the spirochetes following a 2-day temperature shift with blood and without blood. Using combined data from three independent RNA isolations we demonstrated that the addition of blood led to a differential expression of 154 genes. Of these, 75 genes were upregulated, with 49 (65%) of them encoded on plasmids. Blood supplementation of cultures also resulted in the downregulation of 79 genes, where 56 (70%) were plasmid encoded. We verified our results by reverse transcriptase PCR of several genes in both flat and feeding ticks. In the 2-day experiment we observed the effect that exposure to increased temperature and blood combined had on B. burgdorferi gene expression at this crucial time when the spirochetes begin to move from the vector to a new vertebrate host. These changes, among others, coincide with the upregulation of the chemotaxis and sensing regulons, of the lp38-encoded ABC transporter, of proteases capable of remodeling the outer surface of the spirochetes, and of the recombination genes of cp32 as a transient or initial part of the stress response of the phage. These are all functions that could cause or facilitate the changes that spirochetes undergo following a blood meal in the tick.

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