The effect of blood and bacterial proteases on gene expression and transmission of Borrelia burgdorferi

The spirochete Borrelia burgdorferi, the agent of Lyme disease, utilizes differential gene expression during its passage from the tick vector to a vertebrate host. It has been hypothesized that these changes allow this pathogen to adapt and survive in two different hosts. We utilized a whole genome DNA array to study the gene expression of B. burgdorferi in conditions which mimic the environment within feeding ticks. We compared the transcriptomes of the spirochetes following a two-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. The majority of the genes were encoded on plasmids. We verified our results by RT-PCR in flat and feeding ticks to show that the in vitro changes correlate to in vivo conditions. We then pursued studies with genes found to be upregulated on the array, one of which was lon-1, encoding an ATP-dependent protease. We inactivated lon-1 in a virulent strain of B. burgdorferi and found that lon-1 deficiency results in attenuated virulence in mice. Only half of mice infected with the lon-1 mutant contained spirochetes after 21 days, and spirochete numbers in infected mice were lower that wild type. The mutant also showed a deficiency of survival/dissemination in the skin. We also observed differences in outer surface profile in the lon-1 mutant when cultured with blood, and these changes may affect the survival of the mutant in vivo. Our results show that Lon-1 protease is important for pathogenesis of B. burgdorferi.