A fundamental tenet of microbial pathogenesis is that bacterial pathogens must overcome host iron limitation to establish a successful infection. Surprisingly, the Lyme disease pathogenBorrelia burgdorferi has bypassed this host defense by eliminating the need for iron. B. burgdorferi grew normally and did not alter gene expression in the presence of iron chelators. Furthermore, typical bacterial iron-containing proteins were not detected in cell lysates, nor were the genes encoding such proteins identified in the genome sequence. The intracellular concentration of iron in B. burgdorferi was estimated to be less than 10 atoms per cell, well below a physiologically relevant concentration.