Bacillus anthracis Sterne produced a catecholate siderophore named anthrachelin that was based on 3,4-dihydroxybenzoic acid (3,4-DHB, or protocatechuic acid), a catechol moiety previously unreported as a siderophore component. During iron restriction, both anthrachelin and free 3,4-DHB were excreted. Growth at 37degreesC (as compared with 23degreesC) decreased excretion of anthrachelin but not its precursor 3,4-DHB, suggesting that anthrachelin assembly is temperature regulated. A plasmidless strain also produced anthrachelin in an iron-and temperature-regulated fashion, indicating that anthrachelin genes are chromosomal. In addition to anthrachelin-mediated iron delivery, B. anthracis also used heme, hemoproteins, iron-transferrin, and certain heterologous siderophores (xenosiderophores) produced by other microorganisms as iron sources. Downregulation of anthrachelin production at the temperature of the mammalian host (which triggers toxin production in this pathogen) may focus the B. anthracis iron acquisition systems to exploit the iron sources prevailing in the infected host.