Bacillus anthracis secretes two siderophores, petrobactin (PB) and bacillibactin (BB). These siderophores were temporally produced during germination and outgrowth of spores (the usual infectious form of B. anthracis) in low-iron medium. The siderophore PB was made first while BB secretion began several hours later. Spore outgrowth early in an infection may require PB, whereas delayed BB production suggests a role for BB in the later stages of the infection. Incubation of cultures (inoculated as vegetative cells) at 37 degrees C, as compared to 2 degrees C, increased PB production and decreased secretion of BB, suggesting that the production of PB and BB responded to the host temperature signal. The dual siderophores of B. anthracis may fulfill independent roles in the life cycle of B. anthracis.