The AsbB enzyme, which is involved in the biosynthesis of the virulence-conferring siderophore petrobactin in Bacillus anthracis, is shown to catalyze efficient ATP-dependent condensation of spermidine, but not N1-(3,4-dihydroxbenzoyl)-spermidine, with N8-citryl-spermidine or N1-(3,4-dihydroxbenzoyl)-N8-citryl-spermidine, suggesting that N1-(3,4-dihydroxbenzoyl)-spermidine is very unlikely to be a significant intermediate in petrobactin biosynthesis, contrary to previous suggestions.