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Background    Bacillus anthracis  produces two catecholate siderophores, petrobactin and bacillibactin, under iron-limited conditions. Here, we investigate how variable iron and oxygen concentrations influence the biosynthetic output of both siderophores in  B. anthracis . In addition, we describe the differential levels of transcription of select genes within the  B. anthracis  siderophore biosynthetic operons that are responsible for synthesis of petrobactin and bacillibactin, during variable growth conditions.    Methodology/Principal Findings  Accumulation of bacillibactin in  B. anthracis  Sterne (34F 2 ) and in a mutant lacking the major superoxide dismutase ( ΔsodA1 ) was almost completely repressed by the addition of 20 µM of iron. In contrast, petrobactin synthesis in both strains continued up to 20 µM of iron. Accumulation of petrobactin and bacillibactin showed a slight increase with addition of low levels of paraquat-induced oxidative stress in wild type  B. anthracis  Sterne. Cultures grown with high aeration resulted in greater accumulation of petrobactin relative to low aeration cultures, and delayed the repressive effect of added iron. Conversely, iron-depleted cultures grown with low aeration resulted in increased levels of bacillibactin. No difference was found in overall superoxide dismutase (SOD) activity or transcriptional levels of the  sodA1  and  sodA2  genes between iron-depleted and iron-replete conditions at high or low aeration, suggesting that SOD regulation and iron metabolism are separate in  B. anthracis . The highest transcription of the gene  asbB,  part of the petrobactin biosynthetic operon, occurred under iron-limitation with high aeration, but transcription was readily detectable even under iron-replete conditions and in low aeration. The gene  dhbC , a member of the bacillibactin biosynthetic operon, was only transcribed under conditions of iron-depletion, regardless of growth aeration.    Conclusion  These data suggest that bacillibactin regulation is highly sensitive to iron-concentration. In contrast, although regulation of petrobactin is less dependent on iron, it is likely subject to additional levels of regulation that may contribute to virulence of  B. anthracis .

Regulation of Petrobactin and Bacillibactin Biosynthesis in Bacillus anthracis under Iron and Oxygen Variation