Figure 1.

Summary of predicted and described iron and non-iron metal acquisition systems in A. baumannii. The iron and non-iron systems encoded within the A. baumannii genome vary dramatically between different strains. (A) This panel depicts the iron acquisition and utilization systems identified or described among the different A. baumannii strains. Secretion of enzymes such as phospholipase C and others may contribute to hemolysis and hemoglobin release from red blood cells. At least five clusters of genes for siderophore synthesis and secretion are dispersed among different strains. These include acinetobactin, fimsbactins A-F, the strain 8399 (om73-entD) siderophore, as well as siderophores produced by the ACICU1672-1683 and the ABAYE1888-1899 clusters. EntA is required for iron acquisition due to its involvement in the biosynthesis of the acinetobactin precursor 2,3-dihydroxybenzoic acid. Siderophores are recognized by TonB-dependent receptors, of which 8–22 have been discovered among the sequenced strains. At least one of multiple encoded TonB/ExbB/ExbD systems generates and provides energy to the TonB-dependent receptors, and at least one FeoAB ferrous iron transport system has been identified in all sequenced strains. Finally, 1–2 heme uptake systems have been identified among A. baumannii strains. NfuA is a Fe-S cluster protein required for iron utilization in the cytoplasm. Regulation of these numerous systems likely depends on the conserved Fur repressor. (B) This panel depicts the zinc acquisition and metabolism systems described in A. baumannii ATCC 17978. Outer membrane transport of zinc may occur via two zinc-regulated TonB-dependent receptors, energized by a zinc-regulated TonB/ExbB/ExbD system. Inner membrane transport is mediated by the ZnuABC transporter. Zur is a conserved repressor that controls the expression of znuABC and likely the other identified zinc systems.