FIGURE 3: Schematic models of haem piracy from haemoglobin by the different Gram negative pathogens described herein. (A) Some pathogens, such as S. dysenteriae, employ monopartite TonB-dependent transporters (TBDT). These β-barrel outer membrane proteins (ShuA in the case of S. dysenteriae), enable active transport of haem from the host haemoprotein into the periplasm from where the haem is transported into the cytoplasm by a haem specific ABC transporter. (B) Some TBDTs, such as the gonococcal HpuB, work in conjunction with an associated surface exposed lipoprotein (HpuA) to extract haem from Hb or Hb:Hp and transport it into the periplasm. (C) Other TBDT systems, such as the A. baumannii HphR/HphA are a different kind of bipartite system. HphA is a secreted protein that is reported to bind Hb and brings its haem to the membrane for transport into the periplasm by HphR.