Pathogenic (disease-causing) bacteria require iron for growth and survival. But iron is also essential to animal hosts, which lock away iron using high-affinity soluble proteins such as heme and transferrin. Therefore, in order for bacteria to successfully invade and infect a host, they must utilize their own special iron-gathering protein systems to compete with the host for iron. Specialized transport proteins called Siderophore Receptors (siderophore is Greek for iron carrier) are located on the outer surface of bacteria. Siderophore receptors belong to a class of tube-shaped proteins called Porins, which transport nutrients through the bacterial cell wall. Together, Siderophore Receptors and Porins (SRP®) can be extracted from bacteria to form a purified protein vaccine. Pictured here is the ferric hydroxamate uptake receptor (FHUA) from E. coli, in complex with the much smaller siderophore protein, ferrichrome. The 3-D structure of siderophore receptors can be visualized using the RCSB Protein Data Bank website.