The outer membrane of Gram-negative bacteria constitutes a permeability barrier that protects the cell from exterior hazards, but also complicates the uptake of nutrients. In the case of iron, the challenge is even greater, because of the scarcity of this indispensable element in the cell's surroundings. To solve this dilemma, bacteria have evolved sophisticated mechanisms whereby the concerted actions of receptor, transporter and energy-transducing proteins ensure that there is a sufficient supply of iron-containing compounds, such as siderophores.