A relatively small hydrophobic molecule that facilitates the transport of ions across lipid membranes. Most ionophores are produced by microorganisms. There are two types of ionophore: channel formers, which combine to form a channel in the membrane through which ions can flow; and mobile ion carriers, which transport ions across a membrane by forming a complex with the ion. Examples of ionophores include vancomycin, nystatin, and 2,4-dinitrophenol.