A major advance in the concept of the fluid mosaic model of biological membranes in recent years has been the appreciation of the domain structure of membranes. This concept is now well developed with mammalian plasma membranes but is an emerging focus with regard to bacterial membranes. In addition to bacterial domains that form spontaneously, it is possible to induce the separation of anionic and zwitterionic lipids with certain cationic antimicrobial agents. As a consequence, the bacterial species for which these agents will be toxic can be predicted on the basis of the lipid composition of the bacterial membrane.