Lipid domains in bacterial membranes and the action of antimicrobial agents
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There has been increasing interest in recent years in describing the lateral organization of membranes and the formation of membrane domains. Much of the focus in this area has been on the formation of cholesterol-rich domains in mammalian membranes. However, it is likely that there are domains in all biological membranes. One of the challenges has been to define the chemical composition, lifetime and size of these domains. There is evidence that bacteria have domains that are enriched in cardiolipin. In addition, the formation of lipid domains can be induced in bacteria by clustering negatively charged lipids with polycationic substances. Many antimicrobial compounds have multiple positive charges. Such polycationic compounds can sequester anionic lipids to induce lipid phase separation. The molecular interactions among lipids and their lateral packing density will be different in a domain from its environment. This will lead to phase boundary defects that will lower the permeability barrier between the cell and its surroundings. The formation of these clusters of anionic lipids may also alter the stability or composition of existing membrane domains that may affect bacterial function. Interestingly many antimicrobial agents are polycationic and therefore likely have some effect in promoting lipid phase segregation between anionic and zwitterionic lipids. However, this mechanism is expected to be most important for substances with sequential positive charges contained within a flexible molecule that can adapt to the arrangement of charged groups on the surface of the bacterial cell. When this mechanism is dominant it can allow the prediction of the bacterial species that will be most affected by the agent as a consequence of the nature of the lipid composition of the bacterial membrane.
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