Cell Membrane Agents

The cell membrane (plasma membrane) of a bacterium controls what enters and leaves the cell. It is a phospholipid bilayer, like the membranes of human cells, but its lipid composition is different. Gram-negative bacteria also have an outer membrane containing lipopolysaccharide (LPS).Polymyxins, such as polymyxin B and colistin, are the main example of antibiotics that act on membranes. They are positively charged molecules with a fatty tail. The positive part binds to the negatively charged lipopolysaccharide of the outer membrane, and the fatty tail pushes between the phospholipids. This disrupts both the outer membrane and the cell membrane, making them much more permeable.The membrane can no longer act as a barrier. Ions, nucleotides and other cell contents leak out, the cell cannot keep the conditions it needs for metabolism, and it dies. Polymyxins are therefore bactericidal. Unlike penicillin, they kill bacteria whether or not they are growing.Because polymyxins bind to LPS, they act mainly on Gram-negative bacteria, so they are narrow-spectrum.Limited selective toxicity. Bacterial and human membranes are built in a similar way, so membrane agents are more toxic to human cells than most other antibiotics. Taken internally, polymyxins can damage the kidneys and nerves. For this reason they are often used on the skin or in eye and ear drops, and colistin is kept as a last-resort antibiotic for Gram-negative bacteria that are resistant to other drugs.