Many bacteria alter the acyl chains of their membrane phospholipids in response to changing environmental conditions. Two of these modifications are reviewed: cis→trans isomerization and cyclopropanation of double bonds. The isomerization reaction is now known to be catalyzed by a periplasmic protein that contains a covalently bound heme. Cyclopropanation has been shown to play a role in Mycobacterium tuberculosis pathogenesis and, in Escherichia coli, plays an important role in resistance to acidic conditions.