Polymorphic behavior of gram-negative bacteria membranes

Freeze-fracture and ultrathin section electron microscopy as well as31P-NMR spectroscopy and light scattering ofEscherichia coli andPseudomonas putida cells under conditions promoting the ability of cells to take up exogenous DNA's (high concentrations of divalent cations and a specific temperature regime) reveal the extensive polymorphic changes and the formation of various structural defects in cellular membranes. Polymorphic changes occur during the heat shock at 42 to 44°C of the cells preincubated at 0°C in the presence of high concentration of Ca2+ or Ba2+ cations and include the formation of various vesicle- and tube-like structures, intermembrane and intercellular contacts followed by membrane fusion and sometimes even by cell fusion. The results obtained suggest the occurrence of phospholipid-enriched zones in the outer leaflet ofE. coli outer membrane. This suggestion is verified and confirmed with the help of phospholipase C, a specific phospholipid binding and digesting enzyme. The presented experimental evidence directly supports the suggestion of Ahkong et al. (Nature253:194–195, 1975) on the identity of the mechanisms of membrane contact formation and membrane fusion in model and cellular membranes. The biological relevance of the polymorphic changes observed is shortly discussed.