Liposome-mediated transfer of macromolecules into flagellated cell envelopes from bacteria

We have studied the interaction between flagellated cell envelopes from Escherichia coli and liposomes. Oligolamellar liposomes of ca. 0.45-micron diameter, composed of azolectin, phosphatidylserine, and cholesterol at a molar ratio of 7:1:2, were prepared by freezing and thawing and subsequent extrusion through polycarbonate filters. These liposomes exhibited high entrapment capacity and low leakiness. Liposome-cell envelope interaction was monitored flow cytometrically in a fluorescence-activated cell sorter with a fluorescent aqueous space marker and by a filtration assay with radiolabels for the lipid phase and the liposomal aqueous space. Maximal association of liposomes with the envelopes was observed in both assays after ca. 25 min at 30 degrees C. After such period of time, it seems that up to 200 liposomes (depending on the liposome to envelope ratio) were associated with a single cell envelope, as calculated from the radiotracer studies. Fluorometric measurements of the transfer of liposomal contents and the intermixing of membrane lipids indicated that at least 20% of the envelope-associated liposomes had delivered their content into the envelopes, possibly by fusion. Electron microscopic observations confirmed the transfer of liposome-encapsulated ferritin molecules into the cell envelopes. Our data suggest that liposomal carriers might be employed to deliver cytoplasmic, chemotaxis-related macromolecules into bacterial cell envelopes.