Studies on the uptake of exogenous phosphoglycerides by Escherichia coli B cells

The uptake of exogenous lipids by Escherichia coli B was studied under various conditions. Lysophosphoglycerides were absorbed more readily than diacyl analogues when sonicated dispersions of a single lipid were used. When these same lipids were admixed with coliform lipids and dispersed as vesicles, the uptake of lysophosphoglyceride diminished and was approximately equal to that of diacyl analogues. Neutral lipids such as diglyceride, fatty acid, and cholesterol were also absorbed when admixed and dispersed as vesicles with coliform lipids. The uptake of lysophosphoglyceride was stimulated slightly by all divalent cations tested except Mg2+; monovalent cations were ineffective. Uptake was accompanied by conversion of lysophosphoglyceride to diacyl analogues. In the presence of Ca2+, lysophosphatidylethanolamine also formed a more polar lipid product yet unidentified. The uptake of lipid did not cause release of 3H label into the medium from cells that had been grown in [3H]acetate-containing medium. Also, the [3H]phosphoglyceride content of such labelled cells remained constant. Thus the uptake process did not involve exchange of membrane lipid with the medium or an enhanced hydrolysis of endogenous lipids, but it represented a net gain of lipid by the cell. The uptake did not seem to involve a stable adsorption of lipid at the surface of the cell as could be judged from electron microscopic examination of the cells after incubation; the cell surfaces were devoid of adsorbed vesicle or liposomal types of structures and did not display evidence of expansion. [3H]Cholesterol-[32P]phospholipid mixtures were taken up without a change in isotopic ratio. This result together with the other evidence presented indicate a net uptake of exogenous lipid by a process likely involving fusion.