Optimizing the membrane composition of immobilized giant vesicles for effective entrapment and observation of motile bacteria

Artificial lipid vesicles are an elegant way of producing auto-assembled customizable chambers, and they have been widely studied for entrapping small molecules in the context of drug delivery. However, their use for microorganism entrapment remains largely unexplored. One obstacle is the generation of giant vesicles (GVs) large and rigid enough to contain potentially mobile micron-size organisms in a way that results in efficient cell encapsulation. To address this challenge we developed a modified version of the previously described gel-assisted swelling method, in which immobilized GVs are formed by hydrating a lipid film deposited on agar with a heated solution. We used solutions containing Magnetospirillum magneticum strain AMB-1 cells to test for bacterial entrapment. Systematically exploring POPC:DOPS:cholesterol ternary mixtures to vary the surface charge and rigidity of the GVs’ membrane allowed the determination of specific conditions leading to the efficient entrapment of these highly mobile cells. Lipid order measurements with the fluorescent membrane probe Laurdan show that entrapment is associated with a significant decrease in membrane rigidity between the swelling and entrapment steps. The high sensitivity of successful entrapment on DOPS concentration further suggests that membrane fluctuations play a major role in allowing the micron-size bacteria to push through the membrane and break in and out of the GVs.