Mixed liposomes containing gram-positive bacteria lipids: Lipoteichoic acid (LTA) induced structural changes

Lipoteichoic acid (LTA), a surface associated polymer amphiphile tethered directly to the Gram-positive bacterial cytoplasmic membrane, is a key structural and functional membrane component. Its composition in the membrane is regulated by bacteria under different physiological conditions. How such LTA compositional variations modulate the membrane structural stability and integrity is poorly understood. Here, we have investigated structural changes in mixed liposomes mimicking the lipid composition of Gram-positive bacteria membranes, in which the concentration of Bacillus Subtilis LTA was varied between 0-15 mol%. Small-angle neutron scattering (SANS) and dynamic light scattering (DLS) measurements indicated formation of mixed unilamellar vesicles, presumably stabilized by the negatively charged LTA polyphosphates. The vesicle size increased with the LTA molar concentration up to similar to 6.5 mol%, accompanied by a broadened size distribution, and further increasing the LTA concentration led to a decrease in the vesicle size. At 80 degrees C, SANS analyses showed the formation of larger vesicles with thinner shells. Complementary Cryo-TEM imaging confirmed the vesicle formation and the size increase with LTA addition, as well as the presence of interconnected spherical aggregates of smaller size at higher LTA concentrations. The results are discussed in light of the steric and electrostatic interactions of the bulky LTA molecules with increased chain fluidity at the higher temperature, which affect the molecular packing and interactions, and thus depend on the LTA composition, in the membrane.