Langmuir, Vol.35, No.48, 15739-15750, 2019
Interactions between Mutant Bacterial Lipopolysaccharide (LPS-Ra) Surface Layers: Surface Vesicles, Membrane Fusion, and Effect of Ca2+ and Temperature
Lipopolysaccharides (LPS) are a major component of the protective outer membrane of Gram-negative bacteria. Understanding how the solution conditions may affect LPS-containing membranes is important to optimizing the design of antibacterial agents (ABAs) which exploit electrostatic and hydrophobic interactions to disrupt the bacteria membrane. Here, interactions between surface layers of LPS (Ra mutants) in aqueous media have been studied using a surface force apparatus (SFA), exploring the effects of Wee temperature and divalent Ca2+ cations. Complementary dynamic light scattering (DLS) characterization suggests that vesicle-like aggregates of diameter , similar to 28-80 nm are formed by LPS-Ra in aqueous media. SFA results show that LPSRa vesicles adsorb weakly onto mica in pure water at room temperature (RT) and the surface layers are readily squeezed out as the two surfaces approach each other. However, upon addition of calcium (Ca2+) cations at near physiological concentration (2.5 mM) at RT, LPS multilayers or deformed LPS liposomes on mica are observed, presumably due to bridging between LPS phosphate groups and between LPS phosphates and negatively charged mica mediated by Ca2+, with a hard wall repulsion at surface separation D-0 similar to 30-40 nm. At 40 degrees C, which is above the LPS-Ra beta-alpha acyl chain melting temperature (T-m = 36 degrees C), fusion events between the surface layers under compression could be observed, evident from delta D similar to 8-10 nm steps in the force- distance profiles attributed to LPS-bilayers being squeezed out due to enhanced fluidity of the LPS aryl-chain, with a final hard wall surface separation D-0 similar to 8-10 nm corresponding to the thickness of a single bilayer confined between the surfaces. These unprecedented SFA results reveal intricate structural responses of LPS surface layers to temperature and Ca2+, with implications to our fundamental understanding of the structures and interactions of bacterial membranes.