The Effect of Lipopolysaccharide Core Oligosaccharide Size on the Electrostatic Binding of Antimicrobial Proteins to Models of the Gram Negative Bacterial Outer Membrane

Abstract

Understanding the electrostatic interactions between bacterial membranes and exogenous proteins is crucial to designing effective antimicrobial agents against Gram-negative bacteria. Here we study, using neutron reflecometry under multiple isotopic contrast conditions, the role of the uncharged sugar groups in the outer core region of lipopolysaccharide (LPS) in protecting the phosphate-rich inner core region from electrostatic interactions with antimicrobial proteins. Models of the asymmetric Gram negative outer membrane on silicon were prepared with phopshatidylcholine (PC) in the inner leaflet (closest to the silicon), whereas rough LPS was used to form the outer leaflet (facing the bulk solution). We show how salt concentration can be used to reversibly alter the binding affinity of a protein antibiotic colicin N (ColN) to the anionic LPS confirming that the interaction is electrostatic in nature. By examining the interaction of ColN with two rough LPS types with different-sized core oligosaccharide regions we demonstrate the role of uncharged sugars in blocking short-range electrostatic interactions between the cationic antibiotics and the vulnerable anionic phosphate groups.

Opposing effects of cationic antimicrobial peptides and divalent cations on bacterial lipopolysaccharides

“…In reality, the conformational properties of peptides in the LPS brush will likely combine both features of rigid cylinders and random coils. While a fuller picture is lacking, this line of reasoning is consistent with the finding that the presence of uncharged sugar groups in the outer core of LPS weakens the binding of cationic antimicrobials to the LPS [14]. A more quantitative study of peptide inclusion in an LPS brush will be useful for improving a coarse-grained approach.…”

“…Use of a more realistic model of LPS will be beneficial for clarifying further OM permeability. Indeed, recent experiments suggest that uncharged sugar groups in LPS reduce the electrostatic binding of cationic antimicrobials to LPS [14]. In principle, this effect can be captured in our approach [Eq.…”

“…(For more recent progress, see Refs. [12][13][14][15][16].) Indeed, increased OM permeability is correlated with enhanced susceptibility of Gram-negative bacteria to AMPs and antibiotics [9][10][11].…”

“…A better understanding of how AMPs reduce OM permeability will benefit our effort for combating these bacteria. Indeed, several attempts have recently been made to advance our understanding of OM permeability [12][13][14][15][16]. For instance, an in vitro model of the Escherichia coli cell surface has been proposed and used in further unravelling the effects of divalent cations on OM permeability [12][13][14].…”

The Interaction between Soft Interfaces: Forces and Structural Aspects

“…[48][49][50][51][52][53][54][55][56][57][58][59] In neutron reflectometry, selective deuteration is commonly used to enhance the SLD contrast between different chemical components and thus to better discriminate between their interfacial distributions. The shape of the reflectivity curve R(q z ) encodes the interfacial SLD profile, so that the latter can be reconstructed.…”