Bacterial Membranes Challenged by Multivalent Cations and Cationic Antimicrobial Peptides

Abstract

Bacteria, with their robust membranes, have evolved extensive resistance mechanisms to protect themselves against antibiotic actions. Acting as a critical barrier, the bacterial membrane plays a pivotal role in mediating the interactions between bacteria and antimicrobial agents. Understanding the membrane structure and function is crucial to addressing multi-drug resistant bacterial pathogens. Gram-negative bacteria have a thinner layer of peptidoglycan in their cell wall, which is surrounded by a highly asymmetric outer membrane containing 75% bacteria-specific lipopolysaccharides (LPS). The headgroup of LPS is abundant in phosphates carboxylates groups, making it anionic in nature. Gram-positive bacteria have a thick layer of peptidoglycan in their cell wall, with a polymer-like amphiphile called lipoteichoic acid (LTA) anchored to the phospholipid membrane. The polyglycerol-phosphate backbone of LTA carries significant negative charges due to the presence of the phosphate groups.
This study has developed and characterized the membrane models incorporating key bacterial membrane components, i.e. Ra-LPS, a rough mutant with truncated headgroups, and BsLTA, extracted from Bacillus Subtilis bacteria species. The influence of monovalent (Na+), divalent (Ca2+) and trivalent (La3+) cations and a quadruply-charged peptide (four-antennary oligoglycines) on membrane structure, elasticity and integrity has been investigated, as a function of ionic strength (3-30 mM) and temperature (25 and 40oC). The classic Langmuir-Blodgett trough has been used to evaluate physicochemical properties of bacterial lipid monolayers, shedding light on the underpinning molecular interactions. In situ Brewster angle microscopy (BAM), synchrotron X-ray reflectivity (XRR), and neutron reflectivity (NR) provided complementary structural insights. These unprecedented results lead to a better understanding of the mechanisms of the bacterial membrane structure and elastic properties changes challenged by cationic species, with fundamental implications to rational design of novel physical approaches to disrupting bacterial membranes and killing bacteria.

Date of Award	18 Jun 2024
Original language	English
Awarding Institution	University of Bristol
Supervisor	Wuge H Briscoe (Supervisor)