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Breaching Bacterial Cell Membranes

Primary Supervisor: Dr Sarah L. Horswell, School of Chemistry

Secondary supervisor: Professor T.R. Dafforn

PhD project title: Breaching Bacterial Cell Membranes

University of Registration: University of Birmingham

Project outline:

Knowledge of the structure of a cell membrane is crucial to understanding its properties. It is established that bacterial cell membranes differ in composition from eukaryotic plasma cell membranes, as well as other membranes within cells, but little is known about the reasons for this diversity. If we are to develop new antimicrobial agents, we need to understand the differences between bacterial and mammalian (or even plant) membranes in order to target bacteria selectively. For example, one class of antimicrobial agents includes small, positively charged peptides, which have an affinity for the negatively charged membranes of many bacteria, such as E. Coli. The key question is whether it is sufficient for the outer half of the membrane to bear a negative charge or whether there is a specific interaction between those negatively charged lipids and the peptides, then how the functional groups facilitate or hinder insertion of that peptide and subsequent aggregation to form pores.

We have developed a means to vary the charge density of a lipid bilayer without changing composition, by supporting the bilayer on an electrode. Several in situ probes can be used to study how the structure and elasticity vary with the charge density, which allows investigation into how both charge and specific molecule structures affect packing and properties. We have considerable experience in these techniques and propose to apply them to the questions of how charge and specific lipid functional groups confer structural properties and are involved in interactions with simple cationic peptides and other small molecules that have been shown to disrupt bacterial cell membranes or cell walls.

In this project we shall carry out structural studies on a range of lipid compositions. We shall then select a series of compositions and study the propensity to interact with cationic peptides, to determine whether these peptides can interact selectively with compositions representative of bacterial membranes. Using monolayers and bilayers as membrane mimics we aim to determine the effects of externally applied charge density and intrinsic charge density on barrier properties and membrane structure. We shall then determine how these properties affect interaction with cationic peptides and whether there are specific interactions that can be targeted or whether the interactions are purely electrostatic

By using our combination of techniques we can build up a detailed picture of the effect of charge, size and shape of charged phospholipid on membrane structure and properties, which will lead to an understanding of the rĂ´les of various lipids in cell membranes. Further, we will shed light on the interaction of potential antimicrobial agents with model bacterial membranes and learn which factors may lead to selectively disrupting the membranes of bacteria over those of plants or animals. If model studies are successful, they have the potential to be used to screen potential target compounds to combat infection in humans, animals or plants and so would find ultimate potential application in protecting crops, animals or reducing the number of compounds tested on animals.

BBSRC Strategic Research Priority: Understanding the Rules of Life: Structural Biology & Integrated Understanding of Health: Ageing & Pharmaceuticals

    Techniques that will be undertaken during the project:

    • Protein production through microbial expression
    • Surface pressure-area isotherm measurements and monolayer transfer
    • Capacitance and impedance measurements
    • Interfacial vibrational spectroscopy (infrared and Raman)
    • Atomic force microscopy
    • Light scattering
    • Neutron and X-ray Scattering
    • Neutron and X-ray Reflectometry, Grazing Incidence X-ray Diffraction
    • There is also potential to visit a collaborator in Germany working on molecular dynamics simulations and to learn these techniques

    Contacts: Dr Sarah Horswell, University of Birmingham