Targeting the bacterial surface for antibiotic delivery

Bacterial infections and antimicrobial resistance present major global public health challenges. Current rates of new antibiotic discovery are unlikely to address the demand for effective antibacterial therapies. Even if drug discovery was accelerated, new antibiotics will inevitably engender resistance in both target and non-target organisms collaterally exposed to the antibacterial agents during therapy, limiting the expected usable life of even novel antibiotics.
In addition to developing novel antibiotics, research is now increasingly focussing on new ways of utilising existing (and already approved) antimicrobials to increase their efficacy and potentially overcome resistance. In our labs we have recently found that, surprisingly, when a specific cyclic peptide antibiotic, colistin (polymyxin B), is displayed at the surface of lipid vesicles, liposomes, its microbicidal effect towards Gram-negative bacteria E Coli is significantly enhanced. Whilst the mechanism of this unexpected effect is currently unknown, and our currently hypothesis if that presentation of multiple copies of this antibiotic peptide may enable stronger binding to bacterial surface lipopolysaccharides (LPS) and deliver higher local doses of bactericidal agents, with perturbation/damage of bacterial membrane.
We also observed that liposomes presenting the antibacterial peptide colistin at their surface are capable of selectively associating with Gram-negative bacteria (E. coli) even amongst a background of greater numbers of Gram-positive B. subtilis. and, surprisingly, inducing killing of E. coli at concentrations that were not expected to be effective. This potentially indicates the importance of a presence of high local concentration of colistin, and/or multivalent phenomena, consequent of liposomal adhesion, for bacterial killing, and opens a potential for a novel formulation strategy where colistin toxicity could be addressed by targeted delivery.
Aims and objectives
Our goal is to enhance the efficacy of antimicrobial against targeted pathogenic species, while reducing collateral damage to non-target bacterial species and host cells. The project will deliver an underlying mechanistic understanding of selective routes for bacterial killing and the 'product attributes' of a delivery technology which (i) achieves preferential targeting to bacterial cells per se, and (ii) increases efficacy against targeted species, while reducing collateral damage to non-target bacterial species and host cells.
Methodology
Task 1: Fabrication of liposomes, surface functionalisation, and biophysical characterisation. A library of liposomes with a spectrum of lipid composition, surface functionality, biophysical properties, and ability to perturb bacterial outer membrane will be generated and characterised. Initially colistin, and members of the polymyxin family of antibacterial peptides, will be used for surface functionalisation of liposomes.
Task 2: Preferential bacterial adhesion (targeting) of functionalised liposomes, outer membrane perturbation, and safety to host epithelium and their selective biorecognition tested in a number of target and non-target bacterial species. These could include commensal and pathogenic E. coli, P. aeruginosa, Salmonella Typhimurium, Bacillus subtilis and S. aureus. Mechanism of bacterial recognition and membrane disruption will be investigated at this stage.
Task 3. Targeted combination drug delivery. If time allows, we will hence test functionalised liposomes with a number of potential antibiotic candidates (tetracycline, rapamycin, erythromycin/azithromycin, kanamycin/amikacin or meropenem). The selection aims to address a question if proposed formulation technology is able to yield benefits when resistance to one antibiotic arises.