Bacterial iron uptake pathways as targets for the development of novel antimicrobials

Antimicrobial resistance is an ever increasing problem. Presently it is estimated to be responsible for 700,000 deaths p.a. and it is predicted that this figure will rise to 10 million by 2050. The problem is compounded by the absence of new classes of antibiotics to treat bacterial infections during the past few decades. Therefore, new strategies to combat bacterial infections are required. Many bacteria that exhibit resistance to a wide range of antimicrobial compounds show very low uptake of the antibiotic across the cell envelope either due to a low permeability to the antibiotic or the presence of an efficient efflux system that rapidly removes it from the cell. One approach to promote the uptake of antimicrobials is to design so-called ‘Trojan horse’ antimicrobials. In this approach, an antimicrobial compound is tethered to a molecule that is efficiently transported into the bacterium, thus tricking the bacterium into actively importing the antibiotic.

Gram-negative bacteria contain an outer membrane in which are embedded transport proteins known as TonB-dependent transporters (TBDTs). Each TBDT is specific for a particular compound or group of closely related compounds and it transports such compounds in an energy dependent manner. These compounds are too big to otherwise diffuse across the outer membrane through porins, and they include metal chelates such as haem, vitamin B12 and ferric-siderophore complexes. Siderophores are compounds made by bacteria, fungi and plants that bind ferric iron with high efficiency. The iron-siderophore complexes are recognised by specific TBDTs and transported very efficiently into the periplasmic space between the two membranes. Following this, some siderophore classes are translocated into the cytoplasm via cytoplasmic membrane transporters.

It has been shown that antibiotics that are conjugated to siderophores can also be efficiently internalised by bacteria, thereby resulting in their death. As a prerequisite for exploring the possibilities for designing Trojan horse antimicrobials that are active against bacterial species that are intrinsically resistant to many antibiotics, the aim of this project is to identify siderophores that can be utilised by such antibiotic resistant pathogens. We will also identify the TBDTs responsible for the uptake of such siderophores and we will also explore the substrate specificity of the identified TBDTs by analysing their involvement in internalising siderophores with related structures.