Characterisation of the mode of action of chelating agents against Gram-negative and Gram-positive bacteria

Abstract

Limiting the availability of metals in an environment is known to restrict bacterial growth and proliferation. For example, humans sequester iron, manganese and zinc to help prevent infection by pathogens, a system termed nutritional immunity. Chelators are small ligands that bind tightly to metals and thus have antibacterial properties that mimic these innate immune processes. However, the modes of action of many of these chelating agents in bacterial growth inhibition and their selectivity in metal deprivation in cellulo remain ill-defined. In this thesis, we expand upon previous work in which the effect of eleven chelators on the cellular metal concentrations of Escherichia coli was examined. Building upon these insights, the antibacterial activity of a group of seven chelants was studied in two Gram-negative species, E. coli and Pseudomonas aeruginosa, and two Gram-positive species, Bacillus subtilis and Staphylococcus aureus. Selected chelant-chelant and chelant-antibiotic combinations were evaluated for efficacy in antibacterial hostility. The influence of these chelating agents on cellular metal concentrations in P. aeruginosa was also determined. An experimental evolution approach was employed with E. coli and S. aureus to obtain mutants which have acquired some level of resistance to EDTA and DTPMP. Changes affecting iron and zinc uptake pathways in E. coli were identified, while alterations in cell wall metabolism and surface charge were found in S. aureus. Our results reveal significant insights into the mechanism of growth inhibition by chelants, highlighting their potential as antibacterial agents and as tools to probe the ways in which bacteria tolerate selective metal deprivation.