Insights into the antibacterial mechanism of action of caprylhydroxamic acid (CHA) against Gram-negative and Gram-positive bacteria.

Metal ion bioavailability exerts a strong selective pressure at the infectious interface, limiting bacterial growth and proliferation. Thereby, vertebrates have developed a plethora of mechanisms to starve bacteria of essential metals or direct metal toxicity against invading pathogens, via a process termed nutritional immunity. Commercially used chelating agents have high metal binding affinities that enable these ligands to mimic the conditions of nutritional immunity and confer antibacterial activities. However, one such chelator, caprylhydroxamic acid (CHA), was recently shown to cause no significant alteration in the metal composition of Escherichia coli cells, throwing into question the extent to which metal sequestration is responsible for its antibacterial properties. To gain a greater understanding of the antibacterial mechanism of action of CHA, the bacterial response to chelant exposure was studied in three Gram-negative species, E. coli, Pseudomonas aeruginosa and Serratia marcescens, and two Gram-positive species, Bacillus subtilis and Staphylococcus aureus. CHA was evaluated as an inhibitor of bacterial growth both individually and in combination with antibiotics. Mutant strains were also assessed to help delineate the genes responsible for tolerance to CHA. Finally, gene and protein expression were analysed to gain further insight into the chelant’s cellular targets or subsidiary effects. This study uncovered that while metal deprivation, particularly of iron and manganese, appears to play some role in the mode of action of CHA, the disruption of membrane integrity is primarily responsible for its antibacterial activities. The results reveal new insights into the mechanism of bacterial growth inhibition by CHA, showcasing its potential as an antibacterial agent, and informing its targeted application in consumer products, industrial processes and healthcare.