Antimicrobial resistance threatens the foundations of modern medicine, with a lack of new antibiotics compounding the crisis. Antimicrobial peptides (AMPs) have attracted substantial attention over many decades as a potential source of new therapeutics, but with the exception of natural product-derived cyclic lipopeptides such as the polymyxins (colistin, polymyxin B) and daptomycin, they have failed to achieve clinical approval.

Our group has research program investigating two classes of AMPs. The octapeptins are cyclic octamer lipopeptides structurally similar to the polymyxins, but which intriguingly retain activity against polymyxin-resistant Gram-negative bacteria, [1]. In contrast, the arenicins are 21-residue amphipathic β-hairpin peptides closely related to tachyplesin and protegrin-1, with arenicin-3 analogs progressed into advanced preclinical testing by Adenium Biotech [2]

As part of a project developing readily derivatisable derivatives of all major classes of antibiotics, we have prepared azide-functionalised versions of polymyxin B, octapeptin C4, tachyplesin and arenicin-3. These were then ‘clicked’ with alkyne-derivatised small fluorophores (nitrobenzoxadiazole, NBD: green, and dimethylaminocoumarin-2-acetic acid, DMACA: blue). Notably, the derivatised antibiotics retained similar activity and resistance profiles as the parent antibiotic, making them useful probes to investigate antibiotic-bacteria interactions.

High resolution microscopy of fluorescently-labelled bacteria demonstrated strikingly different localisation patterns between the four antibiotics, despite all nominally working via membrane disruption. Further work has found that the polymyxin probe can rapidly distinguish between bacteria with varying levels of polymyxin resistance, potentially leading to a useful rapid assay for resistance. The data to date suggests that the probes are valuable tools to aid in understanding antibiotic-bacteria interactions, supporting the fight against antimicrobial resistance.