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 developing derivatised versions of major antibiotics that allow for facile attachment of additional functional groups, including fluorophores [1]. We have prepared fluorescent versions of a range of antimicrobial peptides, including polymyxin B, octapeptin C4, tachyplesin arenicin-3, and daptomycin. These have employed small fluorophores (nitrobenzoxadiazole, NBD: green, and dimethylaminocoumarin-2-acetic acid, DMACA: blue). Notably, the derivatised antibiotics to retain similar activity and resistance profiles as the parent antibiotic, making them useful probes to investigate antibiotic-bacteria interactions.
The octapeptins are cyclic octamer lipopeptides structurally similar to the polymyxins, but which intriguingly retain activity against polymyxin-resistant Gram-negative bacteria [2]. 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 [3].
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 and daptomycin probes can rapidly distinguish between bacteria with varying levels of resistance to the parent antibiotic, potentially leading to a useful rapid assay for resistance via flow cytometry or plate-based analysis. The data to date suggests that the probes are valuable tools to aid in understanding antibiotic-bacteria interactions, supporting the fight against antimicrobial resistance.