Abstract by Johan Storm Jørgensen

The escalating challenge of antibiotic resistance underscores the urgent need for novel antimicrobial agents. Gram-negative pathogens, in particular, pose a serious threat, as the discovery of new antibiotic classes against these has stagnated for over four decades. Antimicrobial peptides (AMPs) represent a promising avenue, given their relatively low propensity for resistance development. Among AMPs, polymyxins have gained attention, with colistin and polymyxin B currently serving as last-resort antibiotics against multidrug-resistant Gram-negative pathogens.

The present study focuses on design and synthesis of a series of polymyxin analogs aimed to enhance antimicrobial activity against colistin-resistant pathogens while maintaining efficacy against colistin-susceptible strains. These analogs were evaluated against a panel comprising Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Acinetobacter baumannii, including colistin-resistant isolates of E. coli, P. aeruginosa, and A. baumannii.

Several compounds, notably 2h, 5d, and 5e, exhibited exceptional antimicrobial activity against both colistin-susceptible and -resistant Gram-negative pathogens. Hydrophobicity emerged as a critical factor influencing antimicrobial activity, particularly against colistin-resistant strains. Importantly, all compounds demonstrated non-hemolytic properties, and hit compounds were also non-cytotoxic at more than 100 times their highest measured Minimal inhibitory concentrations against the entire test panel. Encouraged by these promising findings, compounds 2h, 5d, and 5e were further investigated for their potential to eradicate colistin-resistant P. aeruginosa biofilms, particularly those isolated from cystic fibrosis patients. Compound 5d outperformed colistin in eradicating both colistin-resistant and -susceptible biofilms. Moreover, 5d and colistin demonstrated synergistic interactions with rifampicin in biofilm eradication. To elucidate the impact of structural modifications on membrane disruptive ability, we compared the analogs with colistin. Compounds 2h, 5d, and 5e exhibited membrane disruption similar to colistin, albeit to a greater extent. Additionally, membrane-disruptive ability of oncocins modified with bacteria-penetrating motifs was explored, which revealed no discernible impact on bacterial membranes.

In summary, this research provides valuable insights into the structural determinants influencing the activity of polymyxins against colistin-resistant Gram-negative pathogens. This insight enabled the development and characterization of highly efficient hit compounds, capable of eradicating both planktonic bacteria and biofilm-associated bacteria. These findings are significant for the development of novel antimicrobial agents.