Meseret Alem Damtie, Ajay Kumar Vijay, Mark Willcox
Pseudomonas aeruginosa causes ocular and other infections and quickly acquires antimicrobial resistance. Polymyxin B and colistin are last-line agents against resistant P. aeruginosa, yet even resistance to these is increasing. Antimicrobial peptides (AMPs) are also being developed as new antibiotics, but resistant mechanisms to polymyxins might also cause resistance to these AMPs. This study evaluated whether isolates with differing polymyxin resistances also showed elevated minimum inhibitory concentrations (MICs) to the human cathelicidin LL-37 and a synthetic AMP, Mel4. Forty isolates of P. aeruginosa, mostly collected in India and Australia, were assessed for minimum inhibitory concentrations (MICs) by broth microdilution in cation-adjusted Mueller–Hinton broth. Whole genome sequences were analyzed using NCBI BLAST (version 2.17.0). SNPs vs. MIC associations were evaluated with Fisher’s exact test. Sixty-five percent of isolates were resistant to polymyxin B, and 80% to colistin. Polymyxin B MICs ranged from 0.5 to 512 µg/mL, with 32.5% showing intermediate resistance and 22.5% being highly resistant (MIC ≥ 256 µg/mL). MICs for polymyxin B and colistin were strongly correlated with each other (Spearman’s R ≥ 0.6; n = 40; p ≤ 0.001). LL-37 showed moderate correlations with polymyxin B, colistin, and Mel4, whereas Mel4 showed weaker correlations with polymyxin B or colistin (R < 0.4). Genomic analysis identified SNPs in mipB (V469M, G441S) as being associated with the MICs to all the antimicrobials. Strains with MICs between 64 and 512 µg/mL were significantly more likely to harbor nalC (E153Q/D) or the mipB variants (p < 0.05). Higher polymyxin MICs were associated with elevated MICs to LL-37 and, to a lesser extent, Mel4, suggesting partial shared resistance among membrane active peptides. Defining the effect of the SNPs and clinical relevance of AMP cross-resistance may inform future therapies and safer contact lenses.