Bacteria Resistant to Last-Resort Antibiotics May Also Resist Antimicrobial Peptides | Research

The takeaway

Pseudomonas aeruginosa strains resistant to polymyxin antibiotics also showed elevated resistance to the human antimicrobial peptide LL-37 and a synthetic peptide Mel4, with shared genetic mutations suggesting cross-resistance between these membrane-active agents.

Cross-resistance confirmed
65% of Pseudomonas isolates resistant to polymyxin B also showed elevated MICs to human LL-37 and synthetic Mel4 antimicrobial peptides, with nalC and mipB mutations associated with resistance to all agents

What the researchers found

Among 40 Pseudomonas aeruginosa isolates (from India and Australia):

• 65% were resistant to polymyxin B; 80% to colistin

• Polymyxin B MICs ranged from 0.5 to 512 µg/mL, with 22.5% being highly resistant (MIC ≥256 µg/mL)

• Polymyxin B and colistin MICs were strongly correlated (Spearman's R ≥0.6, p≤0.001)

• LL-37 (human cathelicidin) showed moderate correlation with polymyxin B, colistin, and Mel4

• Mel4 (synthetic AMP) showed weaker correlations with polymyxins

• SNP analysis identified nalC (E153Q/D) and mipB (V469M, G441S) variants as associated with MICs to all antimicrobials

• Strains with MICs 64-512 µg/mL were significantly more likely to harbor these variants (p<0.05)

Why it matters

Antimicrobial peptides are one of the most promising alternatives to traditional antibiotics, but if bacteria already resistant to polymyxins can also resist AMPs, this undermines the entire strategy. The finding of shared resistance mechanisms between polymyxins and both natural human AMPs (LL-37) and synthetic peptides means that the superbug problem could extend to the next generation of antimicrobial peptide drugs before they even reach the market.

How the study worked

Forty Pseudomonas aeruginosa isolates, mostly from India and Australia, were tested for MICs to polymyxin B, colistin, LL-37 (human cathelicidin), and Mel4 (synthetic AMP) using broth microdilution in cation-adjusted Mueller-Hinton broth. Whole genome sequencing was performed and analyzed using NCBI BLAST. SNP-MIC associations were evaluated with Fisher's exact test. Correlation analysis used Spearman's rank correlation.

What this study cannot tell us

Relatively small sample size (40 isolates). Isolates were predominantly from India and Australia, limiting geographic generalizability. The correlation between polymyxin and AMP resistance does not prove a causal shared mechanism. LL-37 and Mel4 are only two of many AMPs in development — other AMPs with different mechanisms may not show the same cross-resistance. Clinical relevance of in vitro MIC changes for AMPs has not been established.

How to read the evidence

This is an in vitro microbiological study combining phenotypic resistance testing with whole genome sequencing. The sample size (40 isolates) is adequate for initial observation but insufficient for definitive resistance mechanism characterization. The findings represent a significant safety signal for antimicrobial peptide development.

When this study was published

Published in 2025, this is a very current study addressing a critical concern for the antimicrobial peptide field. As AMP drug candidates advance through clinical development, cross-resistance with existing antibiotics is an increasingly important consideration.

The bigger picture

The antibiotic resistance crisis is one of the greatest threats to global health, and antimicrobial peptides have been positioned as a solution because their membrane-disrupting mechanisms were thought to be harder for bacteria to resist. This study challenges that assumption by showing cross-resistance between conventional polymyxin antibiotics and AMPs, suggesting shared membrane defense mechanisms. If bacteria can develop broad resistance to membrane-active agents, the antimicrobial peptide pipeline faces a more significant challenge than previously appreciated.

Questions still open

Can antimicrobial peptides be designed to circumvent the shared resistance mechanisms identified in this study?
Do the nalC and mipB mutations represent a general membrane defense strategy that could affect all membrane-active antimicrobials?
Should AMP drug candidates be screened against polymyxin-resistant strains early in development to identify cross-resistance risks?

Common questions

What are antimicrobial peptides and why are they important?

Antimicrobial peptides (AMPs) are small proteins that kill bacteria primarily by disrupting their cell membranes. Your body naturally produces them (like LL-37) as part of your immune defense. Scientists are also developing synthetic AMPs as new antibiotics because traditional antibiotics are losing effectiveness against superbugs. This study raises concern that some bacteria may already have defenses against both old antibiotics and these new peptide alternatives.

Why is cross-resistance between polymyxins and AMPs worrying?

Polymyxins (like colistin) are the 'last resort' antibiotics — used only when nothing else works against resistant bacteria. Antimicrobial peptides were supposed to be the next line of defense. But if the same genetic mutations make bacteria resistant to both, then superbugs that survive polymyxins may already be resistant to the peptide drugs being developed to replace them, potentially leaving us with even fewer treatment options.

Read the original research

Detection of Genes Associated with Polymyxin and Antimicrobial Peptide Resistance in Isolates of Pseudomonas aeruginosa.

International journal of molecular sciences, 26(21)

View on PubMed ↗Publisher’s paper ↗

Citation

Damtie, Meseret Alem; Vijay, Ajay Kumar; Willcox, Mark Duncan Perry. (2025). Detection of Genes Associated with Polymyxin and Antimicrobial Peptide Resistance in Isolates of Pseudomonas aeruginosa.. International journal of molecular sciences, 26(21). https://doi.org/10.3390/ijms262110499