Rational Design of Antimicrobial Peptides Based on Bacterial Type‑I Toxins AapA1, IbsC, and Fst1

The antibacterial properties of 43 modified peptide toxins derived from three distinct type-I toxin-antitoxin (TA) systems, comprising aapA1-isoA1, fst1-rnaII, and ibsC-sibC, were studied. Modifications of truncated toxins included an adjustment of overall charge and hydrophobicity. In the AapA1 and Fst1 series, the effects of altered charge per residue (via insertion of cationic blocks at the termini and/or within the sequences) were examined. In the IbsC series, an Arg₄ block was also introduced to study amphipathicity, whereas fatty acyl moieties of varying length were incorporated to assess the influence of hydrophobicity on cell selectivity. Several peptides in the AapA1 series demonstrated moderate to high antibacterial activity (inhibition at 1-8 μM), while a few peptides in the Fst1 series were almost as potent. The best peptides in the IbsC series exerted antibacterial activity at 1-8 μM, but unexpectedly, introduction of N-terminal fatty acyl moieties conferred reduced potency. Analogues with an Arg-rich motif possessed more potent antibacterial activity than the corresponding Lys-containing analogues. Generally, antibacterial activity was absent when hydrophobicity was below a critical threshold, and for most peptides within each subset, higher hydrophobicity conferred increased hemolytic properties. Selected peptides underwent further studies, including a comparison with all-D versions and analogues displaying polar substitutions. Intriguingly, several low-hemolytic peptides exhibited potent synergistic interactions when applied in combination with rifampicin, azithromycin, or oritavancin.