TisB toxin-derived antimicrobial peptides show potent activity against Gram-negative bacteria and synergize with azithromycin

The escalating global crisis of antimicrobial resistance (AMR) necessitates the discovery of novel anti-infective agents. Conventional antibiotics are increasingly ineffective against multidrug-resistant pathogens, leaving a critical gap in treatment options. Antimicrobial peptides (AMPs) offer a promising alternative due to their distinct mechanisms of action, often targeting bacterial membranes. This study explores toxin-antitoxin (TA) systems, specifically the TisB type-I system, as an underexplored source for developing new antibiotic candidates, aiming to address the urgent need for effective antibacterial therapies.

Researchers synthesized 44 antimicrobial peptides derived from the TisB type-I TA system. They systematically modified these peptides through sequence truncation, cationic modifications, incorporation of ultrashort PEG-like flexible linkers, Ala scanning, and partial or full incorporation of D-amino acids to establish structure-activity relationships. The peptides were tested for antibacterial activity against Gram-negative bacteria, including E. coli, K. pneumoniae, P. aeruginosa, and A. baumannii, and Gram-positive bacteria like S. aureus and E. faecalis. Primary endpoints included Minimum Inhibitory Concentrations (MICs) and hemolytic properties (at 400 μM). Synergy studies were conducted with azithromycin against Gram-negative strains.

Several TisB-derived peptides demonstrated promising antibacterial activity against Gram-negative bacteria, with MICs in the range of 2-16 μM. Their hemolytic properties were maintained at an acceptable level, showing approximately 20-30% hemolysis at 400 μM. In contrast, activity against Gram-positive bacteria, specifically S. aureus and E. faecalis, was lower, with MICs ranging from 8-64 μM. Intriguingly, synergy studies revealed a significant finding: > Low micromolar concentrations (0.25-8 μM) of essentially nonhemolytic TisB-derived peptides (exhibiting less than 10% hemolysis at 400 μM) effectively reduced the MICs of azithromycin to therapeutically relevant levels in E. coli, K. pneumoniae, and P. aeruginosa. These results highlight the potential of TA system toxins as a novel source for developing potent antibiotic candidates.