Antimicrobial Peptides Based on the TisB Toxin: Toward Enhanced Activity and Synergy with Antibiotics

Antimicrobial resistance is a growing global health crisis, necessitating novel anti-infective agents. The present study explores the potential of toxin-antitoxin (TA) systems as a source of antibiotic candidates. Thus, 44 antimicrobial peptides derived from the TisB type-I TA system were employed to establish initial structure-activity relationships via sequence truncation, cationic modifications, and incorporation of ultrashort PEG-like flexible linkers as well as Ala scanning and partial or full incorporation of D-amino acids. This led to the identification of several hits with promising antibacterial activity (MICs in the range 2-16 μM) against the Gram-negative E. coli, K. pneumoniae, P. aeruginosa, and A. baumannii, while their hemolytic properties were kept at an acceptable level (i.e., approximately 20-30% hemolysis at 400 μM). In contrast, the activity in the Gram-positive bacteria S. aureus and E. faecalis was lower (MICs in the range 8-64 μM). Intriguingly, synergy studies revealed that low micromolar concentrations (0.25-8 μM) of essentially nonhemolytic TisB-derived peptides (i.e., less than 10% hemolysis at 400 μM) could reduce the minimum inhibitory concentrations of azithromycin to therapeutically relevant levels in E. coli, K. pneumoniae, and P. aeruginosa. These findings highlight the potential of the TA system toxins as a source of antibiotic candidates.