Discovery of a potent ornithine-modified gramicidin S analogue against drug-resistant Staphylococcus aureus and Enterococcus faecalis with minimal red blood cell toxicity

Abstract

The high haemolytic toxicity of Gramicidin S restricts its therapeutic use to topical applications. Given the growing need for new antibiotics and drawing inspiration from the cyclic structure and druggable characteristics of Gramicidin S, we have synthesized 15 ornithine (Orn) modified analogous peptides systematically and investigated their antimicrobial activity and cytotoxicity. Results revealed that mono- ornithine residue replaced with tryptophan (11) and arginine (12) peptides showed improved activity against multidrug resistant bacterial strains of Staphylococcus aureus and Enterococcus faecalis (MIC 4–8 μg/mL) compared with comparators vancomycin (MIC >64 μg/mL), levofloxacin (MIC 32–64 μg/mL) and meropenem (MIC 8–64 μg/mL). Cytotoxicity data demonstrated that peptide 11 (HC₅₀ = 112.1 μg/mL) and 12 (HC₅₀ = 186 μg/mL) exhibited greatly reduced haemolytic activity, as compared with Gramicidin S (HC₅₀ = 35.13 μg/mL). The concentration-dependent time-kill kinetic assay resulted the active peptide 12 represents better bactericidal effect compared with 11 and vancomycin. Scanning electron microscope analysis shows that GS and the modified peptide 12 disrupt the bacterial cell surface, causing damage and leading to bacterial cell death. 2D NOESY data of 12 showed that the arginine residue side-chain guanidinium ion and tryptophan indole form a cation-π interaction. This interaction between arginine and tryptophan stabilizes the β-sheet conformation, selectively targets bacterial membranes, hence exhibiting reduced red blood cell toxicity. The overall study suggests that the peptide 12 may be further developed as an antibiotic for systematic use against infections caused due to S. aureus.