β-Sheet Antimicrobial and Antimycobacterial Peptides with a High Therapeutic Index by Employing the Repeats of a 5-Mer Segment of α-Helical Cytolytic Melittin

Despite strong antimicrobial activity, high haemolytic/cytotoxic properties, associated with its non-cell-selective membrane-lytic property and its large size prohibits considering bee-venom, α-helical, 26-residue, melittin as a lead molecule for antimicrobial-drug development. Herein, we have explored the design of shorter non-haemolytic/non-cytotoxic antimicrobial peptides with low hydrophobic/amphipathic property and β-sheet secondary structure by using a 5-mer carboxy-terminus melittin segment, “WIKRK”, which contains hydrophobic and polar-cationic residues at its two ends. Three peptides, 10-mer, MLT-1, 15-mer, MLT-2, and 20-mer, MLT-3 were designed by taking two, three, and four repeats of “WIKRK”, respectively. MLT-2 and MLT-3 showed β-sheet secondary structure in bacterial membrane-mimetic lipid-vesicles, exhibited negligible haemolytic/cytotoxic properties in comparison to melittin, demonstrated significant antibacterial activities with high therapeutic indexes, and showed anti-mycobacterial activities against a fast/slow-growing strain. Further, MLT-2 and MLT-3 showed appreciable in vivo efficacy against Escherichia coli American Type Culture Collection 25922-infection in mice in the intravenous route and inhibited transcription/translation in in vitro assays suggesting their intracellular mode of action.