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The emergence and dissemination of multidrug resistant (MDR) bacteria are major challenges for antimicrobial chemotherapy of bacterial infections. In this critical condition, cationic antimicrobial peptides are 'novel' promising candidate antibiotics to overcome the issue. In this study, we investigated the antibacterial mechanism of new melittin-derived peptides (i.e., MDP1 and MDP2) against multidrug resistant Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa. MDP1 was designed with deletion of three amino acid residues, i.e., S, W, and I, from the end of second hydrophobic motif of melittin. In the next step, VLTTG in MDP1 sequence was substituted with tryptophan residue. MDP1 and MDP2 had a high-antibacterial activity against MDR and reference strains of S. aureus, E. coli, and P. aeruginosa. DNA and calcein release and flow cytometry assays indicate a time-dependent antibacterial activity on the examined bacteria affected by both MDP1 and MDP2. Finally, SEM analyses highlighted dose- and time-dependent effects of MDP1 and MDP2 on S. aureus and E. coli bacteria by induction of vesicle or pore formation as well as cell lysis. In this study we successfully showed that rational truncation of large hydrophobic motifs can lead to significant reduction in toxicity against human RBCs and improving the antibacterial activity as well. Analyses of data from DNA release, fluorometry, flow cytometry, and morphological assays demonstrated that the MDP1 and MDP2 altered the integrity of both Gram-positive and Gram-negative bacterial membranes and killed the bacteria via membrane damages.

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