C-Terminal Amino Acids of Alpha-Melanocyte-Stimulating Hormone Are Requisite for Its Antibacterial Activity against Staphylococcus aureus

Alpha-melanocyte-stimulating hormone (alpha-MSH) is an endogenous neuropeptide that is known for its anti-inflammatory and antipyretic activities. We recently demonstrated that alpha-MSH possesses staphylocidal activity and causes bacterial membrane damage. To understand the role of its amino acid sequences in the staphylocidal mechanism, in the present study we investigated the antimicrobial activities of different fragments of alpha-MSH, i.e., alpha-MSH(6-13), alpha-MSH(11-13), and alpha-MSH(1-5), and compared them with that of the entire peptide. Our results showed that peptides containing the C-terminal region of alpha-MSH, namely, alpha-MSH(6-13) and alpha-MSH(11-13), efficiently killed > 90% of both methicillin-sensitive and -resistant Staphylococcus aureus cells in the micromolar range and similar to 50% of these cells in the nanomolar range; their efficiency was comparable to that of the entire alpha-MSH, whereas the peptide containing the N-terminal region, alpha-MSH(1-5), was found to be ineffective against S. aureus. The antimicrobial activity of alpha-MSH and its C-terminal fragments was not affected by the presence of NaCl or even divalent cations such as Ca(2+) and Mg(2+). Similar to the case for the parent peptide, alpha-MSH(6-13) and alpha-MSH(11-13) also depolarized and permeabilized Staphylococcus cells (similar to 70 to 80% of the cells were depolarized and lysed after 2 h of peptide exposure at micromolar concentrations). Furthermore, scanning and transmission electron microscopy showed remarkable morphological and ultrastructural changes on S. aureus cell surface due to exposure to alpha-MSH-based peptides. Thus, our observations indicate that C-terminal fragments of alpha-MSH retain the antimicrobial activity of entire peptide and that their mechanism of action is similar to that of full-length peptide. These observations are important and are critical in the rational design of alpha-MSH-based therapeutics with optimal efficacy.