C-terminal amino acids of alpha-melanocyte-stimulating hormone are requisite for its antibacterial activity against Staphylococcus aureus.

Alpha-melanocyte-stimulating hormone (α-MSH) is an endogenous neuropeptide that is known for its anti-inflammatory and antipyretic activities. We recently demonstrated that α-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 α-MSH, i.e., α-MSH(6-13), α-MSH(11-13), and α-MSH(1-5), and compared them with that of the entire peptide. Our results showed that peptides containing the C-terminal region of α-MSH, namely, α-MSH(6-13) and α-MSH(11-13), efficiently killed >90% of both methicillin-sensitive and -resistant Staphylococcus aureus cells in the micromolar range and ∼50% of these cells in the nanomolar range; their efficiency was comparable to that of the entire α-MSH, whereas the peptide containing the N-terminal region, α-MSH(1-5), was found to be ineffective against S. aureus. The antimicrobial activity of α-MSH and its C-terminal fragments was not affected by the presence of NaCl or even divalent cations such as Ca2+ and Mg2+. Similar to the case for the parent peptide, α-MSH(6-13) and α-MSH(11-13) also depolarized and permeabilized Staphylococcus cells (∼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 α-MSH-based peptides. Thus, our observations indicate that C-terminal fragments of α-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 α-MSH-based therapeutics with optimal efficacy.