The antimicrobial activity of the anionic peptide, AP1 (GEQGALAQFGEWL), was investigated. AP1 was found to kill Staphylocoecus aureus with an MLC of 3 mM and to induce maximal surface pressure changes of 3.8 mN m(-1) over 1200 s in monolayers formed from lipid extract of S. aureus membranes. FTIR spectroscopy showed the peptide to be alpha-helical (100%) in the presence of vesicles formed from this lipid extract and to induce increases in their fluidity (Delta v circa 0.5 cm(-1)). These combined data show that AP1 is able to function as an alpha-helical antimicrobial peptide against Gram-positive bacteria and suggest that the killing mechanism used by the peptide involves interactions with the membrane lipid headgroup region. Moreover, this killing mechanism differs strongly from that previously reported for AP1 against Gram-negative bacteria, indicating the importance of considering the effects of membrane lipid composition when investigating the structure/function relationships of antimicrobial peptides. (c) 2006 Elsevier Inc. All rights reserved.