PGYa and PGAa are synthetic, amphipathic, alpha-helical peptides that were designed using a novel "sequence template" approach. Their antimicrobial activity was tested against several pathogenic clinical isolates, most of which were multiply resistant to conventional antibiotics. PGYa appeared to be more active towards Gram-positive species (MIC = 0.5-4 microM), towards such Gram negative species as P. aeruginosa, X. maltophilia, E. coli, K. pneumoniae and S. enteritidis (MIC = 1.4 microM), and towards the filamentous fungus A. niger (MIC = 8 microM). Conversely, PGAa showed the greater activity towards three Candida species (MIC = 2.16 microM). The peptides were shown to have a bactericidal activity, resulting in a decrease of viability for both Gram-positive and -negative bacteria of 3-6 logs within 60 min. Scanning electron microscopy of S. aureus and E. coli treated with PGYa shows considerable roughening and blebbing of the bacterial surfaces providing conclusive evidence that the peptide is membrane active.