In-vitro activity and killing effect of polycationic peptides on methicillin-resistant Staphylococcus aureus and interactions with clinically used antibiotics

The in-vitro activity of nisin, a 34-residue peptide produced by several La ctococcus lactis strains, and ranalexin, a 20-residue peptide isolated from the skin of the bullfrog Rana catesbeiana, alone and in combination with a moxycillin, amoxycillin-clavulanate, imipenem, clarithromycin, ciprofloxaci n, rifampin and vancomycin was investigated against 40 nosocomial isolates of methicillin-resistant Staphylococcus aureus (MRSA). ALI isolates were in hibited at concentrations of 1 to 32 mu g/ml. Synergy was observed when the peptides were combined with other agents, with the exception of the beta-l actams. Finally, the consecutive exposures to each peptide did not result i n selection of stable mutants with decreased susceptibility. Our finding sh ow that nisin and ranalexin are active against MRSA, and that their activit y is enhanced when they are combined with several antimicrobial agents. (C) 2000 Elsevier Science Inc. All rights reserved.