Potent, synergistic inhibition of Staphylococcus aureus upon exposure to a combination of the endopeptidase lysostaphin and the cationic peptide ranalexin

Objectives: Successful treatment of infections involving multiply drug-resistant methicillin-resistant Staphylococcus aureus (MRSA) is becoming increasingly difficult. In this work, we have investigated the potential of combining lysostaphin with cationic antimicrobial peptides to effectively inhibit Staphylococcus aureus. Methods: S. aureus strains were grown in 96-well plates in the presence of increasing concentrations of lysostaphin and the peptides ranalexin, dermaseptin S3(1-16) or magainin 2. Growth was determined visually after 48 h and the plates imaged, or by automated optical density readings in a plate reader. Susceptibility to the combination of lysostaphin and ranalexin was also determined by viable cell counts. The efficacy of combined lysostaphin and ranalexin on a solid surface was tested via disc diffusion assays. Results and conclusions: Combination of lysostaphin with ranalexin resulted in potent, synergistic inhibition of S. aureus MSSA476 and MRSA252. Synergistic inhibition was specific for lysostaphin-susceptible staphylococci and was not observed with clinical isolates of the Gram-negative Escherichia coli, or other Gram-positive organisms, such as Enterococcus faecalis. Lysostaphin was not specifically synergistic with ranalexin alone. Synergy was also observed with two other cationic antimicrobial peptides, magainin 2 and dermaseptin s3(1-16); although combination with ranalexin was most potent. Synergistic inhibition by ranalexin in combination with lysostaphin resulted in an enhanced bactericidal effect. Importantly, synergy between lysostaphin and ranalexin was also observed after impregnation and drying in filter paper discs that clearly inhibited growth of S. aureus on the surface of agar; a solid, porous matrix. Thus, the combination could represent a novel route to target wounds infected with drug-resistant MRSA via dressings impregnated with the two compounds.