Short antibacterial peptides and erythromycin act synergically against Escherichia coli

Five different peptides (6-18 residues) with chain lengths shorter than the required minimum to span the bacterial cell membrane as monomeric helices were designed in order to elucidate whether variation in chain length exerted differences in their mode of action. To gain a better understanding of the possible mode of action of these peptides, they were studied in combination with clinically used antibiotics with different targets. Antibiotic-peptide combinations were tested against Escherichia coli ATCC 25922 and Staphylococcus aureus ATCC 25923. No synergy was observed between the peptides and antibiotics when tested against S. aureus. Synergic interactions between all peptides and erythromycin were observed when tested against E. coli. Synergy was also observed with rifampicin and two peptides against E. coli. There was no clear-cut correlation between the ability to interact synergically or antagonistically and the number of residues. We further investigated the combined action of our peptides and PGLa, to elucidate peptide-peptide interactions. In contrast to previously reported synergy between magainin 2 and PGLa, our peptides did not show any synergy when combined with PGLa. Thus, our results indicate an alternative mode of action of these antibacterial peptides as compared with peptides such as magainin 2.