The bactericidal activity of magainins against Pseudomonas aeruginosa and Enterococcus faecium

The antimicrobial activities of two synthetic magainins, MSI 94 and MSI 93, were investigated in vitro against 20 clinical isolates each of Pseudomonas aeruginosa and Enterococcus faecium. The ranges of the MICs of both agents were 6.25-50 mg/L for P. aeruginosa and 3.13-12.5 mg/L for E. faecium. In time-kill studies, the magainins demonstrated rapid, concentration-dependent bactericidal activity against selected isolates of both species. This bactericidal effect was inoculum-dependent for P. aeruginosa in both the logarithmic and stationary phases of growth. Bacterial regrowth was regularly observed after 24 h of incubation in the time-kill studies; this was due to loss of antimicrobial activity during overnight incubation rather than to the development of resistance. More rapid and sustained bactericidal activity was noted when the magainins were combined with either silver nitrate (AgNO3) or gentamicin against P. aeruginosa and with gentamicin against E. faecium. Neither agent produced a post-antibiotic effect on P. aeruginosa.