Animal studies are ongoing to determine the in vivo activity of a new class of antimicrobial peptides from the skin of the African frog Xenopus laevis, the magainins. This family of peptides consists of two closely related peptides, each of which is 23 amino acids long, that inhibit the growth of numerous species of bacteria and fungi in vitro. An animal model of experimental bacterial keratitis induced in adult New Zealand white rabbits was used to determine the in vivo relevance of the antimicrobial activity of magainins. Pseudomonas aeruginosa was primarily considered because it causes the most destructive and the most difficult to treat corneal infection in humans. Topical treatment with magainin drops was started 1 hour after inoculation of the organisms, and control animals were treated with either vehicle or gentamicin. These studies have shown that magainin by itself was inferior to gentamicin in terms of its minimum inhibitory concentration, and in the resolution of chemosis, congestion, and keratitis induced by the Pseudomonas organism. On the other hand, the erythromycin is proving to be efficacious in terms of reducing the amount of keratitis induced by P. aeruginosa, as well as in improving its minimum inhibitory concentration. Pseudomonas is a gram-negative organism, thus erythromycin alone is not effective. It is postulated that erythromycin opens membrane channels large enough for the magainin molecule to squeeze through. The synergistic effect of magainin and erythromycin looks promising and is worthy of further study.
