Amphibians are constantly beset with microbial pathogens. In particular, the chytrid fungus Batrachochytrium dendrobatidis is known to infect over 350 species of frogs and has lead to widespread declines in many populations. The pathogen infects the mouths of larvae and the skin of adults leading to osmotic imbalance, salt loss, and eventually death.
The African clawed frog has evolved skin secretions that render it immune to the fungus. The toad secretes many compounds in its skin mucus including the peptides magainin-1, magainin-2, and PGLa. When exposed to the outer membrane of fungal cells, these peptides self-assemble into structures that interrupt the normal protective functioning of the membrane leading to fungal death. This strategy is also quite effective against a broad variety of microbes including many that are resistant to antibiotics. What is remarkable is that magainin and PGLa alone are relatively weak antimicrobial agents; however, when combined, they exhibit synergy which increases their antimicrobial effectiveness more than 30 fold.
"Skin secretions from many, but by no means all, species of Anura (frogs and toads) contain host-defense peptides with broad-spectrum antibacterial and antifungal activities and the ability to permeabilize mammalian cells…with few exceptions, the peptides are cationic and contain at least 50% hydrophobic amino acids. Circular dichroism and NMR studies have shown that they generally lack stable secondary structure in aqueous solutions but have the propensity to form an amphipathic α-helix in the environment of a phospholipid vesicle or in a membrane-mimetic solvent such as 50% trifluoroethanol–water. There is no single mechanism by which peptides produce cell death but their action does not involve binding to a specific receptor rather a non-specific interaction with the bacterial cell membrane that results in permeabilization and ultimate disintegration. Consequently, the frog skin peptides are usually active against microorganisms that are resistant to currently licensed antibiotics due to their markedly different and highly destructive mode of action...the magainins in the skin of African clawed frog, X. laevis...multiple peptides derived from the post-translational processing of the biosynthetic precursors of caerulein, xenopsin, and peptide glycine-leucine-amide (PGLa)." (Conlon et al. 2012:514)
"[T]he pathogenic chytrid fungus Batrachochytrium dendrobatidis has led to widespread declines in frog populations...This aquatic fungus parasitizes the mouthparts of larvae and the keratinized epidermis of post-meta- morphic amphibians producing electrolyte depletion and osmotic imbalance leading to death...The ability of an infected animal to mount a cell-mediated or humoral adaptive immune response is poor, suggesting that the skin-associated system of innate immunity may be of particular importance in host defense...Host-defense peptides can in principle provide protection for X. laevis against infection by the chytrid." (Conlon et al. 2012:516)
"[M]againin-1, magainin-2, PGLa, and CPF-2 from X. laevis inhibit the growth of B. dendrobatidis zoospores. The MIC values of the peptides tested individually are relatively high (50– 200 μM) but they act synergistically so that a mixture of magainin-2 and PGLa in a ratio of 1:1 inhibited growth at concentrations as low as 6.25 μM of each peptide." (Conlon et al. 2012:517)