Frogs and salamanders have several innate defenses against chytrid and other pathogens found in their environment. One is the secretion of mucus from mucous and granular glands in their skin. Mucous glands help to keep amphibian skin moist and slippery, protecting them from the elements, as well as from predators (many amphibians use their “sliminess” to slip out of the clutches of predators, as anybody who has ever tried to catch a frog knows). Granular glands, on the other hand, often contain antimicrobial peptides, which are short amino acid chains able to prevent microbes from colonizing an amphibian’s skin. Granular glands can also contain toxins, as in the case of Bufonidae (toads) or Dendrobatidae (poison dart frogs). As a result, the mucus produced by amphibians usually has antimicrobial peptides mixed in with it, preventing the growth of fungi as well as pathogenic bacteria.
A second line of defense is the immune system. Like humans, frogs have an immune system containing lymphocytes, or white blood cells, that attack unwanted invaders. Studies have shown that the suppression of lymphocyte activity can result in increased susceptibility to the amphibian chytrid fungus.
Thirdly, symbiotic bacteria present on the skin of frogs may serve to outcompete other skin pathogens like the chytrid fungus. Many of these produce their own antifungal or antimicrobial peptides. The laboratory that I work in at Virginia Tech studies this particular defense mode. Bacteria can be isolated from the skin of frogs or salamanders and then assayed with Bd to determine if they can prevent the amphibian chytrid fungus in vitro, which essentially means “in the laboratory”, or outside of natural conditions. Already, studies have shown that there is potential in applying bacteria that produce metabolites able to prevent chytrid onto frogs in the wild. The bacteria Janthinobacterium lividum produces a metabolite known as violacein, that has been used to successfully inoculate Mountain Yellow-legged Frogs (Rana muscosa) against Batrachochytrium dendrobatidis in the laboratory.
With so many defenses, you might ask why amphibians still succumb to the chytrid fungus. The answer is that, as is usual in life, the relationship between frog and fungus is far more complicated than it might seem. For instance, it is thought that frogs are more susceptible to heavy infection by the chytrid fungus during metamorphosis, since their entire immune system and outer skin changes during the transformation. The ability of amphibians to defend against Bd is also affected by environmental conditions such as temperature, stress, exposure to pollutants and infection by parasites or other pathogenic organisms.
Frog: 1, Fungus: 2