This Frog Outsourced Its Defense Against a Species-Killing Fungus

A small Brazilian frog, Haddadus binotatus, shrugs off a fungus that has driven at least 90 frog species extinct. Strip away the bacteria living on its skin, and it stops shrugging.

In lab tests, frogs whose skin bacteria were suppressed with antibiotics became more likely to develop signs of disease. The bacteria aren't incidental passengers. They are, functionally, a second immune system.

The chytrid fungus Batrachochytrium dendrobatidis, or Bd, damages amphibian skin and eventually stops the heart. Researchers had long noticed that H. binotatus rarely got infected in the wild. A Penn State-led team working in Brazil has now tested why, rather than just observing it. The results appear in the journal Animal Microbiome.

The mechanism is chemical. Lead author Laura Schuck cultured roughly 700 bacterial strains from frog skin, exposed Bd to each one's chemical byproducts, tracked fungal growth over a week, then sequenced each culture and built a reference database linking bacterial identity to how strongly it inhibited the fungus. What mattered wasn't how many kinds of bacteria a frog carried. It was whether the right ones were there.

The second species, Ischnocnema henselii, got sick at high rates whether its bacteria were intact or wiped out. Why one frog assembles a protective crew and the other doesn't is still an open question, and it's the one the team plans to chase next.

The bacterial armor is real. The instruction manual for building it has not been found.

Read the full story at Popular Science, September 24, 2026

Hot Take: Strictly speaking, the frog isn't fighting the fungus at all. Its tenants are, which makes H. binotatus less a survivor than a landlord with exceptionally good tenants.