Peptides Protect From Fungal Infection

a dark green and brown spotted frog faces forward with brown eyes standing on pebbles

Biological Strategy

African clawed frog

AskNature Team

Image: Benedikt Rauscher / Wikimedia / CC BY SA - Creative Commons Attribution + ShareAlike

Protect From Fungi

Fungi–which include molds and mushrooms, amongst others–play important roles in every ecosystem, but can also create diseases in plants and animals. Fungi can rapidly reproduce, so living systems must fend them off early to avoid their spread and potential, physical harm. Living systems use both physical and chemical strategies to protect from fungi, and must ensure that these strategies don’t harm the threatened living system itself. For example, a pumpkin’s skin has antifungal proteins to prevent fungal growth without harming the pumpkin itself.

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Protect From Microbes

In living systems, microbes play important roles, such as breaking down organic matter and maintaining personal and system health. But they also pose threats. Bacteria can be pathogens that cause diseases. Some bacteria create colonies called biofilms that can coat surfaces, reducing their effectiveness–for example, inhibiting a leaf’s ability to photosynthesize. Living systems must have strategies for protecting from microbes that cause disease or become so numerous that they create an imbalance in the system. At the same time, living systems must continue living in harmony with other microbes. Some living systems kill microbes. Others repel without killing to reduce the chances that microbes will adapt to the lethal strategy and become resistant to it. For example, some pea seedlings exude a chemical that inhibits biofilm buildup.

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Amphibians

Class Amphibia (“to live a double life”): Frogs, salamanders, toads

Amphibians live a “double life” by being able to survive both on land and in water. Unlike reptiles with dry, scaly skin, amphibians have smooth skin that needs to be kept moist. Many live in the water through development and start life with gills, and then transition to breathing with lungs. They can get even more oxygen through cutaneous respiration, or breathing through their skin. This class represents the first animal group to be able to efficiently walk on land—aided by the well-defined limbs that gave them a “leg up” on their finned competitors.

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Peptides on the skin of African clawed frog protect from fungal infection by having a semiselective binding nature to bacterial pathogen cells affording each peptide the ability to bind to a variety of pathogens.

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.