Lipoproteins released by bacteria on frog skin protects frogs from fungal disease.

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Frogs live in humid environments such as tropical forests. Because of the high humidity, there is a lot of water in the air, and this helps fungi to grow more easily. Fungi can infect frogs and cause infections, similar to how humans can get fungal infections such as ‘Athlete’s Foot’. Some frog species are on the decline or have become extinct as a result of fungal infections. However, certain frog species have developed a defense against these fungi. Bacteria living on the skin of frogs of the Anuran family in Panama release compounds that protect the frogs from fungi. One specific compound, viscosin, can stop fungi growth. 

Viscosin belongs to a class of compounds called lipopeptides. Lipopeptides are compounds made by different types of bacteria that can kill other bacteria by creating holes in their cell membrane. The cell membrane protects the cell from the outside environment and only allows certain molecules to pass through. For example, the flow of potassium ions into and out of the cell is maintained by the membrane and must be very tightly controlled. If a hole is created in the membrane, the cell can lose or gain more of an ion from the environment, causing an imbalance. As a result, the cells will die. 

Scientists have recently shown that viscosin can also inhibit fungal infections. A major difference between bacteria and fungi is their cell wall. Gram-negative bacteria are a type of bacteria that make up the majority of infections. They have an outer membrane and an inner membrane. In contrast, a fungus cell has an outer wall made up of sugar proteins and chitin, and an inner membrane. Chitin is a strong material that protects the fungi cells and is much stronger compared to the bacterial membrane. Scientists think that viscosin works by breaking down chitin to disrupt fungus cell function, similar to bacteria. This has not been studied before and may lead to a new understanding of how different compounds kill fungi cells.  

Researchers have been working to discover new antifungal medicine because certain types of fungi cannot be killed using current medicines. The discovery of new compounds, such as viscosin, can help us in our fight against fungal diseases.

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References

“The families Pseudomonadaceae and Bacillaceae had the highest number of active bacteria against A. fumigatus. In particular, P. cichorii, isolated from the skin of C. crassidigitus, showed the strongest inhibition of A. fumigatus by agar diffusion assays… We report for the first time the presence of cyclic lipopeptides produced by skin-associated bacteria of Panamanian frogs. Among the cyclic lipopeptides produced by P. cichorii, we identified viscosin and massetolides (A, F, G, and H)…  we found that viscosin, at 62.50 µg/mL, inhibited the growth of A. fumigatus by 80%. Therefore, we suggest future studies on viscosin as a possible β-glucan and chitin biosynthesis inhibitor, considering that these are essential components of the cell wall of Aspergillus spp… Although viscosin is known for its antibiotic effects, it had not been previously isolated from cutaneous bacteria of amphibians nor tested for effects on B. dendrobatidis. Future studies on chitin synthesis inhibition by viscosin-like lipopeptides could be an interesting subject of research, taking into account that chitin is the major component of chytridiomycete cell walls” (Martin H et. al. 2019:7)

Journal article
Viscosin-like lipopeptides from frog skin bacteria inhibit Aspergillus fumigatus and Batrachochytrium dendrobatidis detected by imaging mass spectrometry and molecular networkingScientific ReportsFebruary 28, 2019
Christian Martin H., Roberto Ibáñez, Louis-Félix Nothias, Cristopher A. Boya P., Laura K. Reinert, Louise A. Rollins-Smith, Pieter C. Dorrestein & Marcelino Gutiérrez

“The proposed primary mode of action of LPs is pore formation in membranes, leading to an imbalance in transmembrane ion fluxes and cell death (Bender et al., 1999; Baltz, 2009).” (Raaijmakers et al. 2010: 1037)

“When tested in vitro,LPs of Pseudomonas and Bacillus species indeed exhibit lytic and growth-inhibitory activities against a broad range of microorganisms, includ- ing viruses, mycoplasmas, bacteria, fungi and oomycetes.” (Raaijmakers et al. 2010: 1044)

Journal article
Natural functions of lipopeptides from Bacillus and Pseudomonas: more than surfactants and antibiotics. FEMS Microbiology ReviewsApril 20, 2010
Jos M. Raaijmakers, Irene de Bruijn, Ole Nybroe & Marc Ongena

“b | Gram-positive bacteria have a single lipid membrane surrounded by a cell wall composed of a thick layer of peptidoglycan and lipoteichoic acid, which is anchored to the cell membrane by diacylglycerol… d | A single plasma membrane is also present in fungi, surrounded by a cell wall consisting of various layers of the polysaccharides chitin, β-glucan and mannan (in the form of mannoproteins). “Brown et al. 2015: 622)

Journal article
Through the wall: extracellular vesicles in Gram-positive bacteria, mycobacteria and fungi.Nature ReviewsSeptember 1, 2015
Lisa Brown, Julie M. Wolf, Rafael Prados-Rosales and Arturo Casadevall

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