#FEMSmicroBlog: How bacteria can save a frog’s skin

02-06-2022

Bacteria living in and on animals have many important and beneficial functions, such as shielding their host from infection through ‘colonization resistance’. The study “An experimental test of disease resistance function in the skin-associated bacterial communities of three tropical amphibian species” in FEMS Microbiology Ecology showed that specific bacteria living on the skin of frogs can help to protect them from a deadly fungus. Vilhelmiina Haavisto explains for the #FEMSmicroBlog how experimental studies like this one can help us learn more about the microbes living alongside us. #FascinatingMicrobes

 

Bacteria and fungi fighting on a frog’s skin

One of the many ways by which microorganisms serve multicellular beings (like ourselves) is by providing colonization resistance, whereby communities living in and on animal hosts protect from infection by invading pathogens.

One of the benefits of understanding exactly how microorganisms accomplish this feat is to treat infections by targeting the microbiota. A factor contributing to its efficiency is the composition of the microbiota, and whether it contains species that can directly or indirectly shut out pathogens.

The study “An experimental test of disease resistance function in the skin-associated bacterial communities of three tropical amphibian species” in FEMS Microbiology Ecology sought to understand how the skin microbiota of frogs is involved in determining their susceptibility to infection by the fungus Batrachochytrium dendrobatidis, or Bd.

Sometimes termed the ‘amphibian plague’, this pathogen causes a potentially fatal skin condition called chytridiomycosis and is a major player in the ongoing worldwide decline of amphibians.

A scanning electron micrograph of the fungus Batrachochytrium dendrobatidis (Bd), sometimes referred to as the ‘amphibian plague’ | Picture by Alex Hyatt, CSIRO (CC BY 3.0)

 

It’s all about the ‘right’ microbes

The presence (or absence) of certain microbes can influence how effectively the microbiota protects its host from Bd. Bacteria that can inhibit the growth of Bd have been isolated from many species of amphibians. The study investigated how the frog skin microbiota changes upon exposure to Bd, and whether specific bacteria are linked to disease resistance or susceptibility.

The study focused on three species of tropical frogs (Agalychnis callidryas, Craugastor fitzingeri and Dendropsophus ebraccatus) from Soberanía National Park in Panama, where Bd is endemic. Samples of the skin microbiota before and after exposure of the frogs to Bd in the laboratory was throughout a Bd infection cycle in the laboratory, and abundances of different bacteria (inferred from 16S rRNA gene sequencing), indicated severity of the infection.

One of the frogs used in the study by Hughey et al. in FEMS Microbiology Ecology, Craugastor fitzingeri, is vulnerable to infection by the the fungus Batrachochytrium dendrobatidis (Bd) | Picture by Brian Gratwicke (CC BY 2.0)

Although the diversity or structure of the skin microbiota did not change following Bd infection in any of the frog species, presence of some bacteria can predict a host’s response. For example, C. fitzingeri frogs with higher abundances of Kaistia bacteria in their skin microbiota tended to experience lower infection rates.

Conversely, C. fitzingeri without Pseudomonas bacteria seemed to have higher infection rates. Pseudomonas was previously classified as Bd-inhibitory, together with two others from the other frog species.

 

Colonization resistance to fight pathogens

The study also provided evidence that communities that have previously been challenged by Bd are less likely to be disturbed by re-exposure. As Bd and the tropical frogs studied are sympatric (i.e. endemic in the same geographic area), their bacterial communities may already be primed to resist the pathogen.

This would explain why the study did not show changes in the skin microbiota following infection. However, this may also have been due to the laboratory conditions in which the experiments were conducted, which differ from those the frogs experience in their native rainforest.

Dendropsophus ebraccatus is one of the frogs used in the study by Hughey et al. in FEMS Microbiology Ecology | Picture by Brian Gratwicke (CC BY 2.0)

Host-associated microbial communities are notoriously complicated to study because of their complexity, and the sheer number of factors influencing their composition and function in their natural habitats. Experimental approaches such as the one employed in the study can help describe how host-associated microbial communities confer their many benefits.

Similar results have been reported in organisms from bats to corals and humans, showcasing the stunning diversity of microbes living in and on us.

 

About the authors of this blog

Vilhelmiina Haavisto is a MSc student in Microbiology & Immunology at ETH Zurich (Switzerland), where she works with freshwater microbial communities. She is also a freelance science writer focusing largely on microbiological topics.

 

About this blog section

The section #FascinatingMicrobes for the #FEMSmicroBlog explains the science behind a paper and highlights the significance and broader context of a recent finding. One of the main goals is to share the fascinating spectrum of microbes across all fields of microbiology.

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