Beyond the Test Tube: How the Amphibian Microbiome is Shaping Real-World Conservation

In the damp, misty night of a Costa Rican montane forest, finding a green-eyed frog (Lithobates vibicarius) is nothing short of a miracle. During the catastrophic amphibian declines of the late 1980s, this once abundant highland endemic seemed to vanish entirely from the mountain ranges of Tilarán, Cordillera Central, and Talamanca. Like so many other amphibian species, L. vibicarius was pushed to the brink, likely by chytridiomycosis, the lethal skin disease caused by the invasive chytrid fungus Batrachochytrium dendrobatidis (Bd).

For years, many feared L. vibicarius was extinct. But years later, relict populations were persisting in isolated places of the misty mountains, reproducing and surviving despite the continued presence of Bd.

How was the green-eyed frog surviving an encounter that proved fatal to so many others?

Swabbing the skin of the green-eyed frog in the field revealed a microbial world harboring diverse bacterial strains, such as species of Pseudomonas and Acinetobacter, with the potential to suppress Bd growth (1). Yet these findings also highlighted how fragile this shield could be: environmental stressors, from local forest disturbance to airborne agricultural fungicides like chlorothalonil, disrupt this skin microbiome, suppressing or stripping away key protective bacteria (2,3).

While species like Lithobates vibicarius show us that some wild populations can eventually adapt and persist alongside Bd, many other threatened amphibians remain on the brink of extinction from chytridiomycosis. Compounding this crisis, salamander populations now face the additional, lethal threat of Bsal (Batrachochytrium salamandrivorans). Waiting for natural adaptation under the pressure of these single or combined pathogens is a risk we cannot afford to take. These species simply do not have the luxury of evolutionary time; they need an active, science-based push of support right now to survive.

Across the globe, dedicated teams of researchers are generating an impressive wealth of scientific knowledge on amphibian microbiomes (4–6). This growing body of research is providing relevant insights into host health and pathogen dynamics. Bridging the gap between sophisticated laboratory insights and impactful on-the-ground conservation action is far from easy. Yet, it represents one of the most exciting opportunities for field practitioners today: an opportunity to translate cutting-edge microbiome science into concrete strategies to save threatened species from chytridiomycosis.

The environmental "Bacterial Pharmacy"

To understand how skin microbes protect an animal, we must first ask a series of fundamental questions. These include: Where do these beneficial bacteria come from, and what determines whether they persist on amphibian skin or disappear over time?

To address this, together with Smithsonian researchers Carly Muletz-Wolz and Brian Gratwicke, we investigated salamander species across the Appalachian Mountains and examined their distinct microhabitats — from forest soils to mountain streams and ponds (7). These salamander species have been living with Bd in their environment and often infecting their species for decades, with records of Bd in the Appalachians as early as the 1960s.

What we found improved our understanding of host-microbiome interactions. The environment an amphibian inhabits is not merely a passive backdrop; it acts as a vast reservoir of protective bacteria — a natural "bacterial pharmacy" that continually complements skin defenses. In Appalachian forests and streams, a high percentage of the potential anti-fungal bacteria found on salamanders were also present in the surrounding soil and water.

Interestingly, habitat types shaped these defensive profiles. Terrestrial environments like forest soils naturally harbor a diverse community of microscopic environmental fungi. To survive on land, land-dwelling red-backed salamanders recruit a broader array of antifungal bacteria onto their skin than their aquatic counterparts, the eastern newts.

This finding delivers an important message for land managers and field biologists: protecting soil and water quality does far more than just reduce stress on animals. It safeguards the environmental reservoir of protective bacteria that amphibians rely on to replenish and reinforce their biological armor.

Host biology and microbial chance

While the surrounding environment offers a shared pool of potential microbial allies, how those microbes integrate into an animal's skin defense depends on the host's unique biology.

Across our Appalachian study sites, different species demonstrated different strategies for assembling their microbiomes:

• The Picky Selector (Two-lined Salamander): This stream-dwelling species selects its skin bacteria based on local climate and environmental parameters, having a tailored community responsive to external conditions.
• The Random Recruiter (Red-backed Salamander): Living in forest leaf litter, the red-backed salamander relies far more on chance events to determine its microbiome. Its skin microbiome shifts frequently, reflecting subtle local fluctuations in the surrounding soil.
• The Fragile Host (Eastern Newt): Aquatic newts host a bacterial community characterized by lower diversity than the ponds they swim in, making them vulnerable to random microbial shifts. When Bd overcomes their initial defenses and infects a newt, the pathogen seems to trigger dysbiosis, making the host skin surface lose its regulatory control and inducing competition among remaining microbes.

These contrasting patterns highlight why "one-size-fits-all" disease management can be failing in the field. A strategy that supports disease resistance in a forest-dwelling salamander may prove ineffective for a pond-breeding newt.

Rethinking probiotics: Generalists vs. Specialists

For more than a decade, one of the most discussed concepts in disease mitigation has been bioaugmentation — isolating "super-bacteria" in a laboratory, culturing it, and spraying it onto threatened wild amphibians as a probiotic bath (8).

However, field trials of this approach have often met with disappointing results. Our mapping of host-environment-bacterial relationships revealed a key ecological reason why this could be happening (7).

By categorizing skin microbes based on their ecological niches, we identified two potential main groups of anti-Bd allies:

• Specialist Bacteria (e.g., Pedobacter): These species adapted to live almost exclusively on amphibian skin and are rarely abundant in the surrounding soil or water.
• Generalist Bacteria (e.g., Variovorax): These "all-terrain" microbes thrive in soil, stream water, and across multiple amphibian host species with their potential ability to inhibit Bd.

This distinction could mark a paradigm shift for applied conservation. Probiotic research has been focusing on identifying strong specialist strains. But when a specialist bacterium is applied to wild animals, it lacks environmental reservoirs to replenish its population and is quickly outcompeted or washed away.

In contrast, generalist bacteria represent far stronger, more resilient candidates for field interventions. Because they persist naturally in the surrounding habitat, generalists might re-colonize host skin, maintaining long-term defensive coverage in wild populations.

Turning research into opportunities for field action

Achieving tangible conservation outcomes through microbiome science is no easy task. It demands patience, rigorous field testing, and interdisciplinary collaboration. However, the incredible work currently being generated by researchers worldwide provides us with unprecedented leverage. Here are four opportunities for field practitioners to turn this research into action:

1. Treat habitat management as microbiome management

Because amphibians continuously draw beneficial microbes from leaf litter, soil, and water, preserving intact environmental reservoirs is a direct disease intervention. Reducing soil compaction, preventing agricultural pesticide runoff (which, as seen in Costa Rica, strips away protective skin bacteria), and maintaining natural water flow supports the microbial pharmacy.

2. Prioritize generalist taxa in bioaugmentation

If a program is developing probiotic interventions, such as pre-release baths for captive-bred animals, it should focus on locally sourced, anti-fungal generalist bacterial strains. These "all-terrain" microbes can have a higher chance of establishing lasting protection once animals return to natural environments.

3. Use microbiome profiling as an early warning system

Swabbing skin microbiomes in wild populations provides a non-invasive diagnostic tool. Tracking microbial diversity and detecting signs of dysbiosis can be used to alert managers about population distress or pathogen pressure before mass mortality events occur.

4. Protect micro-refugia and habitat transitions

Since life stages and habitat transitions alter skin microbial composition — as seen in L. vibicarius tadpoles transitioning to terrestrial adults — protecting unbroken ecological corridors between aquatic breeding sites and forest habitats ensures uninterrupted microbial acquisition throughout an animal's lifecycle.

Conservation beyond the Test Tube

When Lithobates vibicarius vanished from Costa Rica's cloud forests, it felt like another tragedy in the story of amphibian extinctions. Its surprising survival alongside Bd showed us that nature has its own mechanisms of resilience, mechanisms driven in large part by invisible microbial partnerships.

However, we cannot rely on natural resilience alone. While species like L. vibicarius appear to have managed to hold their ground, more species are balancing on the brink of extinction, unable to recover without human assistance. For these species, microbiome management is not just a theoretical concept; it could represent the targeted push they need to bridge the gap between decline and recovery.

Bridging the gap between microbial research and field implementation is undeniably complex, but the momentum created by researchers around the world proves that this approach is far more than theoretical. By protecting healthy, functioning habitats, field practitioners could ensure that wild amphibians retain access to the rich environmental remedies they need to maintain their natural biological shield.

Acknowledgements

This salamanders' project was funded by a Washington Biologists' Field Club Research Award to Carly R. Muletz-Wolz, a Smithsonian Scholarly Studies grant to Carly R. Muletz-Wolz and Brian Gratwicke, and a National Science Foundation grant (NSF: IOS-2131060) to Carly R. Muletz-Wolz and Brian Gratwicke.

References

1. Jiménez, R.R., Alvarado, G., Estrella, J., & Sommer, S. 2019. Moving beyond the host: unraveling the skin microbiome of endangered Costa Rican amphibians. Frontiers in Microbiology 10: 2060. https://doi.org/10.3389/fmicb.2019.02060
2. Jiménez, R.R., Alvarado, G., Ruepert, C., Ballestero, E., & Sommer, S. 2021. The fungicide chlorothalonil changes the amphibian skin microbiome: a potential factor disrupting a host disease-protective trait. Applied Microbiology 1(1): 26-37. https://doi.org/10.3390/applmicrobiol1010004
3. Jiménez, R.R., Alvarado, G., Sandoval, J., & Sommer, S. 2020. Habitat disturbance influences the skin microbiome of a rediscovered neotropical-montane frog. BMC Microbiology 20: 292. https://doi.org/10.1186/s12866-020-01979-1
4. Rebollar, E.A., Martínez-Ugalde, E., & Orta, A.H. 2020. The amphibian skin microbiome and its protective role against chytridiomycosis. Herpetologica 76(2): 167-177. https://doi.org/10.1655/0018-0831-76.2.167
5. Jiménez, R.R., & Sommer, S. 2017. The amphibian microbiome: natural range of variation, pathogenic dysbiosis, and role in conservation. Biodiversity and Conservation 26(4): 763-786. https://doi.org/10.1007/s10531-016-1272-x
6. Wang, Z., et al. 2025. Research status and prospect of amphibian symbiotic microbiota. Animals 15(7): 934. https://doi.org/10.3390/ani15070934
7. Jiménez, R.R., Thacher, P., Osborne, O.G., Gratwicke, B., & Muletz-Wolz, C.R. 2026. Microbial metacommunity ecology: using null models and networks to understand assembly in a salamander disease system. Ecosphere 17(7): e70699. https://doi.org/10.1002/ecs2.70699
8. Miller, A.J., & Voyles, J. 2026. The impact of microbiome diversity and composition on host health and susceptibility to disease across amphibian life stages. Frontiers in Amphibian and Reptile Science 3: 1666714. https://doi.org/10.3389/famrs.2025.1666714