Some tropical frogs may be developing resistance to a deadly fungal disease – but now salamanders are at risk

Panamanian golden frogs (Atelopus zeteki) are listed as critically endangered, and may be extinct in the wild.
Jeff Kubina, CC BY-SA

Louise Rollins-Smith, Vanderbilt University

My office is filled with colorful images of frogs, toads and salamanders from around the world, some of which I have collected over 40 years as an immunologist and microbiologist, studying amphibian immunity and diseases. These jewels of nature are mostly silent working members of many aquatic ecosystems.

The exception to the silence is when male frogs and toads call to entice females to mate. These noisy creatures are often wonderful little ventriloquists. They can be calling barely inches from your nose, and yet blend so completely into the environment that they are unseen. I have seen tropical frogs in Panama and native frogs of Tennessee perform this trick, seemingly mocking my attempts to capture them.

My current research is focused on interactions between amphibians and two novel chytrid pathogens that are linked to global amphibian declines. One, Batrachochytrium dendrobatidis ( abbreviated as Bd), has caused mass frog dieoffs around the world. Recently my lab group contributed to a study showing that some species of amphibians in Panama that had declined due to Bd infections are recovering. Although the pathogen has not changed, these species appear to have developed better skin defenses than members of the same species had when Bd first appeared.

This is very good news, but those who love amphibians need to remain vigilant and continue to monitor these recovering populations. A second reason for concern is the discovery of a closely related chytrid, Batrachochytrium salamandrivorans (Bsal), which seems to be more harmful to salamanders and newts.

Amphibian chytrid fungus has been detected in at least 52 countries and 516 species worldwide.
USDA Forest Service

Global frog decline

More than a decade ago, an epidemic of a deadly disease called chytridiomycosis swept through amphibian populations in Panama. The infection was caused by a chytrid fungus, Batrachochytrium dendrobatidis. Scientists from a number of universities, working with the Smithsonian Tropical Research Institute in Panama, reported that chytridiomycosis was moving predictably from west to east from Costa Rica across Panama toward Colombia.

I was part of an international group of scientists, funded by the National Science Foundation, who were trying to understand the disease and whether amphibians had effective immune defenses against the fungus. Two members of my lab group traveled to Panama yearly from 2004 through 2008, and were able to look at skin secretions from multiple frog species before and after the epidemic of chytridiomycosis hit.

Many amphibians have granular glands in their skin that synthesize and sequester antimicrobial peptides (AMPs) and other defensive molecules. When the animal is alarmed or injured, the defensive molecules are released to cleanse and protect the skin.

Through mechanisms that remain a mystery, we observed that these skin defenses seemed to improve after the pathogen entered the amphibian communities. Still, many frog populations in this area suffered severe declines. A global assessment published in 2004 showed that 43 percent of amphibian species were declining and 32 percent of species were threatened.

In Panama, Smithsonian scientists operate the largest amphibian conservation facility of its kind in the world.

Signs of resistance

In 2012-2013, my colleagues ventured to some of the same sites in Panama at which amphibians had disappeared. To our great delight, some of the species were partially recovering, at least enough so that they could be found and sampled again.

We wanted to know whether this was happening because the pathogen had become less virulent, or for some other reason, including the possibility that the frogs were developing more effective responses. To find out, we analyzed multiple measures of Bd‘s virulence, including its ability to infect frogs that had never been exposed to it; its rate of growth in culture; whether it had undergone genetic changes that would show loss of some possible virulence characteristics; and its ability to inhibit frogs’ immune cells.

As our group recently reported, we found that the pathogen had not changed. However, we were able to show that for some species, frog skin secretions we collected from frogs in populations that had persisted were better able to inhibit the fungus in a culture system than those from frogs that had never been exposed to the fungus.

The prospect that some frog species in some places in Panama are recovering in spite of the continuing presence of this virulent pathogen is fantastic news, but it is too soon to celebrate. The recovery process is very slow, and scientists need to continue monitoring the frogs and learn more about their immune defenses. Protecting their habitat, which is threatened by deforestation and water pollution, will also be a key factor for the long-term survival of these unique amphibian species in Panama.

If Bsal fungus spreads to North America, it could wipe out species like this Northern Slimy Salamander (Plethodon glutinosus).
Marshal Hedin, CC BY

Salamanders (and frogs) at risk

On a global scale, Bd is not the only threat. A second pathogenic chytrid fungus called Batrachochytrium salamandrivorans (abbreviated as Bsal) was recently identified in Europe, and has decimated some salamander populations in the Netherlands and Belgium. This sister species probably was accidentally imported into Europe from Asia, and seems to be a greater threat to salamanders than to frogs or toads.

Bsal has not yet been detected in North America. I am part of a new consortium of scientists that has formed a Bsal task force to study whether it could become invasive here, and which species might be most adversely affected.

In January 2016 the U.S. Fish and Wildlife Service listed 201 salamander species as potentially injurious to wildlife because of their their potential to introduce Bsal into the United States. This step made it illegal to import or ship any of these species between the continental United States, the District of Columbia, Hawaii, the Commonwealth of Puerto Rico or any possession of the United States.

The Bsal task force is currently developing a strategic plan that lists the most urgent research needs to prevent accidental introduction and monitor vulnerable populations. In October 2017 a group of scientists and conservation organizations urged the U.S. government to suspend all imports of frogs and salamanders to the United States.

In short, it is too early to relax. There also are many other potential stressors of amphibian populations including climate change, decreasing habitats and disease. Those of us who cherish amphibian diversity will continue to worry for some time to come.

Louise Rollins-Smith, Associate Professor of Pathology, Microbiology and Immunology, Vanderbilt University

This article was originally published on The Conversation. Read the original article.

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New research suggests common herbicides are linked to antibiotic resistance

New Zealand researchers have found that the active ingredients in commonly-used weed killers like Round-up and Kamba can cause bacteria to become less susceptible to antibiotics.
from http://www.shutterstock.com, CC BY-ND

Jack Heinemann

Antibiotics are losing their ability to kill bacteria.

One of the main reasons for the rise in antibiotic resistance is the improper use of antibiotics, but our latest research shows that the ingredients in commonly-used weed killers like Round-up and Kamba can also cause bacteria to become less susceptible to antibiotics.

Herbicides induce gene activity

Already, about 700,000 deaths are attributable each year to infections by drug-resistant bacteria. A recent report projected that by 2050, 10 million people a year will die from previously treatable bacterial infections, with a cumulative cost to the world economy of $US100 trillion.

The bacteria we study are potential human pathogens. Seventy years ago pathogens were uniformly susceptible to antibiotics used in medicine and agriculture. That has changed. Now some are resistant to all but one or two remaining antibiotics. Some strains are resistant to all.

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Read more: Drug resistance: how we keep track of whether antibiotics are being used responsibly

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When bacteria were exposed to commercial herbicide formulations based
on 2,4-D, dicamba or glyphosate, the lethal concentration of various antibiotics
changed. Often it took more antibiotic to kill them, but sometimes it took less.
We showed that one effect of the herbicides was to induce certain genes that they all carry, but don’t always use.

These genes are part of the so-called “adaptive response”. The main elements of this response are proteins that “pump” toxins out of the cell, keeping intracellular concentrations sublethal. We knew this because the addition of a chemical inhibitor of the pumps eliminated the protective effect of the herbicide.

In our latest work, we tested this by using gene “knockout” bacteria, which had been engineered to lose just one pump gene. We found that most of the effect of the herbicide was explained by these pumps.

Reduced antibiotic use may not fix the problem

For decades we have put our faith in inventing new antibiotics above the wisdom
of preserving the effectiveness of existing ones. We have applied the same invention incentives to the commercialisation of antibiotics as those used with mobile phones. Those incentives maximise the rate of product sales. They have saturated the market with phones, and they saturate the earth with antibiotic resistant bacteria.

Improper use of antibiotics is a powerful driver of the widespread resistance.
Knowing this naturally leads to the hypothesis that proper and lower use will make the world right again. Unfortunately, the science is not fully on the side of that hypothesis.

Studies following rates of resistance do generally find a decrease in resistance to specific drugs when their use is banned or decreased. However, the effect is not a restoration of a pre-antibiotic susceptibility, characterised by multi-year effectiveness of the antibiotic. Instead, resistance returns rapidly when the drug is used again.

This tells us that once resistance has stablised in populations of bacteria, suspended use may change the ratio of resistant to susceptible but it does not eliminate resistant types. Very small numbers of resistant bacteria can undermine the antibiotic when it is used again.

Herbicides and other pollutants mimic antibiotics

What keeps these resistant minorities around? Recall that bacteria are very
small, but there are lots of them; you carry 100 trillion of them. They are also found deep underground to high up in the atmosphere.

Because antibiotics are so powerful, they eliminate bacteria that are susceptible and leave the few resistant ones to repopulate. Having done so, we now have lots of bacteria, and lots of resistance genes, to get rid of, and that takes a lot of time.

As our work suggests, the story is even more complicated. We are inclined to think of antibiotics as medicine and agrichemicals, hand soaps, bug sprays and preservatives as different. Bacteria don’t do this. To them, they are all toxic.

Some are really toxic (antibiotics) and some not so much (herbicides). Bacteria are among the longest lived organisms on earth. Nearly four billion years of survival has taught them how to deal with toxins.

Pesticides as antibiotic vaccines

Our hypothesis is that herbicides immunise the bacteria from more toxic
toxins like antibiotics. Since all bacteria have these protections, the use of widely used products to which they are exposed is particularly problematic. So these products, among others, might keep bacteria ready for antibiotics whether or not we are using them.

We found that both the purified active ingredients and potential inert ingredients in weed killers caused a change in antibiotic response. Those inert ingredients are also found in processed foods and common household products. Resistance was caused below legally allowed food concentrations.

What does this all mean? Well for starters we may have to think more carefully about how to regulate chemical commerce. With approximately eight million manufactured chemicals in commerce, 140,000 new since 1950, and limited knowledge of their combination effects and breakdown products, this won’t be easy.

But neither is it easy to watch someone die from an infection we lost the power to cure.

Jack Heinemann, Professor of Molecular Biology and Genetics

This article was originally published on The Conversation. Read the original article.