Florida woodrat nests are laced with antibiotic-producing bacteria

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This juvenile Key Largo woodrat was captured and released in Crocodile Lake National Wildlife Refuge in Key Largo, Florida, as part of long-term monitoring efforts.

Michael Cove

Key Largo woodrats—small rodents found only in the Florida Keys—build their nests in old cars, abandoned jet skis, and tiny plastic “houses” scattered throughout some of the last remaining forest on their namesake island. The dwellings, covered in feces and urine, seemed potentially risky places to live. Now, a new study suggests the opposite: Not only are the nests free of common rodent diseases, but they are also chock-full of antibiotic-producing bacteria.

This may be the first time such bacteria have been found in wild mammal nests, says microbial ecologist Megan Thoemmes, a postdoctoral researcher at the University of California, San Diego, and lead author of a new study. That makes the 1-meter-high nests a potential new source of antibiotics, she says. “It’s a pretty incredibly unique environment.”

The study is part of a growing field of interest in antibiotic-producing bacteria found in the normal flora of healthy animals, which may help prevent infection by certain pathogens and could one day help humans stay healthy, says Barbara Rehermann, a microbiologist and infectious disease expert at the National Institutes of Health who was not involved in the study.

Starting in the late 1800s, Key Largo’s hardwood forests were slashed and burned to make way for pineapple farms. Woodrats managed to hold on in patches of remaining forest, but most of the mahogany and other slow-growing tropical hardwoods trees where woodrats make their home were decimated. Around 1980, the forests started to make a comeback on the northern end of the island. Today, a protected patch of forest of less than 1000 hectares serves as the last remaining stronghold for the island’s few thousand woodrats.

On an island with few options for trash disposal, the forested area became a dumping ground for old cars and washing machines. Then, something unexpected happened: Woodrats began to move into the trash. “You would come up to a car and it would be all corroded and rotted out inside, but the car would be full of sticks [from woodrat nests],” says co-author Michael Cove, a conservation ecologist and mammalogist at the North Carolina Museum of Natural Sciences. Conservationists soon took note and began to provide them with garbage in areas lacking natural materials. They eventually built more than 1000 homes made out of rocks and plastic culvert pipes.

To find out whether these humanmade nests were creating an environment ripe for pathogens, Cove and Thoemmes swabbed 10 of the supplemental nests to see which bacterial species were growing. For comparison, they also sampled 10 “natural” nests made of sticks and vegetation, nine forest floors near the natural nests, and the skin of nine woodrats. Next, they sequenced the bacteria’s DNA to identify what was there.

Surprisingly, the researchers found no bacterial diseases in any of the nests—including common rodent pathogens like plague and leptospirosis, they report today in Ecosphere. They also found a low prevalence of disease on the woodrats themselves. Instead, both the artificial and natural nests were rich in Pseudonocardiaceae and Streptomycetaceae, bacterial families that produce many common antibiotics, including erythromycin.

The researchers don’t yet know whether the presence of antibiotic-producing bacteria in the woodrat nests is what keeps pathogens out. But they say the nests’ rich bacterial diversity and abundance are likely contributing factors.

The study shows how important it is to know how conservation efforts affect the diversity of animal and environmental microbiomes, Rehermann says. Luckily for the woodrats, she adds, the research suggests the artificial nests did not disrupt normal bacterial composition. Some bacteria that live with humans also help produce antibiotics to keep out pathogenic microbes, she adds. Those antibiotics work “with greater specificity and less interruption of the normal [bacterial species] than broadband antibiotics,” she says. Understanding how this happens in other mammals could one day help researchers boost humans’ own pathogen-fighting bacteria.

Cove hopes the new work will increase interest in conserving the Key Largo woodrat and other species across the Southeast, which could also harbor antibiotics, he says. Cove and Thoemmes hope to sample woodrat nests across more species in search of antibiotics. “We should be doing this at a broader scale to learn about mammals all over, especially rodents,” he says. The researchers will also investigate how woodrats go about packing their nests with antibiotics. Do they ooze the bacteria from their skin? Does it travel on vegetation gathered for the nests? Only time and more swabs will tell, Cove says.

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