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Larval nematode captured by sticky fungal nets, by George Barron

When Threatened By Worms, Bacteria Summon Killer Fungi

When you’re the size of a human, you worry about lions and tigers and bears. But if you’re a bacterium, a tiny nematode worm, just a millimetre long, can be a vicious predator. Nematodes are among the most common animals on the planet, and many of them hunt bacteria in soil and water. The microbes, in turn, have evolved many defences. Some secrete toxins. Others gather in large, invulnerable swarms*.

Now, a team of Chinese scientists have discovered the most outlandish strategy yet: some bacteria transform fungi into worm-killers.

Fungi aren’t known for their speed or mobility, but around 200 species have evolved ways of killing nematodes nonetheless. They use traps, including sticky nets and microscopic lassos made of single coiled cells. Once they ensnare a worm, they grow into it and digest it from the inside out.

These fungi aren’t always killers. One of the most common and best-studied species—Arthrobotrys oligospora—usually feeds on decaying vegetation. It only produces its deadly traps when nematodes are around. Two years ago, one team of scientists showed that it knows when to do this because it can smell its prey, detecting chemicals that the worms can’t help but produce.

But these chemicals aren’t always necessary. Earlier studies have shown that the fungi can also change from death-eaters to death-bringers when they’re exposed to fresh cow dung. Xin Wang, Guo-Hong Li, and Cheng-Gang Zou from Yunnan University reasoned that bacteria in the dung were responsible.

The team isolated a few species of bacteria that could transform the fungi on their own, and identified the chemical that they use—urea. The bacteria mass-produce an enzyme that churns out urea, which then diffuses through the soil. The fungi absorb it and convert it into ammonia, and the ammonia triggers their lifestyle switch. If the team disrupted any part of this pathway, by blocking the various enzymes that make, absorb, or process urea, the fungi no longer set their traps.

The team confirmed the importance of urea by setting up deathmatches in plates of sterilised soil. They seeded the plates with different microbes, unleashed waves of nematodes upon them, and then added the spores of the killer fungi.

Sure enough, the nematodes gorged themselves on the bacteria and their populations bloomed—that is, until the fungi started culling them. If the bacteria couldn’t make urea, the fungi built their traps in a leisurely way and killed the nematodes slowly. But if the team used urea-making microbes, or deliberately added urea to those that couldn’t produce it, the fungi mobilised more quickly and almost completely wiped out the nematodes. By making urea, the bacteria can rally the nematodes’ nemeses to their aid.

These kinds of interaction are common in the natural world. Many plants, for example, defend themselves against very hungry caterpillars by releasing chemicals that summon parasitic wasps, which kill the pests by implanting them with eggs.

We’ve only started to appreciate the scope of these chemical alarms because they are invisible to us, and are often produced by creatures beneath our notice. But they are certainly there. Go for a walk, look at a field, and picture millions upon millions of calls for help, drifting in the wind and cascading through the soil.

Reference: Wang, Li, Zou, Ji, Liu, Zhao, Liang, Xu, An, Zheng, Qin, Tian, Xu, Ma, Yu, Huang, Liu, Niu, Yang, Yuang & Zhang. 2014. Bacteria can mobilize nematode-trapping fungi to kill nematodes. Nature Communications

* PS Both of these traits can coincidentally make bacteria better at infiltrating our bodies and causing disease. Every year, people die because of microbes that were just trying to escape from worms.

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