How bacteria-eating bacteria could help win the war against germs

Henry N. Williams' favorite movie-action sequence unfolds on a strip of glass just a few millimeters across.

It's a cinematic showdown between two bacterial cells: Vibrio coralliilyticus, a large, rod-shaped marine microbe, and a petite pursuant, Halobacteriovorax, that has latched onto the bigger bacterium. The Vibrio, desperate to jettison its assailant, wriggles and whirls in futility before finally coming to a "screeching halt," as Williams described it.

Then the Halobacteriovorax starts its dirty work: It punctures the Vibrio's exterior and begins to bore inside, where it will gorge on its host, clone itself many times over and burst free to find its next meal. "Lions, sharks, tigers — these are all predators that have gotten our attention," said Williams, a microbiologist at Florida A&M University. "But there also exists a much smaller predator that is just as ferocious."

Predatory bacteria, a group of other microbial assassins, carry immense promise in an extraordinarily small package. Deployed under the right circumstances, they could help beat back harmful microbes in the environment, or purge pathogens from the food supply. Some experts think they could someday serve as a living therapeutic that could help clear drug-resistant germs from patients in whom all other treatments have failed.

But even the small community of researchers who study predatory bacteria have not figured out how these cells select and slaughter their hosts. Teasing out those answers could reveal ways to tackle stubborn infections, and provide a window onto predator-prey dynamics at their most microscopic.

To potentially use this group of microbes as "a living antibiotic, we need to know how it grows," said Terrens Saaki, a microbiologist at the de Duve Institute in Belgium. "We can't use it if we don't understand it."

Predatory bacteria were discovered by accident. Scientists stumbled upon them more than a half-century ago while hunting for another type of murderous microbe called a bacteriophage, or phage, a virus that can infect and kill bacteria. Before then, Williams said, "it was not known that a bacterium would prey on other bacteria in this fashion."

And scientists are identifying more of these predators each year — a striking parallel to the world's diversity of phages.

But phages and predatory bacteria are very different beasts. Phages tend to target a narrow range of hosts, whereas many predatory bacteria are far less finicky. Some predatory bacteria are amenable to eating dozens, if not hundreds, of bacterial species, enabling them to thrive in most habitats. And whereas phages work quickly, massacring entire populations within hours, predatory bacteria are plodding, sometimes taking weeks to grow in the lab.

Still, their predatory lifestyle is so fruitful that it appears to have evolved more than once. Some, such as the leechlike Micavibrio, grab onto their victims like vampires and suck them dry. Others, like Myxococcus, are sharpshooters that operate from afar, releasing a deluge of enzymes that can dissolve their prey at a distance. Some Myxococcus cells even band together to hunt, attacking in a coordinated swarm.

Perhaps the most notorious of the bunch, a group called Bdellovibrio, shares a modus operandi with Halobacteriovorax: They penetrate their hosts and annihilate them from within. Most predatory bacteria experts call these perforating predators BALOs, an acronym for Bdellovibrio and like organisms.

As a whole, predatory bacteria are "very efficient killing machines," said Daniel Kadouri, a microbiologist at Rutgers University. "The first time I saw them, I thought, 'These are the sexiest organisms I have ever seen.' "

Even before latching onto their prey, BALOs are formidable foes, capable of chemical sensing that allows them to "sniff" out their prey and then give chase. "They can swim 100 times their body lengths in a second," Kadouri said. "Pound for pound, that's faster than a cheetah."

In animal studies, predatory bacteria have shown promise in targeting disease-causing germs like Salmonella and Yersinia pestis, which causes the plague. Kadouri and Nancy Connell, a microbial geneticist at the Johns Hopkins Center for Health Security, have dosed Bdellovibrios into the lungs of rats and mice and watched them devour most of the prey at hand.

Predatory bacteria could someday change "how we give basic care," Saaki said.

Predatory bacteria are not exclusively weapons of destruction. In Germany, Julia Johnke, a microbiologist, is working on highlighting the microbes' peacemaking skills in the complex community of bacteria that live in the gut.

Some evidence suggests that "healthy human beings usually have predatory bacteria as part of their microbiome," she said. Little is understood about their role, she added. But they likely maintain order in the gut and ensure that no single species runs amok.

A similar dynamic likely holds true out in nature. Even tiny amounts of predatory bacteria can rejigger the microbial membership of a sample of seawater, Henry Williams said. "They're always present, managing the population of some bacteria," he said.

Many textbooks contain a nod to the importance of predators like phages and protists, a group of mostly single-celled microbes whose cells resemble those of animals. But predatory bacteria have largely been left out, even though they appear to be at least as effective at taming their prey. That perhaps makes these microbes some of the world's least visible underdogs.