A living antibiotic: Bdellovibrio bacteria attack and destroy human pathogens

Hannah joined Drug Discovery News as an assistant editor in 2022. She earned her PhD in neuroscience from the University of Washington in 2017 and completed the Dalla Lana Fellowship in Global Journalism in 2020.

In 1941, one of Charles Fletcher's patients was dying of septicemia. "He had multiple abscesses on his face and his orbits (for which one eye had been removed) … He was in great pain and was desperately and pathetically ill," wrote Fletcher, a physician at the Radcliffe Infirmary in Oxford (1).

At any other hospital, physicians would have been unable to do anything for the patient except try to keep him comfortable. Fletcher, though, knew that Howard Florey, a pharmacologist at the University of Oxford, was eager to test the effectiveness of a new drug he’d been working on. On February 12, Fletcher's patient became the first person to be treated with penicillin.

After a few days, the patient was "vastly better," according to Fletcher. Unfortunately, the researchers could not manufacture the drug quickly enough, and the patient died after the supply ran out. Despite this shaky first attempt, Fletcher went on to witness many patients’ lives saved by this seemingly miraculous drug.

Alas, it wasn't long before bacteria began to fight back. By 1942, researchers had already identified strains of penicillin-resistant Staphylococcus aureus in hospitalized patients (2). Today, it's estimated that antibiotic-resistant bacteria are responsible for more than one million deaths per year globally (3).

As researchers scramble to address the growing antimicrobial resistance crisis, they may find help from an unlikely source: other bacteria. Certain species of bacteria that hunt down and devour fellow bacteria, including human pathogens, could serve as living antibiotics. By attacking pathogens in ways that evade the development of resistance, these predatory bacteria could provide hope in cases where traditional antibiotics have failed.

The species Bdellovibrio bacteriovorus was accidentally discovered in 1962 by scientists who were searching for bacteriophages in soil samples (4). At first glance, Bdellovibrio, as researchers call them, don't look especially notable. In images captured with a transmission electron micrograph, the bacterium looks a bit like a sausage with a long, thin tail.

Yet this bacterium has an ability that most other bacteria do not possess: It is a formidable hunter of other bacteria. Bdellovibrio are tiny — only about half a micron long — and are dwarfed by their much larger prey (5). Once a Bdellovibrio bacterium encounters another bacterium, it attaches itself to the surface of its prey. Through a process that is still not well understood, it assesses the potential prey and determines whether it would make a good meal.

"Once they've committed, their first job is to cut a hole in the outer membrane," said Andrew Lovering, a structural biologist at the University of Birmingham. "Then they pull themselves through the hole. Then of course, they don't want to share all the food, so they seal the hole behind them." Over the next few hours, the predatory bacterium devours its prey from the inside, multiplies, and eventually bursts forth, leaving the lifeless husk of the other bacterium behind.

The unusual behavior and mysterious biological mechanisms of Bdellovibrio have captured the imaginations of a select group of scientists. "Predatory bacteria are beautiful in terms of evolution," said Lovering. Researchers such as Lovering and Elizabeth Sockett, a microbiologist at the University of Nottingham, have come to appreciate this quirk of nature for its antibacterial potential.

Sockett, one of the field's pioneers, came across Bdellovibrio while answering fundamental questions in microbiology. "I was quite interested in the fact that these bacteria could actually collide with other bacteria because usually bacteria avoid each other really well," she said.

As concerns about antimicrobial resistant pathogens grew in the early 2000s, Sockett began to wonder if Bdellovibrio could provide a potential solution as an entirely novel way to kill pathogens that were quickly evolving resistance to available antibiotics. Bdellovibrio prey on Gram negative bacteria, a group that includes many deadly human pathogens. In the lab, at least, pathogens that are resistant to various classes of antibiotics, including colistins, carbapenems, and aminoglycosides are easily killed by Bdellovibrio (6–8).

You could put all the resistance genes known into one bacterium, and the Bdellovibrio would just eat [it] anyway. - Andrew Lovering, University of Birmingham

"They don't care about the antibiotic resistance of the cell they're attacking," said Lovering. "So, you could put all the resistance genes known into one bacterium, and the Bdellovibrio would just eat [it] anyway. They don't work in the same way that a drug works."

Indeed, Bdellovibrio are equipped with many ways to recognize and bind to other bacterial cells using various types of adhesins, or cell surface proteins. "Bdellovibrio don't just come with a grappling hook," said Sockett. "They’ve got a grappling hook, and Blu Tack [adhesive putty], and a hammer and nails… If you knock out a single adhesin, you would not get a Bdellovibrio that couldn't attach to prey. And actually, you could knock out several and not get a big effect." It's a significant challenge for other bacteria to evolve mutations to protect themselves against such a diverse array of weaponry, meaning that it is much more difficult for bacteria to develop resistance to Bdellovibrio than to traditional antibiotics (9).

By the mid-2000s, scientists had conclusively demonstrated Bdellovibrios’ ability to kill gram-negative pathogens, including Escherichia coli and Salmonella species in vitro (10,11). However, the question of whether predatory bacteria could be a useful therapeutic remained open.

There was only one way to find out the answer: Take the experiments in vivo. For the first trial of Bdellovibrio as a therapeutic in warm-blooded animals, Sockett's research group teamed up with Robert Atterbury, another University of Nottingham microbiologist with expertise in zoonotic diseases. The results were promising. The predatory bacteria substantially reduced the amount of Salmonella bacteria in chicks that had been previously colonized. A strain of Bdellovibrio that had been genetically altered to remove its ability to prey on other bacteria did not effectively reduce Salmonella colonization, supporting the idea that its activity is due to predation rather than another mechanism. Finally, although the birds’ gut microbiomes were altered by the treatment, there were no obvious impacts on their growth and overall health (12).

After these encouraging early results, Sockett wanted to learn more about what exactly Bdellovibrio were doing inside the host and how they interacted with the host's immune system, which could have major implications for both the safety and efficacy of predatory bacterial therapeutics. However, it's difficult to visualize cell-cell interactions inside the gut of a chicken. "So, then we met Serge Mostowy — now at the London School of Hygiene and Tropical Medicine — who had pioneered pathogen experimentation in zebrafish larvae, and we went from the more opaque chicken to the transparent zebrafish," said Sockett.

In the future, it would be useful to have bespoke predators that have a defined killing range that would come from our understanding of how they stick to prey. - Andrew Lovering, University of Birmingham

Sockett and Alex Willis, a member of Mostowy's research group, injected pathogenic Shigella flexneri bacteria tagged with green fluorescent protein into the hindbrains of larval zebrafish; in some of these fish, they also injected red-tagged Bdellovibrio. When they examined the Shigella-infected larvae under the microscope, they observed that the green fluorescent pathogen spread until it eventually killed approximately 70 percent of the fish. In the other group treated with the red Bdellovibrio, the predatory bacteria attacked and cleared the Shigella, and approximately 60 percent of the fish survived. The team witnessed the dramatic microscopic spectacle of the smaller red predators finding and invading the larger green pathogens playing out inside the zebrafish itself.

Importantly, the researchers observed that Bdellovibrio and the immune system seemed to work cooperatively rather than against each other. When immunocompromised zebrafish were dosed with the pathogen and the predator, they were only about half as likely to survive as the fish that could fight off the pathogen with both their immune systems and the predatory bacteria (7). The relative harmony between Bdellovibrio and the host immune system is promising: The Bdellovibrio aren't killed so rapidly that they are unable to fight the pathogen, and they don't seem to set off an extreme immune response that ultimately harms the host animal.

Separately from her zebrafish work with Mostowy, Willis, and others, Sockett has collaborated with Lovering to answer important questions about how predatory bacteria adhere to and attack prey, as well as how they so effectively lyse their prey without damaging their own cell components. They are also studying the proteins that Bdellovibrio use to attach themselves to prey, which might one day lead to custom-made predators with proclivities for specific types of prey, said Lovering. "In the future, it would be useful to have bespoke predators that have a defined killing range that would come from our understanding of how they stick to prey," he said.

While transparent zebrafish larvae are excellent for visualizing cell-cell interactions, it is crucial for scientists to understand how Bdellovibrio behave in mammals before they can test them in humans. Daniel Kadouri, a microbiologist at Rutgers University, is up to the task.

In 2015, Kadouri published one of the first in-depth explorations of the safety of Bdellovibrio in live mammals. When Kadouri's research team administered the predatory bacteria to mice intravenously or through the respiratory tract, the bacteria provoked a transient inflammatory response, but they did not appear to have negative effects on the overall health or survival of the mice (13). Next, Kadouri investigated the effects of Bdellovibrio treatment on Klebsiella pneumoniae infections. In humans, this species is a major cause of deadly healthcare-associated infections. In Kadouri's experiments in rats, Bdellovibrio significantly reduced the amount of this pathogen in the lungs (14).

Bdellovibrio bacteria were less effective against infections in the blood, however. When scientists injected Klebsiella into the blood, the predatory bacteria did not reduce the levels of the pathogen in the blood or prevent it from spreading to other organs, such as the liver or spleen (15).

"There are lots of other cells floating around in the blood, and these can actually act as decoys. Bdellovibrio, for the most part, just smack into things, and if they can prey on it, they’ll prey," said Kadouri. Having lots of cells around that they can't prey on, such as human blood cells, can make it harder for them to get to the species they can prey on.

Kadouri said that there might also be factors in the blood that inhibit predation, although it's not yet entirely clear what they might be. In support of this hypothesis, an experiment published by Sockett the same year showed that human serum substantially slowed the rate at which Bdellovibrio could hunt down and kill pathogens (16).

For these reasons, Kadouri believes that predatory bacteria will be most useful for localized infections, including lung and wound infections. In partnership with microbiologist Robert Shanks at the University of Pittsburgh, Kadouri is also exploring the possibility of using Bdellovibrio to treat antibiotic-resistant eye infections.

Researchers are still exploring when, where, and in what combinations predatory bacteria can have the most impact. Sockett believes that Bdellovibrio would be a last resort, "fire extinguisher-like" treatment for those with very serious infections rather than a first-line therapy. Kadouri said that predatory bacteria may not be a standalone treatment, but may work best in combination with other infection control agents, such as bacteriophages.

While early animal studies of Bdellovibrio's safety have been promising, researchers need to further investigate how Bdellovibrio interact with the mammalian immune system and microbiome. "The fate and longevity of Bdellovibrio in the body and the immune stimulation caused by it need a lot of understanding before we can use it as a therapy," said Sockett.

Furthermore, since Bdellovibrio are a living therapeutic, Kadouri anticipates substantial regulatory challenges before they could be approved for medical use. Nevertheless, with continuing research, scientists hope that this therapy will one day be another tool in the arsenal to help humanity fight deadly antibiotic-resistant infections.

Hannah joined Drug Discovery News as an assistant editor in 2022. She earned her PhD in neuroscience from the University of Washington in 2017 and completed the Dalla Lana Fellowship in Global Journalism in 2020.