One of the more likely doomsday scenarios may be a future where our antibiotics become useless against a rising tide of so-called "superbugs." Scientists have found promising potential weapons in new materials, gels, lighting, "smart bombs", and even raw honey, and are now adding a little espionage to the arsenal. According to new research, predatory bacteria could be injected into the body to work with the immune system in hunting down harmful, antibiotic-resistant bugs.

In something akin to the story of the old lady who swallowed a spider to catch the fly, a team of researchers from Imperial College London and the University of Nottingham successfully injected one bacteria into zebrafish larvae to fight off infection of another. In this case, the spider was Bdellovibrio bacteriovorus, and the fly was Shigella flexneri, a common cause of what's known as "traveller's diarrhoea."

The researchers first administered a lethal dose of the multi-drug resistant Shigella flexneri strain M90T into zebrafish larvae, and then followed it up with a shot of Bdellovibrio, which has been known to naturally kill bacteria like E. coli and Salmonella. As expected, Shigella numbers fell in the fish that received the second bacteria, but steadily rose in those that went without. Zebrafish were chosen as test subjects for their translucent skin, which allowed the scientists to watch the tiny battles going on inside.

"This study really shows what a unique and interesting bacterium Bdellovibrio is as it presents this amazing natural synergy with the immune system and persists just long enough to kill prey bacteria before being naturally cleared," says Serge Mostowy, co-lead author of the study. "It's an important milestone in research into the use of a living antibiotic that could be used in animals and humans."

Bdellovibrio even managed to fight the infection in zebrafish that had had their immune systems compromised and as a result had no white blood cells. That said, it was more successful in otherwise healthy subjects, which supports the idea that the bacteria cooperates with the host's immune system.

"This has been a truly ground-breaking collaboration that shows therapeutic Bdellovibrio in action inside the translucent living zebrafish," says Liz Sockett, co-lead author of the study. "The predatory action of the Bdellovibrio breaks the Shigella-pathogen cells and this stimulates the white blood cells; redoubling their 'efforts' against the pathogen and leading to increased survival of the zebrafish 'patients'."

The findings provide a solid base for further research into these kinds of "living antibiotics," with the team hoping to test Bdellovibrio in other vertebrate animals and, eventually, humans.

"It may be unusual to use a bacterium to get rid of another, but in the light of the looming threat from drug resistant infections the potential of beneficial bacteria-animal interactions should not be overlooked," says Michael Chew from Wellcome, who partly funded the study. "We are increasingly relying on last line antibiotics, and this innovative study demonstrates how predatory bacteria could be an important additional tool to drugs in the fight against resistance."

The research was published in the journal Current Biology.