As deadly bacteria grow ever more resistant to modern antibiotics, some researchers have turned to ancient medical manuscripts for clues. And it looks like a medieval salve dating back 1,000 years might succeed where many modern antibiotics are starting to fail.

The "ancientbiotic", as the researchers are calling it, was found in one of the earliest known medical textbooks from medieval England, known as Bald's Leechbook.

While many of the remedies included in this tome have not exactly aged well - including a salve to stop goblins visiting in the night - others, like 'Bald's eyesalve', have stood up better to modern scrutiny.

Building on previous research, scientists have shown this mix of natural ingredients - garlic, onion or leek, cow bile, and wine - may very well have powerful antiseptic properties. In fact, it appears to work against a panel of dangerous bacteria that have grown resistant to multiple modern drugs.

After brewing 75 batches of Bald's eyesalve, including 15 with onion and 15 with leek (just to be absolutely sure about the Old English translation), researchers put the ancient recipe to the test.

In the past, the recipe for this eyesalve has been shown to kill Staphylococcus aureus, the bacterium that causes golden staph. Now, a new study led by Jessica Furner-Pardoe from the University of Warwick backs those results up: Even when the bacteria formed particularly sturdy structures known as biofilms, the salve was effective.

S. aureus are particularly resistant bacteria, in large part because they can form biofilms, or, as one study put it: "slime-enclosed aggregates of sessile bacteria… irreversibly attached to surfaces." That's what makes resistant staph so dangerous.

Once these clusters of microorganisms set up shop, it's incredibly difficult to get an antimicrobial agent in there. Breaking up the biofilm requires concentrations of antibiotics a hundred to a thousand times greater than would be necessary with free-floating forms. In extreme cases, it may even require amputation to avoid blood poisoning.

If this new research is right, however, there's something in Bald's eyesalve that biofilms can't handle.

Even better, the salve appears to work against similarly resistant bacteria, including Acinetobacter baumanii, Stenotrophomonas maltophilia, S. epidermidis and S. pyogenes - all of which can infect wounds, form biofilms, and grow resilient to antibiotics.

"We think it has particular promise for treating diabetic foot infections," University of Warwick microbiologist Freya Harrison told CNN.

"They are the ultimate, super-resistant biofilm infection. They are a huge health and economic burden. They really can become untreatable."

But there's a catch: the salve's ingredients work best as an antimicrobial agent only in the remedy's final form. When researchers separated or purified each element, they were not nearly as effective at killing bacterial strains.

The authors think this might be why herbal remedies haven't stood up to scientific scrutiny thus far. In drug development and research, it's common to isolate single compounds, and yet historically, medicine was more about combining natural matter, bouncing properties off one another.

"Thus, when considering natural products as a potential source of anti-biofilm agents, we must consider the possibility that any efficacy they may possess could rely on creating a cocktail of different products," the authors write.

"Understanding the relationship between combinations of natural products and antimicrobial activity may generate a novel way to create new antibiotics from botanicals."

In this new study, for instance, wine showed little antimicrobial activity on its own, for either free-floating bacteria or biofilms.

But that doesn't mean it was entirely worthless. When the authors removed wine from the salve's recipe, there was a large drop in activity against S. aureus biofilms, which suggests it has some important antimicrobial properties.

In a similar way, the onion and bile showed little antibacterial activity when tested alone, but when removed from the whole remedy, it really showed.

Whether or not Bald's eye salve actually works in a clinical setting is up for further investigation, but the mixture didn't show damage to human cells or mice in the lab, indicating it could be safe to pursue more studies.

"Most antibiotics that we use today are derived from natural compounds, but our work highlights the need to explore not only single compounds but mixtures of natural products for treating biofilm infections," says Harrison.

"We think that future discovery of antibiotics from natural products could be enhanced by studying combinations of ingredients, rather than single plants or compounds."

The study was published in Scientific Reports.