The strongest glue so far found in nature could have applications in engineering and medicine, according to its discoverers. Secreted by a marine microorganism, the glue sticks more than twice as hard as the strongest ‘superglue’ currently available.

Yves Brun, a bacteriologist from Indiana University in Illinois, has been working with physicists and engineers from Indiana and Brown University in Rhode Island to study the bacterium Caulobacter crescentus. One of the first species to colonise surfaces submerged in rivers and streams, Caulobacter secretes a sticky compound through a stalk-like structure at its base called a holdfast. These compounds, which are composed of chains of sugar molecules, allow it to hold fast to the inside of metal pipes.

To assess the strength of the glue, the team allowed Caulobacter to colonise the surface of a thin glass pipette, then used a micromanipulator to pull the bacteria off the surface. In 14 trials, the bacteria needed a force of 0.11 to 2.26µN per cell to detach them from the glass surface. Scaling up, this represents a stress of 70N/mm2, compared with breaking stresses of 18-28N/mm2 for commercial cyanoacrylate-based superglues.

According to Brun, the discovery was largely serendipitous. ‘We wanted to look at how certain bacteria can manage to localise sticky molecules in one section of their structures, and we were using Caulobacter because it’s an easy bacterium to study; it’s very widespread and found in all sorts of water conditions,’ he said. The team engineered some ‘knockout’ versions of the bacteria which couldn’t produce particular proteins in the holdfast. ‘Once we had the bacteria attached to the surface, we tried to wash it off. The knockout versions came away straight away, but no matter how hard we sprayed the water, the natural versions wouldn’t wash off. That got us wondering exactly how strong the attachment was.’

The glue is a polysaccharide, N-acetylglucosamine, which appears to be attached to proteins on the base of the holdfast. ‘The polysaccharide is definitely the main component of the glue; it’s the thing that’s sticky. But part of the strength of the bond might come from some sort of interaction between the polysaccharide and the proteins; there are probably several mechanisms of sticking involving different sorts of chemical and physical interaction,’ Brun says.

Brun and colleagues are now looking for ways to make the bacteria produce more of the saccharide. ‘It’s likely that the polysaccharide is modified somehow by the proteins, which might make it prohibitively expensive to make it de novo [from scratch],’ he said. ‘So the best way to do it might be to genetically engineer the bacteria so they produce more of it, and get them to make it for you in a bioreactor.’ Two possible problems occur to Brun: the glue might not be as sticky if separated from the bacterium and purified; and if it is, it might stick to all the materials used to make it. ‘We tried washing the glue off,’ he said. ‘It didn’t work.’

If it can be developed as a commercial product, Brun thinks it is likely to be used as a medical adhesive, possibly to close wounds in surgery or to aid in the attachment of surgical implants; the glue has the twin advantages of extreme strength and complete biodegradability. ‘You’d have to make sure it doesn’t trigger allergic reactions, but it’s made using entirely non-toxic compounds, unlike all other surgical glues, which would be a major advantage’ he said. ‘And Caulobacter uses it in all sorts of water, including seawater. Body fluids are salty, which can be a problem for some glues, but this one shouldn’t be affected.’