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Biomaterials

Gripping stuff

The aquatic bacterium Caulobacter crescentus can come, quite literally, to a sticky end. As Peter H. Tsang and colleagues report (Proc. Natl Acad. Sci. USA 103, 5764–5768; 2006), the bacterium's long, tail-like anchor sticks it so tightly to a supporting surface that it often tears apart when a detaching force is applied, rather than relinquishing its grip. The adhesion is thought to be the strongest of biological origin yet discovered.

When C. crescentus (two are pictured here, spawning clones) is in its non-motile (stuck) state, its anchor — appropriately named its ‘holdfast’ — binds the bacterium to the surface, and a stalk connects the holdfast to the cell body. The authors used atomic force microscopy to record the force needed to detach a non-motile cell from a micropipette by means of a suction pipette oriented perpendicular to the pulling direction. They measured the area of coverage of the holdfast and other dimensions of the bacterium, working out average geometries and finally applying a mathematical technique known as finite-element analysis to calculate the adhesion strength.

Credit: Y. BRUN

The holdfast enables the bacterium to remain stuck to the surface even in strong jets of water, and Tsang et al. calculate that, were it to cover an area of 1 cm2, it could support a weight of 680 kg, even on a wet surface. That exceeds all other known cell-adhesion capabilities — including the sticking power of the much-studied gecko's foot. And because, owing to the geometry of C. crescentus, the adhesion often failed through fracture of the cell stalk rather than at the adhesion interface, the true adhesion strength at the interface could be still higher.

Polymers of a sugar-based molecule called N-acetylglucosamine are known to be present in the bacterium's adhesive plaque. The authors found that when they treated the polymers with an enzyme to break them down, the strength of the bacterial attachment was reduced. However, the detailed physical and chemical mechanisms of C. crescentus's adhesive abilities remain to be revealed. Their elucidation could trigger the development of a new range of synthetic adhesives.

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