Sticky surfaces

Whether bacteria stick to surfaces or not depends partly on how stiff those surfaces are. Armed with that knowledge, MIT engineers have created ultrathin films made of polymers that could be applied to medical devices that go inside the body, such as stents and other cardiac implants.

The researchers, who described their work in a paper in an upcoming issue of Biomacromolecules, found they could control the extent of bacterial adhesion to surfaces by manipulating the mechanical stiffness of polymer films called polyelectrolyte multilayers. Thus, the films could be designed to prevent accumulation of hazardous bacteria or promote growth of desirable bacteria.

'All other factors being equal, mechanical stiffness of material surfaces increases bacterial adhesion,' said Krystyn Van Vliet, the Thomas Lord Assistant Professor of Materials Science and Engineering.

Van Vliet and her colleagues found the same trend in experiments with three strains of bacteria: Staphylococcus epidermidis, commonly found on skin, and two types of Escherichia coli.

Stiffness has usually been overlooked in studies of how bacteria adhere to surfaces in favour of other traits such as surface charge, roughness, and attraction to or repulsion from water. The new work shows that stiffness should also be taken into account, said Van Vliet.

The new films could be combined with current methods of repelling bacteria to boost their effectiveness, said Michael Rubner, director of MIT's Center for Materials Science and Engineering. Those methods include coating surfaces with antimicrobial chemicals or embedding metal nanoparticles into the surface, which disrupt the bacterial cell walls.

The researchers built their films, which are about 50 nanometres thick, with layers of polyelectrolytes (a class of charged polymer). Alternating layers are added at different pH (acidity) levels, which determines how stiff the material is when hydrated at near-neutral pH, such as water. Polymer films assembled at higher pH (up to six) are stiffer because the polymer chains crosslink readily and the polymers do not swell too much; those added at lower, more acidic pH (down to 2.5) are more compliant.

Van Vliet says the team's results could be explained by the relationship between surfaces and tiny projections from the bacterial cell walls, known as pili. Stiffer surfaces may reinforce stronger, more stable bonds with the bacterial pili.