[Editors] MIT crafts bacteria-resistant films

[Elizabeth Thomson]

For Immediate Release
THURSDAY, MAY 15, 2008
Contact: Elizabeth A. Thomson, MIT News Office -- Phone: 617-258-5402  
-- Email: thomson at mit.edu

PHOTO AVAILABLE

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MIT crafts bacteria-resistant films

--Team finds that microbe adhesion depends on surface stiffness

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CAMBRIDGE, Mass.--Having found that whether bacteria stick to  
surfaces depends partly on how stiff those surfaces are, MIT  
engineers have created ultrathin films made of polymers that could be  
applied to medical devices and other surfaces to control microbe  
accumulation.

The inexpensive, easy-to-produce films could provide a valuable layer  
of protection for the health care industry by helping to reduce the  
spread of hospital-acquired infections, which take the lives of  
100,000 people and cost the United States an estimated $4.5 billion  
annually.

The researchers, who describe their work in an upcoming issue of the  
journal 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  
and the paper's anchor author.

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 favor 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, an author  
of the paper and 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.

“For those bacteria that readily form biofilms, we have no delusions  
that we can prevent bacterial films from starting to form. However,  
if we can limit how much growth occurs, these existing methods can  
become much more effective,” Rubner said.

Jenny Lichter, graduate student in materials science and engineering,  
and Todd Thompson, a graduate student in the Harvard-MIT Division of  
Health Sciences and Technology, are joint lead authors of the paper.  
They note that the films could also be used on medical devices that  
go inside the body, such as stents and other cardiac implants.

“Once a foreign object enters into the body, if you can limit the  
number of bacteria going in with it, this may increase the chances  
that the immune system can defend against that infection,” said  
Thompson.

Another possible application for the films is to promote growth of so- 
called “good bugs” by tuning the mechanical stiffness of the material  
on which these bacteria are cultured. These films could stimulate  
growth of bacteria needed for scientific study, medical testing, or  
industrial uses such as making ethanol.

The researchers built their films, which are about 50 nanometers  
(billionths of a meter) 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 6) 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. The researchers are now  
working on figuring out this mechanism.

The research was funded by the National Science Foundation, National  
Institutes of Health and the Arnold and Mabel Beckman Foundation  
Young Investigator Program.

Maricela Delgadillo, a senior in materials science and engineering,  
and Takehiro Nishikawa, a former postdoctoral researcher at MIT, now  
at the Advanced Medical Engineering Center in Osaka, Japan, are also  
authors of the paper.

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Written by Anne Trafton, MIT News Office