Bacteria like E. coli have hair-like protrusions known as fimbriae with a sticky protein on the tip. This adhesive protein is called FimH and binds in an unusual way to a sugar molecule present on the surfaces of cells. A group of researchers at ETH Zurich and the University of Washington in Seattle have been studying how the bacterium E. coli attaches to surfaces and copes with rapidly changing flow conditions as found in the human body. The research appears in the September issue of PLoS Biology, part of the Public Library of Science, a series of open-access journals available online (www.plos.org).
Bacterial adhesive bonds can get stronger under force
Unexpectedly the FimH-sugar combination makes a "catch bond" that acts like a finger trap and actually gets stronger as drag force is exerted on a bacterium, as opposed to getting weaker as for normal bonds. Rather than being swept away by fluids moving through the human body, the bacterium grips even more tightly, helping it stick around and form an infection. The catch bonds release their grip when there is little or no force on the bacteria. In new research, the scientists have learned that the mechanical properties of the fimbriae also play a key role maintaining E. coli attached to mucousal surfaces. The tiny protrusions are made up of interlocking protein segments in a tightly coiled helix shape, like a seven-nanometers-wide Slinky toy. The researchers found that under force, the fimbriae extend to many times their original length as the protein segments uncoil one by one. If the force on them drops, the fimbriae recoil, keeping tension on the bond between the bacterium and the mucous membrane. "The fimbriae uncoil and coil to dampen sudden changes in forces caused by rough and rapidly changing flow conditions", explained Viola Vogel, Professor in the Department of Materials at ETH Zurich. This process maintains an optimal force required to keep the finger trap-like FimH anchor from breaking loose.
Bungee cords for bacteria
"This system extends similar to a bungee cord, but by a different mechanism as bonds that stabilize the helical structure can break or reform sequentially" added study co-author Prof. Wendy Thomas of the University of Washington. Manu Forero, graduate student at ETH Zurich, found that fimbrial uncoiling and recoiling events balance each other at an intermediate force level that corresponds to the force at which the sticky protein tip forms the most stable bond with the surface. Thus, the mechanical and adhesive features of the system evolved together to help the bacteria persist in tough environments inside a host animal or person.
"Research on these fimbriae uncovers something that's essentially a mechanical nanodevice created by nature, and gives us the opportunity to adapt such a system for biotechnological and even other technical uses," said study co-author Dr. Evgeni Sokurenko of the University of Washington. "It also improves our understanding of how to fight bacteria that persists in turbulent fluid environments, like the human urinary tract or intestines."
Materials provided by Swiss Federal Institute Of Technology Zurich. Note: Content may be edited for style and length.
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