How microbes attach and release from mineral surfaces is fundamental to the way microorganisms are transported through water treatment facilities, the effective use of agrochemicals, and the movement of toxic metals in groundwater, for instance. But the forces between the molecules at the surfaces of microbes and minerals had never been measured.
Now, a Virginia Tech graduate student has invented a technique called biological force microscopy and used it to determine what happens when Shewanella, a microorganism found in most soils, meets goethite, the most important iron oxide in soils worldwide. The research provides evidence of recognition between a living organism and an inanimate object, such as a mineral.
The research was featured in Science on May 18, 2001, in the article, “Bacterial Recognition of Mineral Surfaces: Nanoscale Interactions Between Shewanella and a-FeOOH,” by Steven Lower, Michael F. Hochella Jr., and Terry J. Beveridge. Lower, now an assistant professor at the University of Maryland, College Park, invented the technique while a Ph.D. student in geology at Virginia Tech. Hochella is professor of geochemistry and mineralogy at Tech, and Beveridge is at the University of Guelph.
Respiration is the process by which living things break down carbohydrates to produce energy. Like us, Shewanella use oxygen to breakdown carbohydrates; but, if there is no oxygen, the bacteria use iron3 to breathe. This ability has a significant impact on the way minerals dissolve and the movement of iron in the environment.
So the researchers looked at Shewanella and goethite, the iron oxide known for turning soil yellowish. “The bacteria use the mineral as a terminal electron acceptor,” Hochella says. “That is, the bacteria transfer (carbohydrate) electrons to the mineral. The addition of each electron breaks a bond between atoms on the mineral’s surface and allows the goethite to shed iron. Its surface molecules convert from iron3 (Fe III) to iron2 (Fe II). The result can be iron shed into groundwater. If a toxic metal, such as lead or arsenic, is bound to the goethite, these metals are also shed.”
Complex biomolecules on the surface of Shewanella bacteria facilitate the electron transfer. The researchers have determined that Shewanella have a much greater affinity for Fe III-containing minerals when oxygen is absent. “Shewanella make a special protein that interacts specifically with the surface of the Fe III mineral,” says Lower. “It is as if Shewanella recognize the mineral as beneficial to life. This protein appears to be made specifically to transfer electrons from Shewanella to goethite.”
To study these molecular interactions, Lower modified an atomic force microscope so he could attach a living bacterial cell to an arm and move the cell toward a mineral surface to observe and measure the interaction. “A cell is about one micron, or one-millionth (10-6) of a meter,” says Hochella. “Under this microscope, it looks like a blimp attached to the bottom of a huge pole."
Before the cell touches the mineral, there is an interaction — attraction or repulsion. It is a weak force in terms of measurement, but it determines whether or not the microbe attaches to the mineral, Hochella explains. Lower’s microscope measures the attraction in “nanoNewtons” — a force that is between one millionth and one billionth of the force between a record player’s stylus and a vinyl record when it is being played.
“By measuring the forces of the attraction, we think we can tell which biomolecules are facilitating the process. Some of these biomolecules haven’t even been identified,” Hochella says.
Why is knowing about attraction or repulsion between microbes and minerals important?
“Identification of this protein is the first step in our complete understanding of how a bacteria moves electrons from itself to minerals,” says Lower. “By understanding this process, we may then be able to control related processes, such as the release of metals or other contaminants from a mineral surface, which often go hand-in-hand with the electron transfer reactions.”
Hochella adds, “If you have a pathogenic bacteria in the soil, you want to know if it’s going to travel into and within the ground water and get into drinking water. Or will it attach to a mineral surface and be removed from the water? So, we hope to be able to predict microbial movement.”
Probing the unknown properties of single biomolecules is the subject of the Virginia Tech group’s continued research, for which they have just received a $1.1 million grant from the National Science Foundation nanoscale science and engineering initiative.
Since the nanoscale interactions are mediated by biomolecules and the inorganic complements of the mineral surface, Virginia Tech graduate student Treavor Kendall figured out how to attach a single biomolecule to the arm under the atomic force microscope. “You can’t even see it,” says Hochella.
But the interaction can be measured. “When you use the microscope to pull the biomolecule away from the mineral, it’s like stepping on chewing gum then pulling your foot up. You stretch the biomolecule until it snaps off. By measuring the stretching characteristics, we think we can tell which biomolecule is there. We are literally looking at their nano-mechanical characteristics.”
Larger images of Shewanella are available at www.research.vt.edu/resmag/hochella/Shewanella_image.html
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