May 29, 2001 COLLEGE STATION - The microbes in our guts may prove more useful than we think. Scientists at Texas A&M University are investigating how bacteria may serve as sensors for environmental chemicals, as means to deliver drugs to specific targets, or as tiny turbines.
Michael Manson, professor of biology and of biochemistry and biophysics at Texas A&M, studies how E. coli bacteria move to find food. E coli is attracted to nutrients such as the sugar maltose and repelled by harmful substances like nickel ions. This simple behavior is called chemotaxis.
To migrate toward higher concentrations of "good" chemicals and away from higher concentrations of "bad" ones, a bacterium constantly monitors the levels of these chemicals as it swims. If the concentration of a "good" chemical increases, or a "bad" chemical decreases, the cell keeps swimming ahead. Otherwise, it tries a new direction at random.
Working with the group of Robert Gunsalus, professor of microbiology and molecular genetics at UCLA, Manson's laboratory has created a protein that allows bacteria to swim away from nitrate, one of their main sources of energy. The avoidance of nitrate is actually a disadvantage to E. coli, but the fact that this manufactured protein works suggests that novel biosensors can be created to meet a designer's specifications.
The advantage of this system is that receptors on the surface of E. coli cells could be modified to recognize virtually any small molecule. Manson is pursuing this aspect of protein engineering in collaboration with Frank Raushel, professor of chemistry at Texas A&M.
"The possibilities are limitless," Manson says. "The end result could be a low-cost, effective, flexible, environmentally friendly arsenal of harmless bacteria that carry out chemical-detective assignments unnoticed by anyone who is not in on the secret."
Bacteria may also be employed as microscopic "pack animals" to deliver minuscule cargo to predetermined targets. The difficult part is how to load the cargo onto the bacteria without interfering with their swimming.
To solve this problem, Manson turned to Ry Young, professor of biochemistry and biophysics and of biology at Texas A&M. Young works with a virus called bacteriophage lambda that attaches to and injects its DNA into E. coli.
Normally, phage lambda kills bacteria, but a certain mutant lambda phage, when it infects a particular mutant strain of E. coli, injects its DNA only to have the DNA degraded. Meanwhile, the empty phage particle, whose long stalk and globular head remind one of a lollipop or lunar lander, remains firmly attached to the surface of the cell.
The inactivated phage particle becomes a perfect support on which to bind cargo. The protein that encloses the head of the phage can be engineered to bind tightly to the vitamin biotin. Streptavidin, a protein that interacts strongly with biotin, can then be attached to the cargo, forming the critical link between the burden (streptavidin) and the bearer (phage particle with biotin on it).
Manson's group is working with the laboratory of Arun Majumdar, professor of mechanical engineering at the University of California at Berkeley, to demonstrate a proof-of-concept for this idea. Initially, the cargo will be a bead that contains an easily monitored fluorescent compound in addition to streptavidin. They will see whether bacteria carrying such beads are drawn to the source of an attractant chemical.
Although this project is in its beginning stages, Manson predicts that "bacteria may be used to deliver packets of drugs, hormones, and tumor-killing chemicals efficiently and precisely to the desired targets. They may also be able to deliver 'building materials' to construction sites in nanofabrication applications."
Bacteria may also find application in the new and rapidly growing field of microfluidics. It is often desirable to move liquids through capillaries only a few microns in diameter. (A micron is one-millionth of a meter.) External pumps must generate very high pressures to overcome resistance to flow through such fine tubes, and these pressures can damage devices. Bacteria may be gentler, but just as effective.
Each bacterium has about four flagella, which consist of corkscrew-like propellers, each of which is rotated by an electrical motor at its base, turning at typically several hundred revolutions per second.
Manson and his students are working with Ajay Malshe and Steven Tung, who are in the Department of Mechanical Engineering at the University of Arkansas - Fayetteville, to test whether bacteria can generate rates of flow comparable to those maintained by external pumps. An array of bacteria will be tethered in a microchannel 5 by 20 microns in diameter. (The bacteria themselves are only one micron in diameter and two to four microns in length.) The Arkansas group can manufacture the nanochannels and has developed very sensitive meters that detect flow rates of microliters per minute.
By studying the proteins that make up the motor, Manson is also trying to determine the minimum number of components present in a functional unit. He will work with two groups at Texas A&M, that of Hagan Bayley in the Department of Medical Biochemistry and Genetics and that of Paul Cremer in the Department of Chemistry, to build a flagellar motor outside of a cell. Bayley and Cremer study the properties of proteins that are assembled into artificial as well as natural biological membranes.
"If we can attach some kind of propeller to these reconstituted motors and devise a way to impose a 200 millivolt electrical potential across the membrane to drive one-way proton flow, we will have a controllable turbine," Manson claims. "In addition to direct applications of this technology to microfluidics or nanoscale generation of electrical power, it may be possible to use the bioengineering principles revealed to build non-biological correlates that will usher in a whole new era of nanotechnology." Manson is thinking small, and he is clearly proud of it.
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