Bacteria can "recognize"
minerals
Under oxygen-poor conditions, some bacteria are capable of changing their metabolism so that they can utilize reducible metal species present in solid minerals as their terminal electron acceptor instead of oxygen. They are, in essence, "breathing" minerals. Such bacteria are important players in a variety of geochemical processes, ranging from ore deposition to chemical weathering of rocks to produce soils. Nevertheless, the mechanism(s) by which these so-called "metal-reducing bacteria" manage this trick have been unclear. Using the newly developed technique of "biological force microscopy", OBES-funded researchers Steven Lower and Mike Hochella at the Virginia Polytechnic Institute and State University, working with Terry Beveridge at the University of Guelph have been able to show that in the case of the metal-reducing bacterium, Shewanella oneidensis, the bacterium produces a specific protein under anerobic conditions that mediates the electron transfer (Science, 2001, v292, pp1360-1363).
Biological force microscopy was developed by the VIP&SU/Guelph researchers. In the technique, which is illustrated in the figure above, a single, live cell (green) is attached to the tip of an atomic force microscope (yellow). The deflection of the tip due to interaction forces between the cell and an underlying surface is detected by measuring the associated deflection of a laser beam (red) that is reflected off the tip. The method is capable of measuring changes in forces of less than a nanonewton.
Dr. Lower and coworkers measured the attractive force between live S. oneidensis cells and the surface of the iron oxide mineral, goethite (FeOOH), as a function of distance under aerobic and anerobic conditions. Similar experiments were conducted with the mineral diaspore (AlOOH), which has the same structure as goethite, but does not contain a reducible metal ion. Additional experiments were conducted using dead bacteria. The researchers found that the attractive force is the strongest under anerobic conditions and in the presence of goethite, exactly the conditions under which the bacterium can and does resort to the use of iron as an electron acceptor. Furthermore, the change appears to be associated with the presence of a 150-kilodalton protein on the outer membrane of the cell, which is inferred to play a key role in transferring electrons from the bacterium to the mineral. This protein is not always present; it it only expressed under anerobic conditions and in the presence of goethite. It is as though the bacterium senses both the absence of oxygen and the presence of a surface containing an electron acceptor (Fe3+, in this case), and changes its surface in response to what it is sensing.
