Dueling theories for electricity-breathing microbes

The bacterium Shewanella oneidensis enjoys a unique respiratory versatility: not only can it use oxygen as a terminal electron acceptor, as do humans, but it can also transfer electrons to extracellular minerals such as iron oxides. Researchers hope to use that flow of electrons to, say, power microbial fuel cells or remediate soil contaminants, but they remain conflicted as to how the feat is carried out. Two theories have emerged: that microbes secrete shuttle molecules that diffuse to a metal’s surface, deposit electrons, and then return to start the process anew; and that on contacting a suitable metal, they pass electrons directly across the cell membrane. A team led by Charles Lieber (Harvard University) and Bradley Ringeisen (US Naval Research Laboratory) has produced new evidence in favor of the shuttle hypothesis. The researchers conducted a tiny fuel cell experiment with two types of nanoelectrodes—one exposed via openings too small for microbes to access, shown in the figure, and the other with large, accessible openings. Both yielded roughly equal current. Meanwhile, experiments by Mohamed El-Naggar (University of Southern California), Yuri Gorby (J. Craig Venter Institute), and colleagues support the direct contact mechanism. Applying voltage drops along the bacteria’s pili—long filament-like appendages that might attach to metal surfaces—they observed lengthwise conductivities sufficient to keep pace with bacterial respiration. It’s possible, maintain both teams, that microbes can choose either strategy based on the demands of their environment; the groups are now collaborating to find out. (X. Jiang et al., Proc. Natl. Acad. Sci. USA, 107, 16806, 2010 ; M. Y. El-Naggar et al., Proc. Natl. Acad. Sci. USA, 107, 18127, 2010 .)—Ashley G. Smart