Scientists have made a surprising discovery about how strange bacteria that live in soil and sediment can conduct electricity. The bacteria do so, the researchers determined, through a seamless biological structure never before seen in nature - a structure scientists can co-opt to miniaturize electronics, create powerful-yet-tiny batteries, build pacemakers without wires and develop a host of other medical advances.
Scientists had believed Geobacter sulfurreducens conducted electricity through common, hair-like appendages called pili. Instead, a researchers have determined that the bacteria transmit electricity through immaculately ordered fibers made of an entirely different protein. These proteins surround a core of metal-containing molecules, much like an electric cord contains metal wires. This "nanowire," however, is 100,000 times smaller than the width of a human hair.
The authors report a 3.7 Å resolution cryoelectron microscopy structure, which surprisingly reveals that, rather than PilA, G. sulfurreducens nanowires are assembled by micrometer-long polymerization of the hexaheme cytochrome OmcS, with hemes packed within ∼3.5–6 Å of each other. The inter-subunit interfaces show unique structural elements such as inter-subunit parallel-stacked hemes and axial coordination of heme by histidines from neighboring subunits. Wild-type OmcS filaments show 100-fold greater conductivity than other filaments from a Δ omcS strain, highlighting the importance of OmcS to conductivity in these nanowires.
This tiny-but-tidy structure, the researchers believe, could be tremendously useful for everything from harnessing the power of bioenergy to cleaning up pollution to creating biological sensors. It could actually serve as the bridge between electronics and living cells.
"There are all sorts of implanted medical devices that are connected to tissue, like pacemakers with wires, and this could lead to applications where you have miniature devices that are actually connected by these protein filaments," said the senior author. "We can now imagine the miniaturization of many electronic devices generated by bacteria, which is pretty amazing."
Geobacter bacteria play important roles in the soil, including facilitating mineral turnover and even cleaning up radioactive waste. They survive in environments without oxygen, and they use nanowires to rid themselves of excess electrons in what can be considered their equivalent to breathing. These nanowires have fascinated scientists, but it is only now that researchers have been able to determine how G. sulfurreducens uses these organic wires to transmit electricity.
"The technology [to understand nanowires] didn't exist until about five years ago, when advances in cryo-electron microscopy allowed high resolution," said the senior author. "We have one of these instruments here at UVA, and, therefore, the ability to actually understand at the atomic level the structure of these filaments. ... So this is just one of the many mysteries that we've now been able to solve using this technology, like the virus that can survive in boiling acid, and there will be others."
Biological wiring in bacteria never seen before!
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