Wash. – When Yuri Gorby discovered that a microbe
which transforms toxic metals can sprout tiny electrically
conductive wires from its cell membrane, he reasoned
this anatomical oddity and its metal-changing physiology
must be related.
A colleague who had heard Gorby's presentation at
a scientific meeting later reported that he, too,
was able to coax nanowires from another so-called
metal-reducing bacteria species and further suggested
the wires, called pili, could be used to bioengineer
now turns out that not only are the wires and their
ability to alter metal connected—but that many
other bacteria, including species involved in fermentation
and photosynthesis, can also form wires under a variety
of environmental conditions.
“Earth appears to be hard-wired,” said
Gorby, staff scientist at the Department of Energy's
Pacific Northwest National Laboratory, who documents
the seeming ubiquity of electrically conductive
microbial life in the July 10 advance online Proceedings
of the National Academy of Sciences .
In a series of experiments, Gorby and colleagues
induced nanowires in a variety of bacteria and demonstrated
that they were electrically conductive. The bacterial
nanowires were as small as 10 nanometers in diameter
and formed bundles as wide as 150 nanometers. They
grew to be tens of microns to hundreds of microns
The common thread involved depriving a microbe of
something it needed to shed excess energy in the
form of electrons. For example, Shewanella , of interest
in environmental cleanup for its ability to hasten
the weathering of toxic metals into benign ones,
requires oxygen or other electron acceptors for respiration,
whereas Synechocystis , a cyanobacterium, combines
electrons with carbon dioxide during photosynthesis.
of these “electron acceptors,” bacterial
nanowires “will literally reach out and connect cells
from one to another to form an electrically integrated
community,” Gorby said.
“The physiological and ecological implications for
these interactions are not currently known,” he said, “but
the effect is suggestive of a highly organized form
of energy distribution among members of the oldest
and most sustainable life forms on the planet.”
In one clever twist, Gorby grew pili from mutant
strains developed by collaborators that were unable
to produce select electron transport components called
cytochromes. Sure enough, the nanowires of the mutants
were poor conductors.
“These implicate cytochromes as the electrically
conductive components of nanowires, although this
has yet to be conclusively demonstrated,” Gorby said.
To measure currents as precisely as possible, Gorby
and colleagues from the University of Southern California
have built a microbial fuel cell laboratory at PNNL.
The small bacteria-powered batteries, cultured under
electron-acceptor limitations and fueled by lactate
or light, produce very little power, as measured
by a voltmeter hooked to a laptop computer.
co-author and PNNL scientist Jeff Mclean, who manages
the microbial fuel cell laboratory, said that small
changes in fuel cell design and culture conditions
have already shown large improvements in the efficiency
of the fuel cells. For example, so-called biofilms—a highly interconnected bacterial
community—put out much more energy than other configurations.
The research was funded by the DOE Office of Science
programs Genomics: Genomes to Life and Natural and
Accelerated Bioremediation Research in the Office of
Biological and Environmental Research.
PNNL is a DOE
Office of Science laboratory that solves complex
problems in energy, national security, the environment
and life sciences by advancing the understanding
of physics, chemistry, biology and computation. PNNL
employs 4,200 staff, has an annual budget of more
than $725 million, and has been managed by Ohio-based
Battelle since the lab's inception in 1965.