As scientists work toward making genetically altered bacteria create living
"circuits" to produce a myriad of useful proteins and chemicals, they
have logically assumed that the single-celled organisms would always respond
to an external command in the same way.
 | | In this colony, the bacteria lighting up in green are those being "turned on," while those in red remain "off." Credit: Lingchong You |
Alas, some bacteria apparently have an individualistic streak that makes them
zig when the others zag.
A new set of experiments by Duke University bioengineers has uncovered the
existence of "bistability," in which an individual cell has the potential
to live in either of two states, depending on which state it was in when stimulated.
Taking into account the effects of this phenomenon should greatly enhance the
future efficiency of synthetic circuits, said biomedical engineer Lingchong
You of Duke's Pratt School of Engineering and the Duke Institute for Genome
Sciences & Policy.
In principle, re-programmed bacteria in a synthetic circuit can be useful for
producing proteins, enzymes or chemicals in a coordinated way, or even delivering
different types of drugs or selectively killing cancer cells, the scientists
said.
Researchers in this new field of synthetic biology "program" populations
of genetically altered bacteria to direct their actions in much the same way
that a computer program directs a computer. In this analogy, the genetic alteration
is the software, the cell the computer. The Duke researchers found that not
only does the software drive the computer's actions, but the computer in turn
influences the running of the software.
"In the past, synthetic biologists have often assumed that the components
of the circuit would act in a predictable fashion every time and that the cells
carrying the circuit would just serve as a passive reactor," You said.
"In essence, they have taken a circuit-centric view for the design and
optimization process. This notion is helpful in making the design process more
convenient."
But it's not that simple, say You and his graduate student Cheemeng Tan, who
published the results of their latest experiments early online in the journal
Nature Chemical Biology.
"We found that there can be unintended consequences that haven't been
appreciated before," said You. "In a population of identical cells,
some can act one way while others act in another. However, this process appears
to occur in a predictable manner, which allows us to take into account this
effect when we design circuits."
Bistability is not unique to biology. In electrical engineering, for example,
bistability describes the functioning of a toggle switch, a hinged switch that
can assume either one of two positions – on or off.
"The prevailing wisdom underestimated the complexity of these synthetic
circuits by assuming that the genetic changes would not affect the operation
of the cell itself, as if the cell were a passive chassis," said Tan. "The
expression of the genetic alteration can drastically impact the cell, and therefore
the circuit.
"We now know that when the circuit is activated, it affects the cell,
which in turn acts as an additional feedback loop influencing the circuit,"
Tan said. "The consequences of this interplay have been theorized but not
demonstrated experimentally."
The scientists conducted their experiments using a genetically altered colony
of the bacteria Escherichia coli (E.coli) in a simple synthetic circuit. When
the colony of bacteria was stimulated by external cues, some of the cells went
to the "on" position and grew more slowly, while the rest went to
the "off" position and grew faster.
"It is as if the colony received the command not to expand too fast when
the circuit is on," Tan explained. "Now that we know that this occurs,
we used computer modeling to predict how many of the cells will go to the 'on'
or 'off' state, which turns out to be consistent with experimental measurements"
The experiments were supported by the National Science Foundation, the National
Institutes of Health and a David and Lucille Packard Fellowship. Duke's Philippe
Marguet was also a member of the research team.
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Posted October 4th, 2009
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