Innovative Biological Circuits Developed at MIT
Transistors in electrical circuits function as switches that control the flow of current. A research team at the Massachusetts Institute of Technology (MIT) has made significant strides in emulating this mechanism from a biological perspective, enabling bacteria to act as switches themselves. They have successfully created a “living circuit” capable of performing complex calculations by interconnecting colonies of bacteria.
In this biological circuit, bacterial switches manage the flow of small molecules, which serve as signals for subsequent cells. The researchers assert that using two bacterial strains that function as transistors, along with three strains responsible for signal transmission, it is theoretically possible to construct virtually any circuit. “We have built some of the fundamental components of the computer architecture that are commonly used,” explains Hamid Doost-Hosseini, the lead author and doctoral researcher at MIT. “With these five strains, any arithmetic operation can be constructed.”
Revolutionising Functional Distribution
The research team, led by Doost-Hosseini, utilised the bacterium Pantoea agglomerans, which is commonly found on plant surfaces, to create two types of transistors that can be activated and deactivated by a molecule called “OC-6”. Each transistor detects the presence of a target molecule, “OC-12”, and produces an output molecule, “OHC-14”, depending on the conditions present.
Three relay bacterial strains are responsible for converting the OHC-14 signal into an input for another transistor. This setup allows the transistors to be interconnected, much like components on a printed circuit board. For instance, a bidirectional switch can be constructed using two transistors that detect OC-12, which based on the input from the switch, can relay information to various relay strains. Subsequently, the signal is processed by another transistor.
In this instance, the researchers designed a circuit where signals are transmitted solely to the nearest colony, positioning bacterial colonies approximately 5 mm apart on an agar medium. This arrangement ensures that information flows in a single direction along the intended pathway as it is relayed from one colony to the next.
Achieving Complex Computational Logic
This innovative design has enabled the construction of circuits capable of executing different logical operations depending on the arrangement of bacterial colonies. Moreover, these circuits can process multiple signals simultaneously, featuring demultiplexers that distribute a single signal to multiple destinations based on a control signal. Among these, the most extensive circuit sums two inputs and comprises 24 interconnected bacterial colonies.
Traditionally, synthetic biology focused on consolidating all sensors and computational functions within a single cell. However, this approach posed challenges, such as the limited number of usable transcription factors and the excessive burden placed on the cell’s protein production mechanisms. By distributing functions across several cells and interlinking them, the researchers have managed to create more diverse circuits.
“We have demonstrated that by connecting simple functions of individual cells, more complex functionality can be achieved,” emphasises Christopher Voigt, the lead author and head of the Department of Biological Engineering at MIT. “From a computational perspective, there is nothing an iPhone can do that these circuits cannot.”
Envisioning a Future of Intelligent Plant Systems
Despite these advancements, the processing speed of such bacterial circuits remains far behind that of electronic computer circuits. Completing a single calculation takes approximately eight hours. Nonetheless, the research team does not view this slow processing time as a drawback. They argue that for biological applications, this perception of time is actually advantageous. “We do not aim to replace computers but rather to integrate computational control within biology,” Voigt clarifies. “If we consider that the bacteria present in plant roots or the plants themselves perform these calculations, we can conclude that an overnight timeframe for simple calculations is adequately swift in comparison to the entire growth period.”
In addition to this technology, the researchers envision coating the surfaces of plant leaves and roots with bacterial circuits. If realised, such circuits could detect environmental stressors, such as drought or pest infestations, and initiate an autonomous response. In agricultural contexts, for instance, these circuits could autonomously synthesise fungicides upon detecting stress.
While the biological circuit takes approximately eight hours to complete a calculation, this duration is not particularly slow within the agricultural domain, which operates on seasonal growth cycles. The day when plants can autonomously “calculate” their own states with the help of bacteria and make spontaneous decisions may be closer than we think.