For most of computing history, information has moved through wires, as electrons pushed along a circuit by voltage. Researchers at the Massachusetts Institute of Technology (MIT) have now built a version of that same logic using something far older than any transistor: living bacteria.The team, led by Christopher Voigt, head of MIT’s Department of Biological Engineering, has engineered bacteria that behave like transistors, the basic switching components found in every computer chip. When printed onto a Petri dish and arranged correctly, these bacterial colonies form what the researchers call living “circuit boards,” structures that do not carry electricity, but chemical signals.The study, led by Hamid Doosthosseini, an MIT postdoctoral researcher, was recently published in Nature Chemical Biology.
How a bacterium becomes a switch
In a conventional electrical circuit, a transistor works as a switch. It controls whether current flows or stops, depending on a third input signal. The MIT team recreated this same behaviour chemically, using a bacterium called Pantoea agglomerans, a species that naturally grows on surfaces, including plant leaves and roots.The researchers engineered two types of bacterial transistors. Both respond to a molecule called OC-6, but in opposite ways: one switches on when OC-6 is present, the other switches off. Each transistor also detects a second molecule, OC-12, which acts as the actual signal being processed. Depending on whether OC-12 is present, and whether the switch is active, the transistor produces an output molecule called OHC-14.That output does not travel far on its own. This is where the second part of the system comes in.
Relay strains do the wiring
Alongside the two transistors, the team built three additional bacterial strains that function as relays. Their job is to pick up the OHC-14 signal from one transistor and translate it into a form that can activate the next one, effectively wiring the colonies together the way a circuit board wires its components.Because a transistor and its neighbouring relay are printed only about five millimetres apart, signals travel to the nearest colony and no further. Information moves in one direction only, colony to colony, until it reaches its destination. With just two transistor types and three relay strains, the researchers say they can, in principle, build nearly any circuit.“We’ve built some initial computer architecture components that are commonly used, but any operation can be built with these five strains,” Doosthosseini said, according to MIT News.
What these circuits can already do
Using this five-strain toolkit, the team demonstrated circuits capable of several logic functions, including “or” and “imply” gates, along with more complex operations such as adding two or three inputs together, or acting as a demultiplexer, a circuit that takes a single incoming signal and directs it to one of several possible destinations based on a control input.The largest circuit built so far adds two inputs together and uses 24 bacterial colonies working in coordination.“This work shows that we can get toward more complicated functions by linking up simpler functions in individual cells,” Voigt said. “Computationally, there’s nothing that your iPhone can do that these circuits couldn’t do.”That claim is less about speed than about capability. Which brings up the obvious limitation.
Slow, but on biological time
A single calculation in this system takes about eight hours, compared to the fractions of a second an electronic circuit needs. For most technology, that delay would be disqualifying. But the researchers are not designing this for phones or laptops.“We’re not trying to replace computers, but rather put computational control into biology,” Voigt said. “If you have bacteria on the root of a plant, or the plant itself is doing the computing, running a simple calculation overnight is fast enough relative to a growth season.”A plant does not experience drought or pest attack in microseconds. It experiences these things over days and weeks. A circuit that takes overnight to compute a response is, on a plant’s timeline, still fast.
Why this could matter for agriculture
The intended application is not inside a lab, but on a farm. The researchers hope to eventually coat plant roots or leaves with these bacterial circuits, allowing the plant itself to carry a built-in sensing and response system. Once a circuit detects a specific stress condition, such as drought, it could trigger an output like the synthesis of a fungicide, without any external monitoring or intervention.This is a meaningful shift from most synthetic biology work, where researchers typically cram all the sensing and logic into one engineered cell. That approach runs into a hard ceiling: there are only so many transcription factors available before crosstalk and overburdening the cell’s protein machinery become a problem. By spreading the computation across multiple specialised strains instead of stacking it into one, the MIT team sidesteps that ceiling entirely.The research was funded in part by the US Defense Advanced Research Projects Agency and the US Intelligence Advanced Research Projects Activity, underscoring that the interest in engineered biological computation extends well beyond agriculture.
