To assemble circuits, the researchers printed bacterial colonies onto agar plates, positioning each colony about 5 millimeters from its nearest neighbor. That spacing ensures chemical signals reach only the next colony in the sequence, so information travels through the circuit in a single direction — much like wiring on an electronic board.

Why not put the whole circuit in one cell?

Synthetic biologists usually build circuits by engineering a single cell to produce proteins and transcription factors — molecules that turn genes on and off — that interact with each other. That approach works, but it has a built-in ceiling.

Each operation in a circuit typically needs its own dedicated transcription factor so that signals don't interfere with one another. Only a limited number of suitable transcription factors exist, and loading too many circuits into one cell can overwhelm its protein-making machinery.

The MIT team sidestepped that limit by distributing the work. Instead of cramming an entire circuit into one cell, they engineered individual cells as single components and wired them together in different arrangements.

How powerful are these circuits?

In the study, the same bacterial transistor performed different logic operations — "multi-input," "or," and "imply" gates — depending on where the researchers placed it in a circuit. By connecting multiple transistors, the team built systems that could:

• add two input signals together;
• process several signals simultaneously;
• route a single incoming signal to one of several destinations based on a separate control signal, a function known in electronics as a demultiplexer.

The largest circuit, containing those 24 printed colonies, was designed to add two inputs. The researchers also demonstrated a bidirectional switch that sends information through different relay strains depending on a separate switch input.

"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."

What are the limits?

Speed is the obvious one. Each calculation takes the bacterial circuits about eight hours to complete — vastly slower than any electronic computer. But the researchers argue that biology runs on a different clock.

"We're not trying to replace computers, but rather put computational control into biology. 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," Voigt said.

The agricultural application remains a long-term goal rather than a near-term product. The team has so far demonstrated its circuits only on agar plates in the lab, and the study does not yet show the system functioning on living plants in soil or field conditions. Scaling from a Petri dish to a root system will require further work.

The research received partial funding from the U.S. Defense Advanced Research Projects Agency and the U.S. Intelligence Advanced Research Projects Activity. Former MIT postdoc Haorong Chen is also an author of the paper.