MIT researchers have developed a novel approach to computing by engineering bacteria to function like transistors, the basic building blocks of electronic circuits. These living transistors can be printed onto a growth medium inside a Petri dish, forming what the team calls "living circuit boards." The bacteria regulate the movement of small signaling molecules that carry information, much like electrical signals in traditional circuits. The team created two types of bacterial transistors and three relay strains that help transfer signals between them. Together, these five strains form a modular system that can be arranged to build various types of circuits. The bacteria used in the study, Pantoea agglomerans, is a common surface-dwelling microbe often found on plants. The researchers engineered two versions of bacterial transistors that respond to a molecule called OC 6. One version activates when it detects OC 6, while the other deactivates. Both transistors also respond to a second molecule, OC 12. Depending on whether OC 12 is present and whether the transistor is activated, the bacteria produce an output molecule called OHC 14. This output can then be used as an input for other components in the circuit. To connect different parts of the circuit, the researchers engineered three relay strains of Pantoea agglomerans. These cells take the OHC 14 signal and convert it into a new output that can be used by the next transistor. This allows the system to function like a traditional electronic circuit, with individual components "wired" together. For example, the team created a bidirectional switch using two transistors that detect OC 12. Depending on an external switch input, the system can direct information through different relay strains before sending it to additional transistors for further processing. To construct the circuits, the researchers printed bacterial colonies onto agar plates, which serve as a growth medium. Each colony was placed about 5 millimeters apart to ensure that chemical signals only reach the next colony in the sequence. This controlled spacing allows information to flow through the circuit in a single direction, much like electricity moving through a circuit board. The researchers demonstrated that the same bacterial transistor can perform different logic operations depending on its position in the circuit. These included "multi-input," "or," and "imply" gates. By linking multiple transistors, the team created more complex systems, such as circuits that could add two signals together, process multiple signals simultaneously, or act as a demultiplexer, which routes a single signal to one of several destinations based on a control input. The most complex circuit built in the study involved 24 interconnected bacterial colonies and was designed to add two inputs together. The research was supported by the U.S. Defense Advanced Research Projects Agency and the U.S. Intelligence Advanced Research Projects Activity, and the findings were recently published in the journal Nature Chemical Biology.