Genetically engineered bacteria. Petri dish containing genetically altered Escherichia coli (E. coli) bacteria. The dish contains two forms of the bacteria, 'sender' cells (centre), which produce chemical signals and 'receiver' cells (green, red) that have been genetically engineered to respond to the chemical signals by producing fluorescent proteins. The receiver cells are grown in patterns surrounding the sender cells. The colour of the fluorescence is dependent on the concentration of the chemical signal. This biological system is designed in a similar manner to electronic circuit design, a discipline known as synthetic biology. Such systems could one day be used to create new drugs, biosensors, biological computers or diagnose diseases.