The researchers designed a genetic switch to conditionally control and track gene expression in bacteria. The switch uses the tetracycline repressor system, where the presence of the inducer anhydrotetracycline (aTc) allows expression of a gene of interest. They constructed a plasmid with constitutive red fluorescent protein expression and aTc-inducible green fluorescent protein. Initial versions showed poor repression, but optimizing the ribosome binding site of the tetracycline repressor gene yielded a functional switch with visible induction of green fluorescence with aTc. This switch will allow studying effects of gene expression on bacterial behaviors in future experiments.