Principle of the externally tunable bacterial band-pass filter. a, Schematic representation of the plasmid map and gene circuit relationships. BLA expression plasmid (pDIM-C8-X) and reporter plasmid (pTS1) are compatible. In the absence of sufficient cellular β-lactamase (BLA) activity for hydrolysis of ampicillin (Amp), cell wall synthesis is compromised and cells cannot proliferate. In addition, murein breakdown results in the accumulation of aM-pentapeptide. aM-pentapeptide induces the ampC promoter via interactions with AmpR [11, 12] resulting in the expression of tetC (which confers tetracycline resistance) and gfp (which encodes the green fluorescent protein). However, the level of Amp necessary to induce ampC is lower than the level that prevents the growth of E. coli cells. BLA gene expression is regulated through IPTG-induction of the tac promoter that is repressed by LacI. b, Fluorescence and band-pass growth behaviour on agar plates of E. coli cells that have the band-pass filter circuitry. When cells are spread evenly on plates containing Tet/IPTG and an Amp gradient radiating from the center of the plate, a ring of growth and fluorescence appears. c, The size of the ring radius (defined as the radius of highest fluorescence) depends on the ratio of the amount of Amp added in the center and the β-lactamase activity of the cells. β-lactamase activity is expressed here as the MIC of Amp in liquid culture . Strains are defined in Additional file 1. The MIC for a strain was set by the concentration of IPTG.