Bacterial antibiotic resistance. A loop of bacterial DNA called a plasmid (circle), carries genes for resistance to the antibiotics tetracycline and ampicillin. Using restriction enzymes (blue) the tetracycline resistance is disrupted by inserting a gene (red). The plasmid with disrupted resistance to tetracycline (but functional ampicillin resistance, represented as green) is replicated and transmitted amongst the bacteria. Addition of ampicillin antibiotic kills the bacterial colonies which do not carry the plasmid. Subsequent treatment with tetracycline kills only the colonies which contain the plasmid with disrupted tetracycline resistance.