Stage Four - Identification of host cells using gene markers

• After transformation, we must identify which bacterial cells have taken up the plasmid. This can be achieved by recognising that bacteria have evolved to produce an enzyme that breaks down antibiotics before they destroy the bacterium, giving them antibiotic resistance.
• The genes for the production of enzymes that break down antibiotics are found in bacterial plasmids. Some plasmids can even contain genes for resistance for multiple antibiotics. For example, the R-plasmid provides antibiotic resistance for both ampicillin and tetracycline.