The Effects of Ampicillin versus Tetracycline on the Plasmid Copy Numbers of pBR322

The pBR322 plasmid contains genes encoding for tetracycline and ampicillin resistance. Escherichia coli DH5α cells with pBR322 plasmid were grown and plated on selective plates containing 1) ampicillin only and 2) tetracycline only with varying antibiotic concentrations of half strength, three-fourth strength and full strength (15 μg/mL tetracycline and 50 μg/mL ampicillin for full strength). Colonies from the plates were grown overnight in Luria Broth (LB) and the cells were used for plasmid isolation. Gel electrophoresis was done on the isolated plasmids and pixel counts in the digital gel images were analyzed by Kodak 1D to determine the intensity of the bands on the gel. It was found that ampicillin selection led to a higher digital pixel counts and copy numbers of plasmids per cell than tetracycline selection. Furthermore, there seemed to be a correlation that there was an increase in plasmid copy numbers with an increasing antibiotic concentration.