The Effects of Ampicillin on the Growth of Escherichia coli

Khadija Lawrence and Michelle Anthony North Carolina State University, Department of Microbiology, 4510 Thomas Hall, Campus Box 7615 Raleigh, NC 27695 Abstract Escherichia coli is a Gram-negative, rod-shaped bacterium that normally inhabits the intestines of humans and animals. Not all strains of E. coli are pathogenic, but ones that are contribute to one of the leading causes of foodborne illnesses. (2) Given this information, it would helpful to know if an antibiotic such as Ampicillin has any effect on the growth of E. coli. Ampicillin is a beta-lactam antibiotic that attacks Gram-positive and some Gramnegative bacteria. The amino group in Ampicillin allows it to penetrate the outer membrane of Gramnegative bacteria. It will then become an inhibitor of transpeptidase, which is needed for bacterial cell wall formation, and eventually leads to cell lysis. (1) Figure 1: Ampicillin Structure (1) Methods/Materials Several preliminary experiments were used in order to determine if E. coli was sensitive to Ampicillin. Once this was successfully achieved varying amounts of liquid ampicillin were used to see if the IC50 could be determined. Throughout experiments, the amount of ampicillin added to samples ranged from 0-200 μL. Each sample mixture contained H20, low salt concentration LB and the inoculum, totaling 250 μL per sample. There were approximately 6-8 different samples used for each experiment. After making all sample mixtures, 200 μL of solution was loaded into wells to be measured in the Bioscreen C growth curve analyzer. Results Performing the experiment three times, a trend was discovered between the lag and log phase. Through graphing, as seen in Figure 2, the slope of the log phase was shown to be consistent throughout the varying amounts of ampicillin. However, the lag phase was most affected. As the amount of ampicillin was increased, the distance between the lag phases increased also in a correlating manner. At the higher concentrations, there appeared to be no growth because the lag phase was delayed so much. The graphical correlation of Ampicillin to the length of the lag phase can clearly be seen in Figure 2.