An Undergraduate Laboratory Exploring Mutational Mechanisms in Escherichia coli Based on the Luria-Delbrück Experiment

Understanding the mechanism for DNA mutations is a key concept in most genetics and microbiology courses. In addition, understanding that most mutations occur prior to exposure to selection is an important yet often difficult concept for students to grasp. We developed an undergraduate laboratory activity on mutation mechanisms based on the classic experiment from Luria and Delbrück. The activity uses Escherichia coli as the model organism and the antibiotic streptomycin for selection. Students gain hands-on experience with an important experiment in genetics, and the laboratory contains an investigative component in having students calculate mutation rate for streptomycin resistance and in having the students design a follow-up experiment. E. coli has a knockout collection available, and we used a wild-type strain and a ΔmutS strain in the laboratory exercise. The ΔmutS strain is missing an enzyme in the mismatch repair pathway, and students calculate and compare the mutation rate and frequency for both the wild type and the knockout strain. Assessment of student learning showed that students had a significant gain in understanding of mutational mechanisms. An optional, additional experiment involving PCR and DNA sequencing of streptomycin-resistant mutants is also presented.