Detecting Double Mutants Resistant to both Streptomycin and Ampicillin

In this portion of the lab you will begin with an F (F minus) streptomycin-resistant culture of E. coli (“Strain 1” obtained from the first part of this lab) and an F’ (F prime) E. coli strain that is resistant to ampicillin (“Strain 2”). Each of these strains is resistant to only a single antibiotic. The goal of the experiment is to detect bacteria resistant to both antibiotics – these bacteria are the result of conjugation between the two aforementioned strains.

Each of these strains is inoculated on the surface of three different types of plates – TSA plus streptomycin, TSA plus ampicillin, and TSA plus both streptomycin and ampicillin. The two cultures are then mixed and then allowed to sit at room temperature for 15 minutes. During this time, conjugation should occur between these two strains. The mixture should then be inoculated on the surface of the same three types of plates. These nine plates are then incubated overnight at 37oC and inspected shortly thereafter.

The results of this portion of the experiment are shown below, and all six lab groups obtained the same results (six replications). Strain 1 was inoculated onto the plates of the leftmost column and strain 2 was inoculated onto the plates of the rightmost column. The mixture of the two strains was inoculate into the plates in the middle column. The top row of plates contains streptomycin, the center row of plates contains ampicillin, and the bottom row of plates contains both streptomycin and ampicillin.


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