Detecting Streptomycin-Resistant Mutants

This portion of the lab uses the same broth culture of antibiotic-sensitive E. coli that has been grown overnight, and the goal of this portion of the lab is to find any rare mutants that are resistant to streptomycin.

One milliliter of this overnight culture is transferred into a pair of TSA plates (0.5 ml per plate) containing the antibiotic streptomycin and spread evenly upon the surface of these plates. Only streptomycin-resistant bacteria can survive on these plates. The plates are then incubated at 37oC overnight and inspected shortly thereafter.

Paired plates from thirteen cultures were assessed in the laboratory this semester; eleven of these pairs of plates contained no antibiotic-resistant colonies and two pairs of plates contained colonies of streptomycin-resistant bacteria. One of these pairs of plates contained only a single colony, and the other pair of plates contained six streptomycin-resistant colonies.

From this data and the data in the previous section you should be able to calculate the mutation rate, which is defined as the number of mutations conferring streptomycin-resistance divided by the total number of bacteria. (The total number of bacteria is equal to the total number of cell divisions.)


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