Chapter 7, Problem 6EQ

In your laboratory, you have an ${\text{F}}^{-}$ strain of E. coli that is resistant to streptomycin and is unable to metabolize lactose, but it can metabolize glucose. Therefore, this strain can grow on a medium that contains glucose and streptomycin, but it cannot grow on a medium containing lactose. A researcher has sent you two E. coli strains in two separate tubes. One strain, let's call it strain A, has an F′ factor (an F prime factor) that carries the genes that are required for lactose metabolism. On its chromosome, it also has the genes that are required for glucose metabolism. However, it is sensitive to streptomycin. This strain can grow on a medium containing lactose or glucose, but it cannot grow if streptomycin is added to the medium. The second strain, let's call it strain B, is an ${\text{F}}^{-}$ strain. On its chromosome, it has the genes that are required for lactose and glucose metabolism. Strain B is also sensitive to streptomycin. Unfortunately, when strains A and B were sent to you, the labels had fallen off the tubes. Describe how you could determine which tubes contain strain A and strain B.