Question

Assume that you transform bacteria with a plasmid containing an ampicillin-resistance gene. Instead of directly plating the transformed population as you did in this lab, you set up two liquid cultures of them, one that contains ampicillin and one that does not. You will then assay these cultures on plates at two different times: immediately after you set up the cultures, and then again after the bacteria have been in culture for an extended period. The assays will demonstrate the number of ampicillin-resistant vs. ampicillin-sensitive culture bacteria in each culture at each time. To perform each of the two assays, you prepare serial dilutions of the two cultures and plate them onto LB plates with and without ampicillin (the dilution) is simply to ensure that you will get some plates on which you can distinguish separate colonies).
Describe what you expect to observe in the initial assay and in the second assay. What, if any, differences might you expect in terms of the ratios of ampicillin-resistant and nonresistant bacteria?

Answer 1

In the initial assay with ampicillin there will still be, although in a smaller number, nonresistant bacteria that have survived incubation. After an extended period, ampicillin will kill all nonresistant bacteria and in the second assay, only ampicillin-resistant bacteria will survive.