Problem Set 2A
We have seen that the distinction between cells that adapt to a condition and grow, and
cells that mutate and can grow under new conditions is an important one. The problems
below address this difference and come from real-world examples.
Problem 1. Determine if the Lac+ cells generated in step 1 are mutants, or arose through
adaptation ("adaptants").
1. Take 109 Lac- E. coli cells and plate on M9+lactose plates. 1000 colonies develop. These
are either rare Lac+ mutants or rare Lac+ adaptants.
2. Take one colony and grow in 1 ml of M9 + glucose to 109 cells per ml.
3. Dilute and plate ~1000 cells on a M9 + glucose plate and a M9 + lactose plate:
M9 + glucose: 982 colonies form
M9 + lactose: 942 colonies form
3b. Do the same steps 2-3 for the orignal Lac- cells:
M9 + glucose: 1024 colonies form
M9 + lactose: 2 colonies form
4. Was the colony used in step 2 a Lac+ mutant or a Lac+ adaptant? What is the evidence for
your answer?
The Lac+ cells are mutants. We know this because the change to Lac+ is permanent,
i. e. was not reversed by growth in M9 + glucose in step 2. In addition, all of the cells that
descended from the Lac+ colony in step 2 have retained Lac+ phenotype (see step 3) like
we would expect if a genetic change was responsible for the conversion to Lac+. We saw
that in step 1, that if adaptation happens, it is rare and affects ~1 in every 1000 cells.
The 2 Lac+ colonies that formed in step 3b from the original Lac- strain are mutants
that can use lactose. Remember that this is expected to happen once in every 1000, or so,
cells.
Problem 2. Determine if the Ampr cells generated in step 1 are mutants, or arose through
adaptation ("adaptants").
1. Take 109 w.t. E. coli cells and plate on LB + Amp. 1000 colonies develop. These are either
rare Ampr mutants or rare Ampr adaptants.
2. Take one Ampr colony and grow in 1 ml of LB to 109 cells per ml.
3. Dilute and plate ~1000 cells on a LB plate and a LB + Amp plate:
LB: 987 colonies form
LB + Amp: 1 colony forms
3b. Do the same steps 2-3 for the orignal w.t. E. coli cells:
LB: 974 colonies form
LB + Amp: 0 colonies form
4. Was the colony used in step 2 a Ampr mutant or a Ampr adaptant? What is the evidence
for your answer?
It was an Ampr adaptant. We know this because its phenotype was reversed when it was
grown in LB in step 2. If it had been an Ampr mutant, all 1000 cells in step 3 would have
been Ampr and grown to give 1000 colonies.