|
|
+ growth -- no growth
Table #1: E.coli strains and growth results
Strain I |
Strain I |
Strain II |
Strain II |
|
| Contents of Plates | Expected |
Observed |
Expected |
Observed |
| LB agar | + | + | + | + |
| LB agar + str | + | + | -- | -- |
| LB agar + amp | -- | -- | + | + |
| LB agar + amp + str | -- | -- | -- | -- |
| LB agar + nal | -- | -- | + | + |
Reasons: Growth was expected for the LB agar plates because there was no antibiotic mixed in with the agar. Strain I was resistant to streptomycin, so growth was expected on the agar with streptomycin. Strain II, on the other hand, was resistant to ampicillin, so there was growth on that plate, not on the streptomycin plate. Both strains would not grow of the plate with both antibiotics because each strain would be sensitive to one of the antibiotics. On the naladixic acid plate, the ampicillin resistant strain grew because it had a chromosomal resistance to the naladixic acid.
Table #2: Growth on mating plate (LB agar + amp +str)
| Region | Expected |
Observed |
| I (cells of strain I) | -- | -- |
| II (cells of strain II) | -- | -- |
| III (cells of strain I & II --mating) | + | + |
Table #3: Recombinant Cell Growth (assuming strain II is donor)
Expected |
Expected |
Observed |
Observed |
|
| Contents of Plate | I |
II |
I |
II |
| LB agar + str | + | -- | + | -- |
| LB agar + amp | + | + | + | + |
| LB agar + str + amp | + | -- | + | -- |
| LB agar + nal | -- | + | -- | + |
1. Strain I was the strain that was sensitive to amp, but did have the resistance to str. Because it was sensitive to the amp, it could not grow on that plate.
2. Strain II was resistant to ampicillin, but not to str. Because of the sensitivity to str, it would not grow on that plate.
3. Each strain was sensitive to at least one of the antibiotics in the agar that contained both, so, even if the antibiotic that it was resistant to was present, there was still another antibiotic to prevent the growth on that plate.
4. On regions I and II, where the individual strain were kept separate, there was no growth expected, because that plate contained both str and amp. In region III, however, there should be some growth of one of the strains (more than likely the str resistant, because it would have received the plasmid from the amp resistant strain, therefore making it resistant to both antibiotics, but the amp resistant strain would not have received the chromosome from the str resistant strain to make it resistant to both too).
5. Yes, it does. Because the amp resistance is granted by a plasmid, it is easily given to another strain (versus that which is granted by a chromosome). There was growth on the amp/str plate, so that must mean that, through conjugation, one strain must have gotten another resistance (from either the chromosome or plasmid). That means that that particular strain would then be resistant to both antibiotics and would be able to grow on the plate containing both.
6. It is more likely that the resistance granted was received by the plasmid (meaning that the amp resistant strain gave it resistance to the str resistant strain, making the str resistant strain now immune to both antibiotics). Transferring plasmids is much easier (and more probable) than the transfer of a whole chromosome. So this experiment shows that this resistance is transferred through the plasmid rather than the whole chromosome.