Lab class two

Question 1

In the first prac, what were the antibiotics and bacteria

Answer 1

antibiotic: streptomycin suulfate
bacteria: staphylococcus aureus

Question 2

describe briefly what happened in the first experiment

Answer 2

concentration gradient of streptomycin sulfate was set up in one petri dish, the other had no antibiotic
growth measured
even growth on control
gradient on exp, however, some colonies even when the concentration was high

Question 3

in first prac, explain why there were colonies even at high concentrations.

Answer 3

• increasing conc of stre was a selective pressure
• spontaneous mutations conferring ste-R occurred.
• selective advantage - ie. ste-R s.a were more likely to survive and reproduce than those who didnt have the mutation.
• resulted in resistant colonies appearing even at high concs

Question 4

In the second prac, what were the antibiotics, plasmid and bacteria

Answer 4

bacteria: escherichia coli
antibiotic: ampicillin
plasmid: pUC18 DNA (ampicillin resistant)

Question 5

describe briefly what happened in the second expeirmnet

Answer 5

• e. coli cells treated to CaCl2 to enable competency
• 4 mixtures were made - buffer; buffer + DNase (cut plasmid) + plasmid; buffer + plasmid (diluted/undiluted)
• each was grown on either a plain or ampicillin infused petri dish.
• all grew on the control
• growth only occurred for buffer + plasmid on ampicillin petri dish; more growth for undiluted vs diluted.

Question 6

describe the two cell types in the third experiment

Answer 6

E. coli RS1917 was R+; resistant to streptomycin, tetracycline but not nalidixic acid; genes encoded on plsamid.
E. coli JP990 was R-; resistant to nalidixic acid but not streptomycin or tetracycline.

Question 7

describe what occurred in the third experiment.

Answer 7

petri dishes used:
-one antibiotic
-all antibiotics
-nalidixic acid + tetracycline OR streptomycin.
R+ and R- incubated.
R+, R-, and BOTH R+ and R- were put on each petri dish, and were incubated.

Question 8

describe what happened in terms of conjugation in the third experiment.

Answer 8

the donor cell’s sex pilus contacted a receptor on the recipient cell, bringing the cells together.
a conjugative bridge was formed
the donor cell’s plasmid was nicked, and one strand of it was transferred to the recipient cell.
both strands were repaired, and made double stranded again.
now both cells are F+.