Antibiotics

A drop of a solution containing Escherichia coli bacteria was spread over the surface of nutrient agar in a petri dish, labelled dish A (see Fig. 2 below). Each cell in the original drop reproduced to form a colony of cells on the agar surface.

A sterile wooden cylinder, over which a sterile felt cloth had been stretched, was lightly pressed onto the surface of the agar in dish A so that cells were picked up from each E. coli colony. This procedure is shown in Fig. 1.

The felt covered cylinder was then pressed onto the surface of sterile nutrient agar in several petri dishes, such as B and C shown in Fig. 2. Cells from each colony on dish A were transferred to corresponding positions on the new dishes. Some of these new dishes, such as C, had an antibiotic, streptomycin, added to the nutrient agar.

Typical results are shown in Fig. 2.

Question:

With reference to Fig. 2, explain the results shown by dishes B and C;

[3]

Answer:

the colonies on dishes A and B are in the same positions because cell from each colony on A have reproduced and formed a colony on B;

a small number of the many cells picked up by the felt were resistant to streptomycin;

ref. to mutation;

resistant / mutant cells had been transferred to the streptomycin agar in C;

these cells had reproduced to form three colonies;

other cells transferred to dish C did not reproduce / were killed;

ref to selection;

Question:

With reference to Fig 2, explain why a swab taken from area Y on dish A would be likely to include streptomycin resistant cells, while the probability of finding streptomycin resistant cells in area X is low;

[1]

Answer:

resistant colony developed from Y but not X;

mutation is a rare event;

unlikely that a new mutation occurred during transfer of cells from area X to dish C;

Question:

With reference to Fig. 2, explain why the swab taken from area Y would be unlikely to include cells resistant to the antibiotic penicillin.

[2]

Answer:

the antibiotics are chemically different;

different genes would be involved in resistance;

unlikely that both genes would mutate in cells from the same / neighbouring colony;

Question:

Suggest some implications for users of antibiotics, of the experiment described above.

[2]

Answer:

a large population of bacteria is likely to contain a few mutant cell that are resistant to any antibiotic;

use of an antibiotic will create a selection pressure;

kills sensitive cells while resistant ones survive and reproduce;

antibiotics should not be used in routine / trivial / casual way;

ref to antibiotic resistance in named bacteria;

antibiotics may become, useless / ineffective;

antibiotics should be rotated;

kept for use as last resort;

[Total : 8]