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.

1. Question:
   

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

      [3]
      Question:
      

   2. 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]
      Question:
      

   3. 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]
      
   Question:
   

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

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