The left diagram shows a plate containing a jelly with one species of bacterium evenly spread through it. Placed on the jelly are three discs called P, Q and R. They have been treated as in the key.
The right diagram shows the plate 48 hours later.
Key:
P = disc with antibiotic at strength ¼X
Q = disc with antibiotic at strength ½X
R = disc with antibiotic at strength X
Which of the following is/are possible explanations for the result?
1 Antibiotic at strengths X and ½X are equally as effective.
2 Bacterial resistance to this antibiotic occurs at all three strengths.
3 S may represent the maximum distance the antibiotic has diffused out of the disc.

Answer:

13) E

1 – The fact that the outer circle produced by the antibiotics Q and R are very similar in size means that they could be equally as effective.

2 – Bacterial resistance is not mentioned in the question and is thus irrelevant. The fact of the matter is that the antibiotics are doing their job which means that there is hardly any resistance.

3 – S represents the distance up to which the bacteria are killed. This means that is statement could be correct.