Antibiotic sensitivity tests can be carried out to choose appropriate antibiotics to use for treatment of bacterial diseases. A researcher carried out an antibiotic sensitivity test using two pathogenic bacteria, X and Y. The researcher prepared two Petri dishes containing agar. • A culture of each bacterium was spread over the surface of the agar. • Filter paper discs containing antibiotics were placed on the surface of the agar in each dish. • The Petri dishes were incubated at 25 °C for two days. The results of the test using three antibiotics, P, Q and R, are shown in Fig. 4.1. [Figure 4.1]