Effects of different antibiotics on bacteria

Question 1

Independant variable

Answer 1

Antibiotic

Question 2

Dependant variable

Answer 2

diameter of inhibition zone

Question 3

Other variables to be controlled (6)

Answer 3

• Concentration of antibiotic
• Amount of antibiotic
• Disc size
• Bacterial species
• Temperature
• Ruler

Question 4

Equipment (6)

Answer 4

• Samples of different antibiotics on mast ring or filter paper discs
• Petri dishes
• Agar gel
• Disinfectant
• Bunsen burner
• Forceps
• Marker pen
• Adhesive tape
• incubator

Question 5

Outline the method

Answer 5

Wash hands. For this practical you will need to work in sterile conditions (aseptic technique) i.e. you will need to flame the forceps in the Bunsen after every use. Prepare an agar plate seeded with bacteria. Label the Petri dish on the base at the edge with your name, the date and the type of bacterium it is inoculated with. Flame the forceps and then use them to pick up an antibiotic disc or Mast ring. Raise the lid of the Petri dish and place the Mast ring firmly in the centre of the agar; if individual discs are used they will need to be spaced evenly around the dish. Tape the dish securely with two pieces of adhesive tape (but do not seal it completely), then keep it upside down at 30°C for 48 hours. After incubation, look carefully at the plate but do not open it. Where bacteria have grown the plate will look opaque, but where the antibiotics have inhibited growth, clear zones called inhibition zones will be seen. Measure the diameter of the inhibition zones in millimetres and use this information to decide which antibiotic is most effective at inhibiting the growth of the bacterium.

Question 6

Possible evaluation issues (5)

Answer 6

• Ensuring that the discs are placed evenly on the Petri dish
• Having good aseptic technique to prevent plate contamination
• Age of antibiotic, if the antibiotic used is out of date it is likely to be less effective
• Repeats
• Accuracy of incubation temperature and time