Effect of different antibiotics


The effectiveness of different antibiotics on a specific bacteria can be investigated in the

following practical. Antibiotics kill bacteria or slow their growth and their effectiveness

and this will be shown by the clear area produced around the sample of each antibiotic.


Independent variable: Type of antibiotic used

Dependent variable: Area of inhibition zone


Equipment list

  • 5 different types of antibiotics
  • Agar plate
  • Filter paper cut into 6 equally sized discs
  • Sterile forceps
  • Disinfectant
  • Known bacterial culture
  • Sterile pipette
  • Sterile water
  • Sterile plastic spreader
  • Bunsen burner
  • Ruler
  • Tape


Method

1. Before starting the experiment disinfect the workbench, this helps to prevent

contamination when preparing the bacteria culture.

2. Mark on the agar plate the date, your initials and the bacteria being grown. Also mark

evenly across the plate 6 letters A-F to show where to place the different antibiotic discs.




3. Use a bunsen burner to flame the bottle of the neck of the bacterial flask causing the air to

rise and carry away undesired airborne microorganisms and then use the sterile pipette to

transfer 2 cm3 of the liquid broth to the agar plate.

4. Use the sterile plastic spreader to evenly distribute the bacteria across the agar jelly.

5. Take the first filter paper disc and use the forceps to completely submerge it in the first

antibiotic for 10 seconds. Slightly lift the lid of the agar plate at a angle allowing you to

quickly transfer the disc to the agar jelly.

6. Repeat step 5 for the other 4 antibiotics as well as a control disc soaked in sterile water,

sterilising the forceps in between each antibiotic by holding them in the bunsen burner

flame.

7. Tape the lid on at 4 points making it secure enough but also allowing oxygen to enter so

the bacteria may respire aerobically. Then incubate the dish at 25 °C for 48 hours.

8. Disinfect the work surface again and wash your hands.

9. After the 48 hours have passed use a ruler to measure the diameter of the inhibition zone

(clear zone) created around each disc.


Tips to minimise contamination

  • Have the lid removed from the agar plate for as little time as possible.
  • Work near the bunsen burner flame as this helps to sterilise the air by heating and raising

air containing potential contaminants such as other microorganisms.




Conclusion

The effectiveness of each antibiotic can be compared by looking at the area of the inhibition

zones created. The one with the largest inhibition zone has killed the most bacteria and is

therefore the most effective. The disc dipped in water acts as a control to show that the antibiotics

alone are causing the death of bacteria and not any other factor.