Investigating the effect of two different antibiotics on bacterial growth

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Investigating the effect of two different antibiotics on bacterial growth


Antibiotics are drugs that kill bacteria by inhibiting their growth and slowing down their rate of division. There are many different variations of antibiotics each one designed to kill bacteria which causes infection within the human body. In this investigation two unknown antibiotics will be used on ecoli bacteria, different concentrations of both antibiotics will be used to determine the effectiveness on the bacteria by assessing the size of the clear zone.

Hypothesis: There will be no significant difference in the zone of inhibition by using known concentrations of unknown antibiotics on the agar plate.

The dependent variable that will be measured in the experiment will be the size of the clear zone in mm and the independent variable will be the different concentrations of antibiotics and the type of antibiotic due to testing the growth of bacterial growth with two unknown antibiotics.


  • 2 petri dishes
  • 2 bottles (Molten agar and bacterial culture)
  • 3 syringes 5ml x2 and 10ml x1
  • 4 medium sized bottles
  • 8 Sterile paper discs
  • 2 unknown antibiotics
  • Bunsen Burner
  • Pure water
  • Heat proof mat
  • Tweezers
  • Marker Pen
  • Scissors
  • Sellotape  
  • Cleaning cloth
  • Disinfectant spray
  • Incubator set at 25 degrees Celsius


In the experiment the same size paper discs were used, an increase or decrease in the paper disc size will affect the surface area of the paper disc. For e.g. if the size of the paper disc is larger than normal this will lead to more antibiotic being absorbed as there is a larger surface area, resulting in a greater clear zone produced as more bacteria growth will be prevented. Moreover, another important factor was the temperature of the incubator. All the petri dishes were placed at 25 degrees Celsius ensuring the growth of bacteria was not affected. In-addition, the same volume of bacterial culture was added to each the molten culture, if different volume of bacterial culture is used this will affect how effectiveness of the antibiotic, resulting in a changed inhibition zone.  In-addition, the same type of bacterial culture for each petri dish, this will ensure any effect on the clear zone will be due to the antibiotics, if a different type of bacterial culture was used then there is a possibility the bacteria may be resistant to the antibiotic therefore this will affect the clear zones and the overall results, leading them not to be valid.

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Starting the investigation, the working area was first sprayed with disinfectant and then wiped with a cloth, this was done to kill bacteria on the surface that may lead to contamination. The Bunsen burner was then setup and placed on top of a heat proof mat around the working area ensuring a sterile field to prevent contamination when working with petri dishes, antibiotics and culture medium. A bottle of molten agar was then opened and the neck of the bottle flamed, 1ml of bacterial culture was then mixed with bottle of molten agar and was flamed ...

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