1.    The independent variable for this experiment is the concentration of dettol disinfectant used – 25%, 50% and 75%. The dependent variable is the size of the inhibition zone which refers to the area free of bacteria). This is determined by measuring its diameter  using a ruler. The constants (control variables) are room temperature, amount of sunlight and the ingredients in the Petri dish agar.

2.    Prepare  12 Petri dishes of agar and store them in the  refrigerator.

3.    Three Petri dishes are taken out of the refrigerator and allowed to attain room temperature. Label the dishes  - 25%, 50% and 75%.

4.    Mix the dettol disinfectant with  water in the test tube and  label them as follows :
a.    25% - 25ml of dettol disinfectant mixed with 75ml of water
b.    50% - 50ml of dettol disinfectant mixed with 50ml of water
c.    75% - 75ml of dettol disinfectant mixed with 25ml of water

5.    Mix the fourth test tube with 10ml of E. Coli bacterium and 40ml of water

6.    Punch out circular discs of filter paper  using the hole puncher. The discs are separated into three groups and also labeled as 25%, 50% and 75%.

7.    Wash the swab  using the disinfected water and dip it into the test tube containing the bacteria culture. The swab is then gently rubbed  all over the agar surface of the 3 Petri dishes. It is important to ensure that you cover the entire surface of the petri dish thoroughly. To do this, you should swab in at least 3 directions.

8.    Pick up the filter paper discs  using the forceps and dipthem into the test tubes containing the different concentrations of dettol. They are then removed from the test tubes and placed in the middle of the Petri dishes according to how they are labeled. . Cover the Petri dishes and keep them in a cool shaded place for the next 2 days for the bacteria to grow.

9.    After 2 days there will be a circular area on the agar called the inhibition zone which is the area surrounding the paper disc without any bacteria growth. The diameter of this area is measured and recorded in the table provided below. The concept of the inhibition zone is attributed to the Kirby-Bauer disk diffusion method. This method is used by microbiologists to assess the effectiveness of antimicrobial agents. As the dettol diffuses out from the filter paper into the agar, there will be a higher concentration of dettol nearer the disk. This concentration decreases gradually, as the distance from the edge of the disc increases. As long as the concentration of dettol at any given distance is high enough to properly act as an antibacterial agent, no bacteria will grow at that location. Hence, if the bacteria has increased resistance, a smaller inhibition zone will be seen, and vice versa.

10.    The bacteria living around the border of the inhibition zone is collected using a swab and used to create the next batch of bacteria culture as explained in step 5.

11.    Steps 3 to 10 are repeated another 3 times using the bacteria culture harvested according to step 10 and the results observed are  recorded in the table provided below.