Antimicrobial Effectiveness in Bacterial Growth

Categories: Biology


Only 1% of bacteria are harmful for humans however there are methods of preventing the spread of the harmful bacteria. Chemical substances that are used to either kill or inhibit growth of bacteria are known as antimicrobial agents, of which the three basic types are disinfectants, antiseptics and antibiotics. Disinfectants are chemicals used to inhibit growth or kill bacteria on living tissues, antiseptics inhibit growth or kill bacteria on nonliving things and antibiotics are compounds that block the growth and reproduction of bacteria.

Although microorganisms can develop a resistance to these agents, the effectiveness depends on factors such as environmental conditions etc.


Since the effectiveness varies, if environmental factors and other conditions were kept the same, the antibiotic would be most effective.


  • 1 antibiotic disk (Erythromycin)
  • Distilled water
  • Culture of e-coli (Escherichia coli)
  • 1 sterile filter-paper disk
  • Transparent tape
  • 1 sterile nutrient agar plate
  • Metric ruler
  • Forceps
  • 1 sterile inoculating loop.


Part A: Set up of Nutrient Agar plate

  1. Get an agar plate and carefully turn it over (being careful to not open the dish)
  2. Draw two lines (forming right angles) to separate the dish into quarters.

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    Number the quadrants 1-4 on the edges only. Turn the dish back up. DO NOT SPILL.

  3. Insert inoculating loop by lifting top lid slightly (at an angle).
  4. Swipe the loop over the bacteria culture. DO NOT DIG INTO CULTURE.
  5. Slightly open nutrient agar plate and place tip of inoculating loop on the top center of agar and steak the agar. Turn dish 90o and repeat.

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    Immediately dispose loop.

Part B: Placement of Antimicrobials

  1. Select three antimicrobials
  2. Slightly open inoculated agar plate. Place a disk in the center of one of the quadrants using forceps gently until it sticks and then close petri dish.
  3. Repeat step two with the disks of the other two antimicrobials. Place an empty disk in the remaining quadrant (control).
  4. Tape the petri dish to secure tightly.

Part C: Results

  1. Observe the dish after 48 hours
  2. Measure the area clear of bacteria (the clear area is the zone of inhabitance) using a metric ruler.
  3. Record results and return the dish to teacher for proper disposal. It is important not to open the sterile agar plates so that the bacteria don’t spill and avoid contamination. Its important to only write on the edges of the plate so that it is easy to see the areas needed for observation It is important to use sterile techniques while inoculating the agar plates so that bacteria don’t get on anything nor are added in the experiment. Control disks are used in order to have a basis for comparison. It is important to tape the dish in order to avoid spills and contamination of objects by the bacteria.


We used a control disk in order to base the results on something in order to compare the effectiveness of the antimicrobials. Without the control, we would have no basis for comparison. The antibiotics did not do as good of a job as expected because they need to be continuously taken for a set period of time. Due to this reason, doctors subscribe antibiotics for 10 days and if use is stopped before that period, they can easily divide and multiply. The school bathroom soap showed the best results because it had the most amount of clear area. The use of a control showed that the inhabitance of bacteria observed was due to the antibiotic on the disk rather than the disk itself. If a serious staphylococcus infection developed in the locker room of the school gym and I were in charge, I would take a sample of the bacteria and repeat this experiment in order to find the most effective disinfectant.

I think that antibiotics seem to lose their effectiveness against a particular population of bacteria after a prolonged period of time because they adapt to the change and mutate overtime. It is important to take an antibiotic for the exact period of time it is prescribed because the bacteria could be dormant and if not completely killed, they can easily reproduce and make the person ill again. If I were infected with E-coli I would want to be prescribed a stronger antibiotic than erythromycin because it did not give the best result in the experiment.


All antimicrobials used inhibited the growth of the bacteria to some extent however the school bathroom soap showed the best results and inhibited growth to the furthest extent, which rejected my hypothesis. Although it did inhibit growth better than others, it wasn’t as effective as the school bathroom soap. I thought the antibiotic would work best, as it’s the most common used antimicrobial in hospitals however because it was only used once, it was quite ineffective. The hand sanitizer was least effective as it barely inhibited growth. In the end the hand sanitizer and antibiotic disks were both semi cloudy whereas the bathroom soap was clear. In conclusion the bathroom soap worked much better than I expected and I would surely use it before and after lunch in order to avoid being infected.

Updated: Jan 03, 2024
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Antimicrobial Effectiveness in Bacterial Growth. (2016, Mar 31). Retrieved from

Antimicrobial Effectiveness in Bacterial Growth essay
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