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Investigating Bacterial Growth

Investigating Bacterial Growth

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Science Fair Project Description

The purpose of this experiment is to find out whether bacterial growth is inhibited by either strong acids or strong alkalis, or even both. It is important for us to know about bacteria inhibition in order to develop more efficient antimicrobials.
Complexity level:7
Project cost ($):20
Time required:1 hour to prepare, 24 hours incubation period, 30 minutes to observe and record findings.
Material availability:All materials should be provided by the biology laboratory. However, students should prepare their own goggles and lab coat.
Safety concerns:Safety concerns: All biohazard material should be disposed into the sharps bin. Students must put on safety goggles and lab coat at all times throughout this experiment. Teachers must be on standby especially when students are removing the bacteria culture out of the bottle. Disinfectants must also be prepared in case of any bacteria culture spills.


The stronger the alkali or acid, the higher the inhibition of bacterial growth.


What is acid/alkali?

An acidic solution contains a higher than usual proportion of Hydrogen ions (H+), while an alkaline solution contains a higher than usual proportion of Hydroxide ions (OH-). This occurs because compounds can exist as free moving ions in liquid state.

pH is a type of scale used to measure the acidity or basicity (alkalinity) of a solution. It ranges from 1 to 14. As the pH of a solution decreases, it becomes more acidic. When the pH increases, the solution becomes more alkaline. Hence, When an acid is dissolved in water, the pH will be less than 7 and when an alkali is dissolved in water, the pH will be greater than 7. Pure water has an acidity of about 7.

What are Bacteria?

A bacterium is a unicellular organism, and found in all shapes and sizes. They can be round, rods, or spirals. Though Anonie van Leeuwenhoek first observed them in 1676, the hardy bacteria have been around for billions of years. As the first organism to have life on earth, the bacteria has evolved over the years and there are now approximately five nonillion (5 x 10^30) bacteria on earth.

Bacteria can be beneficial to man. Louis Pasteur showed how bacteria can be used to create vaccines and has paved the way for medical progress and breakthroughs.

Scientific Terms

Microccus luteus, Serratia marcesceus, ions, pH


For this experiment, you will need:

  • Microccus luteus and Serratia marcesceus culture
  • 14 Petri dishes filled with blood agar
  • 2%, 4% and 8% concentrated solution of acid
  • 2%, 4% and 8% concentrated solution of base
  • 12 Paper discs
  • 2 Syringes
  • Incubator
  • Goggles
  • Gloves
  • Lab coat


1.    For this experiment, the independent variable is the concentration of acid or alkali used. The dependent variable for this experiment is the diameter of the zone of inhibition of the bacteria on 2 types of bacteria cultures. In this experiment, there are a few aspects that are kept constant. For example, the amount of bacteria, the type and amount of blood agar, the environmental conditions for incubation and the length of time of incubation.

2.    Prepare concentrations of 2%, 4% and 8% acids in different containers respectively.

3.    Next, prepare concentrations of 2%, 4% and 8% bases in different containers respectively.

4.    Soak each concentration of acids and bases with 2 paper discs each.

5.    Prepare sterile syringe to remove the bacteria culture.

6.    Carefully open the cover of the bottle containing the Microccus luteus  bacteria culture, make sure the culture is not spilled. Once spilled, disinfect the spill immediately with disinfectants and wipe it with disposable paper rolls.

7.    Withdraw 5 ml of Microccus luteus  bacteria culture into the syringe.

8.    Inject the sample of Microccus luteus bacteria intermittently around the Petri dish. Try to spread it out as evenly as possible.

9.    Repeat step 8 for 6 more Petri dishes.

10.    Label these dishes accordingly:
a.    Microccus luteus – 2% acid
b.    Microccus luteus – 4% acid
c.    Microccus luteus – 8% acid
d.    Microccus luteus – 2% alkali
e.     Microccus luteus – 4% alkali
f.    Microccus luteus – 8% alkali
g.    Microccus luteus

11.    After that, place paper discs soaked with different concentrations of acids or alkali into each Petri dish according to the labels.

12.    Leave the 7th dish untouched. This is a control specimen

13.    Repeat Step 6 through 12 with the Serratia marcesceus bacteria culture.

14.    Label the Petri dishes accordingly

15.    Place the Petri dishes into the incubator for 24 hours set at 37 degrees Celsius.



The results obtained are recorded in the table below.


Microccus Luteus
Concentration of acid (%) Diameter of zone of inhibition (mm) Concentration of alkaline (%) Diameter of zone of inhibition (mm)
2 3.3 2 2.4
4 5.5 4 4.6
8 10.1 8 10

 Control for Microccus Luteus: 0mm 

Serratia marcesceus
Concentration of acid (%) Diameter of zone of inhibition (mm) Concentration of alkaline (%) Diameter of zone of inhibition (mm)
2 4.6 2 3.1
4 6.7 4 3.7
8 12.3 8 7.8



he results of the experiment show that the hypothesis should be accepted. The higher the concentration of acid/alkali, the more bacteria it kills.
This experiment is useful because it tells us what conditions which are not suitable for bacteria survival. Scientists can develop alternative methods to eradicate harmful bacteria once they know about this experiment.

Also consider

Will all bacteria react the same way to acid and alkali?
What other methods are able to kill bacteria?
How can you minimize the effect of random error in this experiment?
The following bacteria types can be used as alternatives for this experiment:
-Clostridium Perdringens
-Clostridium Botulinum
-Mycobacterium Tuberculosis


Related videos

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