What Is the Effect of Different Shampoos on the Growth of Bacteria in the Hair?

Research Question

What is the effect of different shampoos on the growth of bacteria in the hair?



  • 9 Petri dishes
  • Agar gel
  • Cotton buds
  • Three different shampoos
  • Test tubes
  • Droppers
  • Test tube rack
  • Labels

Independent variable

Different types of shampoos

Dependent variable

The rate of growth of bacteria on Petri dishes: number of bacterial colonies

Controlled variables

The volume of the shampoos


  1. Rub your hands thoroughly in the first volunteer’s hair to ensure that the required bacteria are attached to the hands; 5-6 rubs will be sufficient.
  2. The bacteria collected should be smeared on the first agar plate 5-6 times and labeled correctly.
  3. Add three drops of the shampoo to the Petri dish containing smeared bacteria.
  4. This procedure should be repeated for the other two volunteers.
  5. Different types of shampoos should be collected and put in their respective test tubes.
  6. All the shampoos should be put under the same temperature in their respective test tubes (40 oC).
  7. The bacterial cultures should be put in the incubators at the same temperatures.
  8. Measure 2 ml of shampoos to be added on the growing bacteria on the Petri dishes.
  9. The volume of the shampoos to be added on the growing bacteria has to be kept constant through out the experiment.
  10. Each test tube should have its own dropper to prevent mixing of the contents in the test tubes.
  11. All test tubes containing the three different shampoos should be well labeled.
  12. The Petri dish and its contents should be kept into the incubator for them to start growing.
  13. All the Petri dishes containing the growing bacteria should be well labeled to avoiding mixing them up and creating confusion.
  14. Place the first Petri dish containing bacteria under growth and add 2 drops of the first shampoo (Pantene) to the bacteria.
  15. Place the second Petri dish containing bacteria and add in it the same volume of a different shampoo (head and shoulders).
  16. Place the third Petri dish containing growing bacteria and add drops of a third shampoo (Baby Johnson) like the other two. The same volume of the shampoos should be maintained for the other remaining samples.
  17. After adding Shampoo to the Petri dish the dropper should be returned to the respective test tube.
  18. Do more trials.
  19. Take all the Petri dishes and their contents back into the incubator at a suitable constant temperature (40 oC) and pressure to facilitate their growth.
  20. After a predetermined duration of time, observations should be made in the Petri dishes.
  21. Count the number of bacterial colonies on the different Petri dishes, and record them in a table.
  22. The different shampoos would have an impact on the growth of the bacterial colonies, and this can be recorded in terms of the differences in growth in the respective Petri dishes.

Data Collection

Recording raw data

Table 1.

Shampoo Brand Volunteer 1 Volunteer 2 Volunteer 3
Pantene 60 ± 3 68 ± 3.4 50 ± 2.5
Baby Johnson 62 ± 3.1 48 ± 2.4 52 ± 2.6
Heads and shoulder 2 ± 0.1 18 ± 0.9 0

There was + 5% uncertainty in data collection and therefore, the measured value was recorded with + 5%. This is shown in the following example.

For Volunteer 1, the number of bacterial colony was 60. The uncertainty in the measurement is was + 5%, which comes out to be 3. Therefore, the actual number of bacterial colonies can be anything in the range [57, 63] or 60+3.

Data Processing

Table 2:

Shampoo Brand Volunteer 1 Volunteer 2 Volunteer 3 Averages
Pantene 64 ± 3.2 50 ± 2.5 50 ± 2.5 54.7 ± 2.7
Baby Johnson 62 ± 3.1 48 ± 2.4 52 ± 2.6 54 ± 2.7
Heads and shoulder 2 ± 0.1 18 ± 0.9 0 6.7 ± 0.3

Average number of bacterial colonies

= Sum total number of bacterial colonies counted in the samples/ the number of samples.

The average number of bacterial colonies keeps into account that in some cases the number of bacterial colonies was affected by chemicals and other substances that might have been applied before collection was done.

The numbers also keep into account that during the application of bacteria to the agar gel some might have been lost for instance being stack on the cotton buds.

The effect of different types of shampoos on the number of bacterial colonies is presented in Figure 1, below.

Effect of Different types of shampoos on bacterial growth

From this chart it is very clear that Head and Shoulder is very effective in controlling bacterial growth, as compared to either Pantene or Baby Johnson, while there is hardly any significant difference in the effect of Pantene and Baby Johnson.

There were notable differences in the bacterial colonies that were observed on the Petri dishes. Some bacterial colonies were large while others and other were small. This can be attributed to the fact that there was collection of different strains of bacteria which differ in size and shape. Color differences were also evident because of the effect of shampoos which had different colors.

Conclusion and Evaluation


Heads and shouldersBaby Johnson

This experiment was intended to evaluate the anti bacterial capabilities of three shampoos – Pantene, Baby Johnson and Head and Shoulder. For this reason, bacterial sources were taken from the scalps of different people, cultured and treated with equal volume of these three shampoos. It is obvious that after treatment with the shampoos the bacteria culture which gives minimum number of bacterial colonies has succeeded in arresting the growth of the bacteria and therefore, is the best anti dandruff shampoo.

This is because, the dandruff is the result of the metabolic by-products of skin micro-organisms and therefore, to control dandruff one needs to control these skin micro-organisms. In these experiments, it has been shown that the three different kind of shampoos restrain the growth of the bacterial colonies in different manner. While only ~ 7 bacterial colonies were found in the samples treated by Head and Shoulder, this number of ~ 54 for the samples treated by the other two samples namely Pantene and Johnson Baby.

Therefore, based on the analysis of the experimental data it can be concluded that Head and Shoulder has shown remarkable performance in controlling the growth of bacteria as compared to the other two types of shampoos – Pantene and Baby Johnson investigated in this study. On the other hand there was no significant difference in abilities of the shampoos – Pantene and Baby Johnson. From this study it can be inferred that Head and Shoulder is very effective in dandruff control, which it achieves by controlling the growth of bacteria on the scalp.


Limitations and sources of errors

The procedure can be said to be reliable for determining the effect of shampoos on the growth of bacterial colonies.


The limitations of this procedure included the handling of materials like the droppers whereby they were releasing more shampoos than required thus interfering with the overall number of bacterial colonies in the Petri dishes.

The collection of the bacteria from the hair was also a weakness in the sense that the rubbing was done all over and may be the bacteria were colonial to some spots in the hair. This means that such types of bacteria were not sufficiently collected and hence this affected the overall counts after growth period of 2 days.

The procedure did not have a mechanism of ensuring that all the bacteria collected from the hair were transferred to the agar gels for growth. Possibilities are high that some bacteria remained on the cotton buds and thus their numbers were left out in the counts after the growth period in the Petri dishes.

The time for performing the experiment was also limiting as it did not allow for more samples which could lead to more reliable results. The growth period of the bacteria also limited the number of bacterial colonies that could be observed. Some bacterial colonies were too small to be counted.

The issue of some bacterial colonies being minute and not being counted is also a weakness that has to be mentioned in this procedure.

There was no sterilization of the cotton buds before collection of bacteria and this might have altered the expected results since some bacteria might have been presents on the cotton buds.

The chemical composition of some shampoos might have killed some bacteria hence altering the number of bacteria after growth consequently leading to no counts and reduced counts.

Some people had washed their hair a day before and some had applied oils and chemicals that might have interfered with the number of bacteria present in their samples. These include things like sprays, curlers and hair gels which may have either increased or reduced the number of bacteria in their hair.

Bacterial colonies

Some bacterial colonies, because of being too small, they could not be counted after the two days period.

While adding the shampoos on the Petri dishes, the number of drops might have differed therefore affecting the number of bacterial colonies.

The number of types the cotton buds were rubbed on the agar gels might also have affected the number of bacterial colonies.

The process of rubbing hands in the hair might also have affected the bacteria as the concentration of bacteria in the hair cannot be determined. Some parts of the hair had different numbers of bacteria for instance on the surface and deep inside the hair, the concentration differs.

The effect of the chemicals in the shampoos is likely to have killed some strains of bacteria and this is likely to have reduced the number of bacterial colonies counted in some Petri dishes.

The source of bacteria or the person from whose hair the bacteria were collected is a variable that affects the growth of bacteria. In this study three subjects (volunteers) were used get the bacteria, however, this variable was not at all considered in this study, as it was beyond the scope of this study. But then, ideally, this variable should have been eliminated altogether by taking multiple samples from same volunteer. This is a very important weakness of this study.

Suggestions for further investigations

For better and more accurate results in this experiment adjustments should be made on the apparatus for instance instead of using droppers graduated measuring cylinder should be used to give out the same volumes of the shampoo.

During the transfer of the bacteria to the agar gels from the cotton buds care should be taken to ensure that adequate rubs or smears are made to avoid the bacteria from going to the agar gel and then back to the cotton buds. This can improve on the number of counts after the growth period as most of the bacteria would remain on the Petri dish.

Future arrangements should be made such that the bacteria are given more time to ensure that all of them are included in the count.

All future improvements should be focused on maximizing on the number of bacteria collection from the hair such that during the counting after growth correct figures are recorded.

A more elaborate procedure can be pursued after this experiment to investigate the number of bacteria that can be killed by different shampoos so as to provide consumers with reliable information concerning the best choice of shampoos.

More number of subjects (volunteers) should be included to nullify the interference from the effect of type of hair and other variables relating to the volunteer. Better still, the variable related to the volunteers like type of hair, susceptibility of the subject toward bacterial growth etc. should also be studied in detail to arrive at a conclusive result.

Now, that it has been established that Head and Shoulder performs much better in controlling the bacterial growth as compared to the other two shampoos namely Pantene and Johnson Baby, it is the time to take the next step and to design the experiments for investigating the reason behind the same. One should conduct experiments to ascertain the chemistry aspect of the shampoo which is responsible for restraining the growth of bacteria. This will take the work into the domain of biochemistry and microbiology.

However, it is important to conduct such a study as it will help in revealing the underlying mechanisms for dandruff control and also in determining if a very good anti dandruff like Head and Shoulder is good for all the different kinds of bacteria causing dandruff or it is good only for certain kind of bacteria. Such an study will go a long way in developing better anti dandruff shampoo.

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