An Investigation into a Factor Affecting the Rate of Bacterial Growth

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An Investigation into a Factor Affecting the Rate of

Bacterial Growth


To find out how different brands of hand sanitizer affect the rate of bacterial growth.



Brands of Hand Sanitizer (Lifebuoy, Al Kamal, World of Wipes, Dettol)


The size of the Zone of Inhibition /mm2


What will be controlled?

How will it be controlled?

How much sanitizer

Use a hole puncher to create disks

Type of Bacteria

Use the bacteria provided

Temperature Bacteria Will Grow at

Use incubator for both petri dishes

Time bacteria grow for

Incubate both dishes for 48 hours

Amount and type of agar

Use the agar provided

Surface are of agar

Use petri dishes of the same size


If the concentration of alcohol in a hand sanitizer is increased, the size of the zone of inhibition will be greater because antiseptics like ethanol and isopropanol “kill germs by dissolving their essential proteins” (Sherwood). Using this information, the Dettol hand gel will kill the most bacteria because it has the highest concentration of alcohol (69.4%)

Equipment set-up:

Hole PuncherFilter PaperLatex GlovesHand Sanitizers

TweezersMarkerTape3 Petri Dishes

E.Coli BacteriaAgarIncubatorGraph Paper


  1. Punch 8 holes in the filter paper using a hole puncher. Keep the small disks created.
  2. Wear Latex gloves for protection and to prevent contamination.
  3. In a petri dish, apply Lifebuoy, Al Kamal, WOW and Dettol sanitizers so that there is enough of them to cover the paper disks but not enough for them to touch.
  4. Use tweezers to submerge 2 filter paper disks in each gel.
  5. Turn a petri dish with bacteria and agar upside down so that the lid is on the bottom.
  6. Lift the base of the petri dish and place a filter paper disk of each gel on the surface using tweezers. Arrange them like so:

  1. Close the petri dish and label each disk.
  2. Tape the dish and repeat steps 5-8 for a second petri dish.
  3. Place both dishes in an incubator set at 37°C for 48 hours.
  4. Take the dishes out and place a graph paper with 1mm squares under the dish.
  5. Use the graph paper to count the area of the zone of inhibition (where the bacteria did not grow) for each hand gel in mm.

Data Observations:

Table 1: Results

Zone of Inhibition /mm

Brand of Sanitizer

Alcohol Percentage (%)

Dish 1

Dish 2


Al Kamal















World of Wipes





Graph 1: Average Zone of Inhibition

Graph 2: Alcohol Percentage vs. Zone of Inhibition


It was predicted that the hand sanitizer with the highest concentration of alcohol would kill the most bacteria and hence created the largest zone of inhibition. The results prove that this is true. There is a clear positive correlation that shows that as the concentration of alcohol increases so does the zone of inhibition. The FDA agrees that 62% and higher levels of (ethyl and isopropyl) alcohol provide for “safe and effective antibacterial protection” (Smith). The data; albeit a bit limited shows this. Sanitizers with 62% and above concentration have more than twice the zone of inhibition. This shows that ethyl and isopropyl alcohols do kill bacteria and the higher the concentration, the better they kill bacteria. This is because these types of alcohol kills bacteria by making the cellular membrane (holds everything together) of the bacteria more soluble in water. This causes it to lose its structure and fall apart. As this happens, the alcohol can penetrate the cell and denature the proteins. Proteins are complex shapes and their structure is linked to the function of that protein. Denatured proteins (such as when they come in contact with these alcohols) lose their structure and thus their function thereby killing the function of the bacteria.


What went wrong?

How could this have affected the data?

Improvements for next time:

The same tweezers were used to handle the filter paper disks in the different hand gels

The tweezers could have contaminated the hand gels with other hand gels. This means that the data is not valid.

Wipe and clean the tweezers before handling each hand gel

The filter paper disks were curved

The surface area that was in contact with the agar was not the same across all the trials. This means that the amount of hand gels in contact with the bacteria was inconsistent.

Use a flat part of filter paper to make the disks

Lack of trials: the sanitizers were only tested two times

The data is not be very reliable. The range of some sanitizers is very big and there can be outliers

Do more trials with more petri dishes

The bacteria strain could have been resistant to one or more of the sanitizers

The data would not be valid and it would be an unfair test

Do more trials with another type of bacteria

The Al Kamal sanitizer had an unknown concentration of alcohol

If the alcohol concentration in the sanitizer is not as estimated it could skew the results and the trend would not be as apparent

Use a sanitizer that states its alcohol concentration

Al Kamal and Lifebuoy had the same average zone of inhibition & Dettol and World of Wipes had the same. If the second trial’s data is switched between Al Kamal and Lifebuoy, the data is more consistent (currently there is a big range). They could have been labelled incorrectly.

The trend and the second graph would be invalid because the averages would even out (like what happened; the two averages are the same)

Label each filter paper disk as they are placed in the agar


The method is not very reliable. It is repeatable and the results seem to be consistent except for what appears to be a switch up between the Al Kamal and Lifebuoy sanitizers. This is shown here:

Al Kamal








The number of trials is quite low, there were only two trials for each gel. This means that the results are not extremely reliable. The 7.75 average is also a little bit unreliable because our original results only measure to 1 decimal place so the average cannot be any more precise.


The aim of this investigation was to find out how different brands of sanitizers affect bacterial growth and to superfluously find out which hand gel would be best for everyday use. As only one type of bacteria was used, the results are not valid for the secondary aim of the investigation. The validity is ok because the conclusion concurs with online sources such as the ones mentioned in the bibliography. On the other hand, there were many things that went wrong that could make the results invalid (such as mentioned in the table above).


Caldwell, A. Michelle. How Does Alcohol Kill Bacteria? 2013. 21 December 2013.

McGraw Hill. Protein Denaturation. n.d. 21 December 2013.

Sherwood, Chris. How Does Hand Sanitizer kill Bacteria? 16 August 2013. 15 December 2013. <>.

Smith, Ann. FDA Recognizes Alcohol-Based Hand Sanitizers as Safe and Effective for Consumer Use. n.d. 2013 December 2013.

UCSB. How does ethyl alcohol kill bacteria? n.d. 21 December 2013.