Why do they put mint in toothpaste? Would garlic be better?

Hypothesis

I already know from reading the passage titled “Antibacterial Chemicals” on activity 4.7 that both mint and garlic contain antibacterial properties. After reading through the procedure I expect to see a larger zone of inhibition caused by mint than by garlic – this is because mint is used in toothpaste and other treatments such as “Bonjela” and contains menthol which can act like a local anaesthetic.

Method

We used the procedure outlined on activity 4.7 but with a few changes:

We used 1.5g of plant material between 4 people and only 5cm3 of methylated spirit instead of what is outlined in step 2.
We dipped the sterilised paper discs into the plant extract using tweezers instead of pipetting it as outlined in step 3
We did not use other plants as outlined in step 5

Also:
We decided that the control should be the methylated spirit
We used sterilised discs cut from filter paper instead of the Whatman antibiotic assay paper discs
The agar jelly had already been poured into sterile Petri dishes for us

Safety

We followed normal laboratory rules e.g. no eating or drinking, and wore lab coats.

We cleared and cleaned the bench with a disinfectant (virkon).
We lit a Bunsen burner so there was an updraft away from the bench.
We wash our hands before and after the practical work.
We collected all the equipment you would need before we began, making sure glassware, Petri dishes, pipettes, agar, etc. were sterile.
A jar/beaker of disinfectant was available on each bench for disposal of contaminated items.
We wore goggles
We incubated the plates at 25oC, which is lower than body temperature so there was a lessened risk of the bacteria affecting us.
The methylated spirit was kept away from the Bunsen flame because it is flammable.
We used aseptic techniques such as flaming bottle necks and burning the methylated spirit off a glass spreader to sterilise it. All the equipment we used had been autoclaved
We did not open the Petri dishes that contained the growing bacteria
The Petri dishes containing the bacteria were autoclaved after we finished the experiment.
When seeding the agar jelly with the bacteria we raised the lid of the Petri dish only enough to give access to the agar jelly.
When sealing the Petri dishes we did not fully seal them, therefore preventing anaerobic conditions and the bacteria that survives in these conditions which can be harmful to humans.

Results (in mm)

Conclusion

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Results (in mm)

Conclusion

My results show me that as both plants had a greater zone of clearance than the control (the methylated spirit) that both of the plants had antibacterial properties and that the substance killing the bacteria is active in the plants and is not the methylated spirit used. My results also show that mint has slightly better antibacterial properties than garlic which supports my hypothesis; although since writing my hypothesis my biological knowledge has grown and I have done some further research using information from textbooks and my teacher and my knowledge now contradicts this. Garlic has an active ingredient called allicin which is produced when the garlic plant is cut or damaged (as we have in the experiment by using the pestle and mortar) and the enzyme allinase converts alliin to allicin. This allicin destroys bacteria such as Campylobacter and Heliobacter and interferes with lipid synthesis and RNA production. Mint produces menthol and carvone which although are toxic to microbes and insects are also used in flavourings by humans. Although mint numbs the human mouth and does kill some bacteria, if garlic was used it would destroy more. But for obvious reasons such as bad breath mint is used instead of garlic.

Validity of Conclusion

Although my conclusion has been written based on my own knowledge and common sense, information from textbooks (e.g. SNAB AS textbook page 162 and 163) and given to me by my teacher, I do not have any data from my experiment to prove it. Therefore my results actually disproved my conclusion and therefore it is not a valid conclusion.

Reliability and validity of the experiment

I used four repeats to calculate my average from and by doing this my average was more accurate and reliable. If I had encountered any results that I thought were anomalous I would not have included them in my averages. Each repeat was also performed using the same method and in the same way therefore any systematic errors were carried forward throughout the experiment meaning all results would be affected by them.
The E.Coli bacterium used would not normally be found in the mouth meaning using this type of bacteria to test toothpaste was unsuitable, although using this type of bacteria in the laboratory was the safest.
When measuring the zone of inhibition on the agar plates (meaning no bacteria had grown there) the zones were not completely circular and therefore it was hard to measure. There were also patches where there was no clear patch and so it was presumed the garlic/mint/methylated spirit had not touched the bacteria.
When dipping the paper discs into the mint/garlic there were often lumps of the matter on the disc – this would increase the antibacterial properties of that disc more than the others.
The concentration and amount of substance on the discs was not controlled as one disc could be saturated and the next barely touched into the substance.
1.5g was also very hard to measure accurately on the scales provided and because of the nature of the garlic and mint in was hard to cut up to be very accurate.
My results are not very precise either as my repeats are far away from each other e.g. 15mm and 20mm for Garlic.

How to make more reliable

If I repeated this experiment I could prevent these errors by using more accurate scales with a smaller percentage error and to a higher decimal place. I would also leave the discs in the solution for a set amount of time to allow them to all become saturated in solution before being placed on the agar jelly. I would not use any discs that had any matter on them. I would also use more sets of data to prove/disprove my conclusion. To measure the zone of clearance more accurately I would agree to use the largest/smallest part or use a piece of string placed on the Petri dish and marked where the zone reached to compare all three against each other. I would also leave the experiment for longer for the active ingredient to diffuse. But I would have to do some preliminary work to see for how long I could leave the bacteria and low long I could leave it before different zones of different substances meet as then the experiment would be invalid.

Why do they put mint in toothpaste? Would garlic be better?

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