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

Equipment

Agar plates seeded with bacteria

Plant material (garlic cloves and mint leaves)

Pestle and mortar

10cm3 industrial methylated spirits

Pipette (sterile)

Paper discs (e.g.  Whatman antibiotic assay paper discs)

Sterile Petri dish

Sterile forceps

Tape

Marker pen

Incubator set at 25⁰C

Method

1. The agar plates had already been seeded and left to set for an hour before our practical by the school science technician.
2. We crushed 3g of a garlic glove using the pestle and mortar and 3g of mint leaves using a clean pestle and mortar. We added 10cm3 of industrial methylated spirit and mixed for 10 minutes.
3. We obtained 3 sterile 13 mm Whatman antibiotic assay paper discs and placed them in an open sterile Petri dish. Then using the pipette we added 0.1 cm3 of each of the 2 extracts on two paper discs, the third paper disc will be the control and it will not have any substance on it
4. The paper discs were let to dry for 10 minutes
5. We then used the sterile forceps to retrieve t he discs and placed each one in the middle of 3 agar plates and marked each one in order to know which is which.
6. The Petri dish was closed and sealed but not completely because this could lead to growth of anaerobic bacteria that can be harmful.
7. The dishes were incubated at 25⁰C for 24 hours.

Safety

Methylated spirits are toxic and highly flammable and was handled carefully and kept away from the bunsens.

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.

We wore goggles

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.

Results

Conclusion

My results show me that as both plants had a greater inhibition zone than the control that both of the plants had antibacterial properties and that the substance killing the bacteria is active in the plants.

My results also show that garlic has a better antibacterial property than mint which supports my hypothesis. 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.

Reliability of experiment

• I used 3 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/control had not touched the bacteria.