Growing Bacteria in Petri Dishes – Part 2

As I mentioned in the “Part One” of this blog post, I did an experiment using nutrient agar plates to grow bacteria. I had one question to answer: How do different household cleaning products affect bacteria growth?

What did I do?

During my experiment, I made a solution using agar agar. I then chose a surface and separated in into five sections : one as the original (without product) and the other four for the cleaning products (Dawn Ultra, Softsoap hand soap, Vim Bathroom Cleaner, and Rubbing Alcohol). After that, I took a sample of the surface without any cleaners. Then, one at a time, I washed each section of the surface with its’ corresponding disinfectant, and took the rest of the samples.

After three weeks of observations, here is what I found…

Observations

In the Original sample, you can see many small, circular colonies. They are white and measure 1 to 2 millimeters in diameter.

In the Dawn sample, you can see bigger colonies, some with irregular shapes. They have a yellowish tint and measure 2 millimeters in diameter (on average).

In the Softsoap sample, you can see different sizes and shapes of colonies. Some are white, some are yellow, but they measure about 3 millimeters in diameter (on average).

In the Vim sample, you can see very few larger colonies with irregular shapes. They have a yellowish colour and measure 3 millimeters in diameter (on average).

In the Rubbing Alcohol sample, you can see very few smaller colonies. They all have a white color and measure about 2 millimeters in diameter.

Explanations

Dawn Ultra

The main ingredient used in Dawn Ultra is something called Sodium Lauryl Sulfate. This product bonds with grease, allowing water to remove it more easily.

Since there are no antibacterial products in Dawn Ultra, there wasn’t much of a difference in the number of colonies compared to the original.

Softsoap hand soap

Softsoap hand soap contains a interesting bond called Benzalkonium Chloride. This is the antibacterial ingredient in this hand soap that prevents the growth of microorganisms.

Since there is an antibacterial ingredient in the Softsoap hand soap, there were less colonies than the Original sample. However, the colonies were much larger.

Vim Bathroom Cleaner

Vim Bathroom Cleaner has ingredients that help dissolve and disintegrate oils from different fabrics and tiles.

Since there are no antibacterial products in Vim Bathroom Cleaner, there should have been a large quantity of colonies. However, that was not the case. Instead, there were considerably less colonies than the Original sample.

Rubbing Alcohol

The Rubbing Alcohol that I chose was made of 70% isopropyl alcohol. This is an ingredient that directly attacks bacteria.

As a result, there was a big difference in the quantity of colonies compared to the Original sample.

Conclusion

In the end, all of these products helped diminish the number of colonies in one way or another. However, most of them increased to size of these colonies.

I found this experiment very interesting and I had a lot of fun doing it. I hope this encourages you to try something like this in the future!

And here’s a tip: check the ingredients of your current cleaning supplies. Does it say anything about killing bacteria? What you find might shock you…

CC BY-NC-ND 4.0
Image photographer: Sarah Wilkinson

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