Mystery TSA Dish by Lana A.

Day 0

I chose to sample my pen, the surface of my phone beneath its case, the school’s laptop keyboard, and the table surface. I chose these objects because human hands often touch them. Hands generally host a wide range of bacteria, some that are found on the human body, and others that are picked up from the environment. Since a keyboard or table surface is not seen as a “dirty” object, I wondered if they would grow nearly as many bacteria as a “dirty” object (e.g. toilet handle) would. I later compared my sample results to those of my classmates, who sampled pretty gross things.

Day 3

 

Fig. 1

The third day yielded very few visible colonies on the 25°C plate, and a considerable amount on the 37°C plate. This was interesting because it may indicate that the bacteria found on the surfaces sampled grow best on and in the human body. Of course, it is possible that the bacteria that grow best at room temperature were not supported on trypticase soy agar; although, this is not very likely considering the wide variety of nutrients TSA provides.

Day 7

Fig. 2

The top dish was incubated at 37°C. It appeared that the table surface sample had the greatest amount of growth in terms of square centimeters, i.e. the bacteria might grow more rapidly. If given time, it could have grown into a lawn. My pen yielded the greatest amount of colonies. When grown at 37°C, the laptop keyboard had a significant amount of growth, similar to my pen. Least of all, my phone had only one colony at 37°C. Three entirely different colony types were present. A fourth colony on this plate appeared similar to a colony type on the 25°C plate. In total, 21 colonies were counted.

Within the 25°C plate—shown on the bottom—six different colony types were present. This plate was interesting because many fungal-like colonies grew under the swab of the table surface. My pen also had a great variety of colony colors—orange, tan and white. Altogether, 30 colonies were visible. Another noteworthy point is the fact that practically no colonies grew under the laptop keyboard at 25°C, yet many were present at 37°C.

Fig. 3

As for identification of the bacterial colonies, simply trying to figure them out visually is a complicated task. Although I searched through several sites, I could not pinpoint any definite species. The image above, taken at 400X magnification, was not very helpful either (the two circles seen are associated with the microscope lens, not the slide itself). The slide itself came from 

This was an interesting experiment, as we don’t often think about how bacteria surround every aspect of our lives. After comparing my plates to those of people who swabbed toilets, shoes, and sink handles, I realized that hands are generally clean. That most likely has to do with extensive hand washing measures taken in our society, as well as good hygiene.