Data Collection:
Data collection will be collected by using an automatic cell counter that counts the bacteria in colonies. Since the cell counter is automatic, it eliminates any subjective error in counting the amount of colonies present in the Petri dish. However, the uncertainties are still present and will depend on each of the four areas tested standard deviations.
Materials:
- Accessibility to a toilet, dishcloth, sink, and a refrigerator handle
- 14 prepared Petri dishes, containing agar medium and nutrients
- Wax pencil (for labeling Petri dishes)
- Masking tape
- 12 sterile swabs or 1 inoculating loop
- Cell counter (either manual or automatic)
- Incubator (if incubator is not accessible a warm location, ideally in an environment around 100° F (37° C) - not in sunlight or on a heating register would be perfect)
- (If using the inoculating loop, an alcohol burner—source of flame—to sterilize the loop between each bacteria collection)
- Bleach
Procedure:
- Prepared Petri dishes should be refrigerated until used and always stored upside down (i.e media in upper dish, cover on bottom). This keeps condensation which forms in the lid from dropping onto and disrupting the bacteria growing surface.
- When ready to use, let dishes come to room temperature before taking samples (about one hour).
- Spit in one Petri dish. Replace cover on dish, tape closed, and label each dish so you know the source of the bacteria. Store upside down so the agar remains moist. (This is the positive control.)
- Do not swab anything in one Petri dish. Replace cover on dish, tape closed, and label each dish so you know the source of the bacteria. Store upside down. (This is the negative control.)
- Collect bacteria from the inside of a toilet using one of the sterile swabs.
- Inoculate each dish by streaking a pattern gently across the entire agar surface without tearing into it.
- Replace cover on dish, tape closed, and label each dish so you know the source of the bacteria. Store upside down.
- Repeat steps 5-7 two more times so in total, the inside of the toilet has three designated Petri dishes. (Make sure to swab DIFFERENT areas of the toilet.)
- Repeat steps 5-8 for the rest of the areas to be tested: the sink, a dishcloth, and the refrigerator handle.
- Place all 14 Petri dishes in the incubator or in the warm environment.
- Check the incubator in approximately 36 hours or, if the latter was chosen, check in approximately 54 hours. (The dishes will begin to smell, thus ensuring bacteria is growing.)
- Write down your observations and record the results as to which Petri dishes had the least bacteria and which Petri dishes had the most bacteria.
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Before disposing of dishes in the trash the bacteria should be destroyed. Pour a small amount of household bleach over the colonies while holding dish over sink. CAUTION—do not allow bleach to touch your skin, eyes or clothes. It will burn!
Data Table:
The positive control (the spit) did produce 264 bacteria colonies, and the negative control (nothing inside the Petri dish) produced 0 bacteria colonies, thus showing my experiment was properly conducted. Since the negative control yielded zero bacteria colonies, the number of bacteria colonies initially counted is the true number of bacteria colonies present. If the negative control did produce some colonies, we would have to subtract this number from the colonies counted upon experimentation.
The uncertainty, which was derived from the standard deviations of each of the four areas tested, is as follows:
Inside of a Toilet: ± 10.4083 bacteria colonies
Dishcloth: ± 9.0738 bacteria colonies
Sink: ± 48.1352 bacteria colonies
Refrigerator Handle: ± 2 bacteria colonies
The spread of data is due to the different areas of the four items I tested. I did not swab each of the four areas in the same place because that would not be a good representation of the item itself.
Analysis of Results:
Number of Bacteria Colonies in each Petri Dish for the Inside of a Toilet, Dishcloth, Sink, and Refrigerator Handle
The trend that this line graph depicts that although the inside of a toilet does not have the lowest number of bacteria colonies out of all the twelve Petri dishes, it remains the cleanest out of the other three areas tested. This graph also shows that there is a great amount of variability in the sink data, but it should not be considered an outlier because variability was intended. In order to have a complete representation of the item, I needed to swab different areas for the item I was testing. The great variability in the sink simply tells me that bacteria seem to congregate toward a particular area of a sink.
Average Number of Bacteria Colonies in the Petri Dishes for the Inside of a Toilet, Dishcloth, Sink, and Refrigerator Handle
The trend that this graph shows is similar to the line graph above. The inside of a toilet is the cleanest, while the dishcloth is the dirtiest, even when averaged out. In order from cleanest to dirtiest, the four areas are as follows: Inside the Toilet, Sink, Refrigerator Handle, Dishcloth. The sink would have the cleanest if it was not for Petri dish 2 that had 248 bacteria colonies, thus significantly increasing the variability of the sink’s bacteria colonies.
All in all, as stated previously, the order from cleanest to dirtiest is as follows: Inside the Toilet, Sink, Refrigerator Handle, Dishcloth.
Conclusion:
My hypothesis that the sink would be the cleanest was proved wrong, as exemplified by both the data table and the two graphs in the previous sections. It was in fact the inside of the toilet that proved to be the cleanest, then followed by the sink, refrigerator handle, and lastly the dishcloth. Both graphs and data table clearly explain this trend, and it should be noted that due to the sink’s great variability was resultant in the sink not being the cleanest out of the four areas tested. If the sink did not have a standard deviation of approximately 48, then it is quite possible the sink would have been the cleanest.
The fact that the inside of a toilet is the cleanest and a dishcloth is the dirtiest comes as no surprise to me because upon initial research findings, it was concluded that one of the cleanest places in a common household (including apartments, condos, etc.) is indeed the inside of the toilet because of the constant flushing and self-cleaning. The dishcloth is the dirtiest because it resides in a perfect environment where bacteria can grow, and a dishcloth, similar to a sponge, is the most ideal place for bacteria to simply nestle in the fibers. Furthermore, since humans believe that the kitchen and all related items are presumably the cleanest, we never completely disinfect the dishcloth by placing it in a bleach solution for about ten minutes.
Error Analysis:
This experiment did require minor mathematical computations, and error was eliminated by double-checking the arithmetic in a calculator. Moreover I utilized an automatic cell counter that eliminated any error in the counting of bacteria colonies.
Procedural errors were kept to a minimum, and the equipment in addition to the time management were suitable for this experiment. The time to collect all the bacteria took seven minutes, and the choice of equipment, as stated above, avoided any possible errors with judgment. Furthermore, the chosen process was the most appropriate for this experiment and thusly eliminated any possible errors involved with the procedures.
Though this experiment yielded results that are credible and are of good quality, there are multiple methods that I could have taken to improve this experiment.
A weakness in this experiment that multiple trials were not conducted. Although I did have three different Petri dishes for each of the four independent variables, I simply tested the four variables out of one common household. To have a better comprehension of the data, I should have ventured out to test other households. This weakness is very significant, and could have dramatically altered the results, possibly inviting a couple of outliers, and a change in the order of cleanest to the dirtiest.
Another weakness in this experiment, also highly significant as the last one, was the fact that I only tested four areas that were concentrated in either the kitchen or the bathroom. To retrieve a better representation of a common living area and to overall improve the experiment, I should have extended the experiment to other areas such as the living room, study room, garage, the attic, and the master bedroom. Spreading out the areas I would have tested could have too altered the results severely.
