Science Buddies: "Ask an Expert"

I am helping my daughter with her science fair project. She has cultured many surfaces around her shcool. The bacteria have been growing for 3 days. Bacteria is visible already on many of the cultures. Some are too many to count. How do we best record and catagorize the data? Should she use percentage of petri dish covered? Or aprox. amount and patterning?
Thanks for your help.
lori

Hi lorivadnais,

Since you list your project question as "Which surface has the most bacteria?," a surface that has too many bacteria to count would qualify as the most, without counting, as long as there is only one such surface. Is there more than one such surface?

Hi Lorivadnais,

With Petri dishes that have too many countable colonies, the traditional result reported is "TNTC," or too numerous to count. Generally, this will be over 300 colonies per plate. Since you can't tell the difference between 400 and 400,000 colonies, for example, with one Petri dish, you just report your results as TNTC or "greater than 300." If you have time, you can repeat your results and dilute the sample in sterile water and plate out a 1:10, 1:100, 1:1000 dilution of the sample. Your results are then multiplied by the dilution factor to report a count. If you don't have time to repeat the experiment, then you report your results as TNTC and explain in the conclusions and discussion section of your board what you would do to make the count more exact (test diluted samples).

This is a common problem in scientific experiments, so it is a good one to learn about. Using just one Petri dish, you would be able to test samples that contained between 0 and 300 bacteria/sample. If you added a 1:10 dilution, then your range would be 0 to 3000; and a 1:100 dilution would give a range of 0 to 30,000. Scientists always like to have a quantitative result, but if your result is outside the range, you have to report is as "less than" or "greater than" the number in the range.

If you have a variety of colonies, you should describe the colonies and use the information from the science buddies website to describe the growth of the colonies.

http://sciencebuddies.com/science-fair- ... ates.shtml

If most or all of your plates are “TNTC,” then try to estimate the percentage area of the plate that is covered by growth.

I hope this helps!


Donna Hardy

Thank you, Donna, for your response.

Besides the dilutions you mentioned, http://www.freesciencefairproject.com/b ... nting.html suggests using a 1:10,000 dilution, in addition, if it's needed. I don't know if this level of dilution will be necessary (of if 1:1,000 is enough), since I don't have direct knowledge in this area.