Science Buddies: "Ask an Expert"

My daughter is analyzing bacteria on a sample of hands and determining if gender plays a role how "germy" those hands are. She plans to collect her samples at school so she can do them all at a consistent time of day in a consistent environment. The challenge is how she can store the swabs until she gets home (6-8 hours later) and can swipe them across the agar in the petri dishes without cross contamination or killing the bacteria. would placing them in a ziplock bag be OK?

also - we bought some agar powder about 2-3 years ago - then found out that she was prohibited from growing bacteria for her project. do you think its still usable - or do we need to purchase a replacement?

Hi there,

Dry storage of the swabs shouldn't be a problem for just a few hours, as long as the swabs are kept out of extreme heat or cold. The easiest and cheapest thing I can imagine is to use a piece of styrofoam like a pincushion and stick the swabs into it (handle first of course) as they are collected, taking care not to let the swab heads touch each other. If she's sterilizing the swabs herself, she could try baking them in individual aluminum foil packets, which could then serve after collection as wrappers for the used swabs. Alternatively, she could store the "business end" of each swab in its own individual sterile (disposable) plastic culture tube or test tube, if she has access to a quantity of these relatively inexpensive (but not as cheap as the first two ideas!) items.

Agar powder doesn't rapidly go bad and as long as it's been kept in a cool dry environment and it still looks like powder (i.e., not caked up or gummy), you can use it with confidence.

Best,

Will Walker, Ph.D
McLaughlin Research Institute

my apologies in the lateness of reply. Thank you for your information, we followed your advice and it worked out well. I do have a few follow up questions.

first is on the Control when the experiment is comparing one group to another (in this case gender). Can she use the average amount of bacteria from al samples combined as the control, then the individual gender as the variable? If not, is there another way to set a control? She did use all 7th graders, in the same school, collected on the same day. Additionally she collected 3 samples from each student so she could replicate the experiment 3 times (something that was suggested in the rubric for attaining the best grade.

Next is something we ran into a problem on. A friend had volunteered a microscope, then at pretty much the last minute couldn't find it. We tried to borrow one from the school and weren't able to. Now we have to try and estimate the amount of bacteria on each of the 18 samples without one. We looked at each dish and did our best to count using a magnifying glass and separating the growth into 3 categories - we counted individual specs and labeled them small. Then there were larger dots, which we labeled as Medium, and then there were milky areas that we labeled as large. we counted each group to try and calculate a total. I have no idea if this is a reliable method, and have no idea about the larger milky areas and that they really represent in terms of bacteria. Any assistance would be most appreciated.

Hi,

It sounds like you did a reasonable job of counting the colonies. The milky areas could have originated from the colonies of several bacteria that grew together, or they could be due to rapidly growing bacteria that formed larger colonies. You probably would have been able to tell what happened, if your daughter had run a diluted sample to reduce the number of microorganisms on each plate.

As long as you counted each plate consistently, however, you can go ahead and compare results. It’s excellent that you have triplicate samples from each test subject. You can try using the student’s t test to determine if there is a difference between your groups:

http://en.wikipedia.org/wiki/Student's_t-test

Please post the data from each plate if you additional questions on analyzing your results.

Donna Hardy

I think we're pretty good on the student test, although one for the three times I repeated it the result was opposite of the other 2 and the overall average.

The project is due tomorrow, but I wanted to double check on the control. Is using the average then comparing the data from the individual genders to it a good control? we couldn't think of any other control for an experiment comparing two genders.

thanks again for your assistance.

Hi,

If your project is due tomorrow, you can report the averages of the groups. The problem is that if you don't use a t-test or other method of analysis, you don't know if different numbers are statistically different or the results are in the same population group. I used to do research using microbial counts, and found that results were not significant unless there was a 2-log (100-fold) difference in the microbial counts. Bacterial counts are amazingly variable. So, if you don't see at least a 5-10 fold difference in the numbers, do be cautious about making conclusions. You can always say results are inconclusive and that you will test more samples to make a definite conclusion.

Good luck!

Donna Hardy