Science Buddies: "Ask an Expert"

I am doing a science project that is mandantory and I have been extremely busy for the past month with personal business regarding family. I am also taking extremely hard classes including AP human geography, Honors English 10 and Honors Biology. So I haven't looked at my expirament with much thought and I am working to the last minute. I have three days left. My expirament is how different kinds of sugars mixed with water effect the growth of bacterial cultures. I used nutrient agar and the different sugars. I placed the petri dishes upside down in a dark box placed in a room heated at 75 degrees f. I am not seeing any results and I am worried. It has been 2 days and I need quick results. My next question is how can I get tangible data? I don't have and can't afford anything fancy and I really don't have alot of time. I understand If you can't help me out but If you have advice it would be greatly appreciated. PLLLEEEEAAASSSEEE help me! I am normally a very hardworking student, and I am very embarassed about this ..... I am just going through a very hard time right now. Thank You So Much!!

I'm sorry your bacteria aren't growing well. 75F should be warm enough, although E coli prefers human body temperature of 98.6 [37C]. Are you using E coli? Where did you get the culture and how old is it? How did you do the experiment? I would have used agar that contains the sugar you are testing--sucrose, fructose, lactose, etc--in place of glucose. But in order to do that you would need to have plain agar and growth medium without glucose.
How much of each bacterial culture did you spread on each agar plate?

The scientific way to quantitate the effect of different sugars would be to plate the bacteria in such a way that they grow in the form of individual colonies. This is done by making serial dilutions of a culture and then spreading a measured volume, say 100 uL, of that on the agar. Is that what you did or did you spread a drop of the whole culture?

If you plated an undiluted culture then you will get a 'lawn' of bacterial growth rather than discrete colonies and you will not be able to make an exact measurement of the effect of different sugars. If the bacteria grew faster with one sugar, then you should see a denser lawn on the plate. Be sure to take good photos of your plates.

Do you have a strong magnifying lens or a microscope with 40X magnification? Take a look at the surface of the agar. If there are tiny colonies, you will be able to see them. Then all you have to do is wait until they get bigger. Wrapping the plates in aluminum foil and putting them inside a box with a lamp [15W incandescent night light] will provide extra heat so the bacteria will grow faster.

Reply with answers to the questions and we will try to help you beat the deadline and get some data. Remember science is an exploration. There is no 'answer' that you are supposed to get. The results you get, no matter what, can be written up in a research report. Describe what you did, present the results and explain your conclusions. That is exactly what scientists do and it is perfectly legitimate.

Good luck!

Sybee

So I got the sample from a cheek swab and I put it on the agar in a squiggle motion not everywhere. I didn't see any other way to test the sugars because alot of the sights were unclear so I basically did an experiment using absorbant paper to soak the sugar and then I put it in on the agar after I put my cheek swab sample on it. I think there is too much moister because I mixed the sugar 1 tsp of water with 1 1/2 tsp of the sugar. I think that might be the problem. I also got a culture counting app but i have no idea it that is reliable. Honestly I think I just need to manually count them. I am dorwning. By the way thank you so much!!!! I put the culture Upside down also because there was so much moister and I think that is interfering.

Oh, now I see what you did. A cheek swab may not have many bacteria, especially if you use an antibacterial mouth rinse. Swabbing the teeth and tongue would have worked better but you still might not have gotten that many bacterial cells.

Did you look at the plates with a magnifying lens? You should see some colonies. Could you tell if there were more or fewer colonies around the paper disks with sugar? The water won't hurt the bacteria unless it has a lot of chlorine. I hope you used distilled water for your experiment!

1 1/2 tsp of sugar in 1 tsp of water is a pretty high concentration and that may inhibit the growth of bacteria: http://www.scientificamerican.com/artic ... sugar-pre/

You do have one plate with no sugar as a control, right? Does that one have any micro-colonies?

You don't have time to do the experiment over so just continue the incubation and try to keep the plates warmer to encourage growth. You will get some colonies eventually on the control plate. The one's with the sugar maybe not if the concentration is so high that it inhibits bacterial growth. So, your hypothesis can be--"High sugar concentrations inhibit bacterial growth." You can explain why this is so and show photos of the plates.

I hope this helps, but if you have more questions I'll be here.

Sybee

Hi Kaitynpsa,

This sounds like a great project and Sybee has given you some excellent advice. The main problem now is that it's likely that the bacteria are growing in solution in the excess moisture on the plates and, if so, they will not be able to form discrete colonies that you can count. Does the excess liquid look cloudy? If, so then you do have bacteria growing. .

Since you are short of time, I recommend that you concentrate on writing up your project. Please refer to the "communicating your results" section in the project guide section of this website. The directions are excellent.

https://www.sciencebuddies.org/science- ... ndex.shtml

Make sure you write up your project completely; perhaps you can compensate for the lack of results with the background research and other required sections. Please refer to your teacher's assignment for the science fair project and make sure you include every detail. When you get to the results section, you might not have quantitative data, but do describe what happened and what you observed. In your conclusion section state whether or not your hypothesis was proven and explain why. In your discussion/future directions section, you can explain what you would do differently if you were to repeat the project.


There is a technique of estimating bacterial concentrations of solutions using a spectrophotometer and measuring the optical density of bacteria at 600 nm. I don't recommend that you actually do this as there are safety concerns about handling unknown bacteria, but you might describe this method and explain how it could be used to measure the bacteria in your samples. Here's a link that describes how to do this.

http://faculty.sdmiramar.edu/dtrubovitz ... hcurve.pdf

You are not allowed to say the project did not work; just describe your experiment and present what happened.

Good luck!

Donna Hardy

Hi Kaitlyn,

How is your project going? I answered your last post but you haven't replied to tell me what happened. Did your colonies finally appear? Were you able to see a difference in number or type of colonies between those with sugar and the one without?

We like to know how your project turned out, so please post again with the results. Donna made good suggestions about the write-up of your report, but if you have some more specific questions don't hesitate to ask.

Sybee