Trouble growing bacteria

[ruzic]

We are doing an experiment to see which hand sanitizer works best. We scraped a q-tip along the floor near the toilet and rubbed it on the agar plates. Then applied sanitizer to a clean q-tip and rubbed it over the bacteria. We kept one control to see what the bacteria looks like with no sanitizer. Nothing grew. Any ideas on what we did wrong?

[donnahardy2]

Edit postDelete postReport this postReply with quoteRe: Help with collecting and studying bacteria
by donnahardy2 » Thu Jan 03, 2013 4:25 pm

Hi Ruzic,

Welcome to Science Buddies!

There are several reasons that may have caused the bacteria not to grow. Here are some questions that might help explain your results.

1. Bacteria will not grow on plates that have dried out. How old were your plates? Was the surface of the agar moist, or did it appear to be slightly dry?
2. What incubation temperature did you use and how long did you incubate the plates? I would expect to see some growth by 48 hours at ambient temperature or within 24 hours at 35 degrees Centigrade. A temperature of over 42 degrees C would inhibit many bacteria.
3. What type of agar did you use? For this type of experiment, a general all-purpose growth medium such a Nutrient Agar would work well. A selective medium such as EMB agar.

You may not have time to repeat this experiment, so with no results, you will need to expand the discussion section and explain what went wrong and what you would do if you had time to set up the experiment again. Don't worry; this is a common outcome with many science projects, and also with experiments done in research labs. Including a positive control in your experiment was an excellent idea.

Please do avoid posting new questions on other students' topics. Post again in this topic if you have any other questions.


Donna Hardy

[ruzic]

Thank you Donna,
We ordered Nutrient Agar Plates from Science Kit & Boreal Labratories. It was hard. I was expecting it to feel more like a gel and it didn't. Before I complain to the company for sending me poor quality, I want to make sure it was not my fault they dried out. We received the box and put them on top of the fridge and left for a 4 day vacation. Did I kill them?

Also, we put the treated plates in a covered enamel dish. It would have been dark, but only about 70 degrees F. Is that too cold?

Thank you for your time.

[donnahardy2]

Hi Ruzic,

Did the plates have an expiration date? Normally plates last for about 2 weeks after they are prepared, provided they are stored upside down in a sealed bag. It's probably better to store the plates in the refrigerator, but I don't think 4 days at ambient temperature would not have damaged the plates. Did you keep them in a sealed bag? Is it possible that the plates froze during shipping to you?

Bacteria grow very well at 70 degrees F. It might take a couple of days for colonies to appear, but it sounds like you incubated them longer than that.

The consistency of the agar should be like stiff Jello, but definitely moist. I would recommend calling the company and provide them with your ordering information and the lot number of the product. You could explain that you obtained no growth on any of the plates and ask for their advice. Hopefully they will want to resolve the problem for you.

Let us know what happened.


Donna Hardy