Science Buddies: "Ask an Expert"

My 6th grade daughter did an experiment on which homemade cleaning solution kills the most bacteria (vinegar and water, lemon oil and water, or just plain water). She used a bacteria testing kit with agar solution & petri dishes. The results were pretty clear -- vinegar worked the best. But how should she report her results? Does she need to count the little bacteria spots? The dish that tested the plain water would be difficult to count because there are so many spots. Also, I forgot to take pictures of the dishes at 48 hours. More bacteria has grown since then, although it has not changed the results. Do we need to redo the experiment? At 48 hours all the spots were kind of a translucent white color, but now a few are black or orange. Does she need to know what that means? Thank you for any help you can provide.

You have an interesting project.

My first qustion is -what was the source of bacteria and number of sbacterial cells per unit volume o solution. You may have to dilute the bacterial suspension to get the numbers down to a countable number. It is important to count the number of colonies per dish to determine the effect of treatment. You present the data in terms of the number of bacteria that survived each treatment. The project can be salvaged by recording the number of colonies on each plate.

This project can be completed in 3-4 days and can easily be repeated if necessary.

If you have further questions please ask.

Matthew W. Mulanax

She used chicken liver blood on the counter to produce the bacteria. Are the dots the actual colonies? The vinegar/water dish has about 40 dots, although half of those are very tiny. At the 48 hour mark, however, there was no visible sign of bacteria on that dish. The lemon oil dish has almost double the number of dots. Again, many of them are tiny. There were signs of bacteria at 48 hours for the lemon oil, but not nearly as much as now. The dish testing the plain water has too many to count (at least easily). It had lots of dots at the 48 hour mark, but since then 3 of the dots have turned black. Even more confusing, some of the dots in the lemon oil dish have a different shape.

Do you have a link for how to properly count and analyze the bacteria? And do we even need to know what kind of bacteria it is? The teacher indicated that we need to have graphs or such to document our results, but since our kit gave no instructions for this it never even occurred to us. I realize now how naive that was. We do have time to repeat the experiment (fair is in a week), but is it really necessary? Is there significance to that 48 hour mark other than to let the bacteria form? Are the dishes of bacteria we already have useless?

Sorry if these are silly questions. Thank you for your help!

Every question is a good question.
Chicken liver blood is a good source. Everyone was wearing rubber gloves?
The vinegar is an acid and bacterial growth is slowed or stopped at low pH of acid.
The lemon oil (the oil portion) is a good food for most bacteria and they grow quite fast.
The black dots are most likely a different species. Bacteria generally occur in mixtures – only occasionally will you find a pure culture (one genus).
You need to count the dots (colonies) on each plate as best as you can and make a table like this with the colony count per plate–

Treatment Plate One Plate Two
Vinegar/water
Lemon oil
Water (Control)

Only one plate per treatment that is OK.
A bar chart would look like this (an example) a graph would not be a dramatic.
Colonies per plate Vinegar/water Lemon oil Water
1000 X
100 X
50 X X
10 X X X
0 X X X
You would use a program like Excel to construct the bar chart.
I hope this helps.
If you have any questions please ask.

Matthew W. Mulanax