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QUESTION: My 5th grade grandson is 48 hours into an experiment that he is conducting for his science project.  He has inoculated three groups of blood agar Petri dishes (using sterile swabs) with microorganisms from the mouths of eighteen volunteers.  (I have placed PetriSeal on all of his dishes and he appears to have nothing hemolytic growing)   Stemming from an experiment that he did last year, he is testing the effectiveness of cold pressed cinnamon oil on mouth microorganisms.  He believes that the cinnamon oil is too strong to be used at 100% since it causes a painful burning sensation to the skin and mouth, so he has mixed it with Canola oil.   Dishes in group one have discs treated with 20% cinnamon oil, group two dishes have 50% cinnamon oil, and group three is a control group using only Canola oil.  At this 48 hour point, he would like to note his observations of what he is seeing in the dishes, but is having difficulty.  He has dishes with dark zigzag lines where he inoculated the blood agar with the swabs.  While the lines are a dark grayish color, they don’t appear to have any colonies of microbes growing in them, as far as he can tell.  What is the proper way for him to describe these dark gray lines and, could there be something growing there that he is, as yet, unable to see that is causing the lines?  In the past, he has never inoculated his Petri dishes in this manner and has always had nice, mostly round or oblong colonies that are easily described be size, color, shape,  elevation, etc.  He has no idea how to describe these and, even worse, how to apply them to a graph or chart.  I think he is beginning to panic, and I know I am because I can’t help him.  Meanwhile, the microbes wait for no one and continue to multiply at their own happy pace.  Again, how can he describe these dark gray lines, are they caused by something growing there, and, how do they fit neatly into a chart or graph consisting of individual round colonies that can be counted?  Can someone help?  Thanks!

ANSWER: Hi lori
I may be able to help here. I was a Medic and a diagnostic bacteriologist while in the army and also worked at a VA hospital. First of all I have qualms about 4th and 5th graders growing mouth bacteria on blood agar. It appears you are using proper precautions however.
I have some questions about the experimental design. There are way too many variables here. Why is he using 18 different subjects If the object is testing cinnamon oil to inhibit bacterial growth. If he wants to find out the effects on a mixed colony the method is ok otherwise  he should isolate the dominant colony and grow a pure culture to test. I suspect he is looking for areas of no growth around the discs. The main hemolytic organism found in the mouth is beta strep and it is not normally there so I can see why you would not find any.. Pathogens are not found in every mouth. There should be no growth zones arout the non pathogens
Since I cannot see these dark grey lines I cannot tell you what they are. Can you send me some photos. Usually on blood media they are partially hemolytic common mouth strep.
Making colony counts requires making a grid with squares of known areas and placing the plate on the grid and counting random squares, finding the average per square and multiplying times the total. This does not seem necessary for this experiment.
Tell him not to panic. It is possible cinnamon will not stop growth. Negative results in science are not bad results. They are information. Without seeing the data I cannot comment on how to record or chart it.
Send me some photos

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Petri Gray Lines
Petri Gray Lines  

Last Year Charts
Last Year Charts  
QUESTION: Thank you so much for taking the time to respond!  I believe my grandson used eighteen volunteers for the purpose of obtaining a random sample of microorganisms found in the human mouth.   He is aware, through researching, that a balanced microbiotia is essential to maintaining a healthy mouth.  I don’t think his concern lies in eliminating pathogenic bacteria, but rather in the excessive build-up of microorganisms that can cause too much plaque and plaque in the wrong places, resulting in gingivitis, dental caries, and periodontal disease.   Dental plaque exists as a biofilm, and most oral anti-microbial agents are fairly ineffective against this.   Cinnamon oil, however, has been shown to have anti-adhesion and biofilm disruption properties, making it a great possibility for testing in the never-ending battle against periodontal disease.  My grandson is required to show his results in at least one graph and preferable two.  His graphs last year were based on the number of colonies per dish and the zones of inhibition (pics attached).  His BIG PROBLEM with his current experiment is this:  How do the dark gray “inoculation lines” fit into these types of graphs?  Or any type of graph, for that matter”, which is based on number of microbe colonies and zones of inhibition?

It would appear that this particular study  does not require the Scientific method and I have no experience with the bacteriology involving tooth decay and the use of cinnamon oils. I cannot tell you much.
The dark lines are strange. I cannot tell you much about the growth there except that common mouth strep appears greenish on blood agar. and there does not appear to be a mixed culture. Since I cannot identify what these are and I cannot tell you their importance to the project. Have made any Gram stains
 This looks like am amazing project for a child this young. Are there dentists in the family


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Walter Hintz


Science teacher for over 50 years. MSc. in biology. I can answer questions in general biology, zoology, botany, anatomy and physiology and biochemistry.


I have a MSc in biology and have been a science teacher for over 50 years. At present I am a faculty member at a college and a science consultant at seven catholic schools.

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