Science Buddies: "Ask an Expert"

Hi - my daughter is doing an experiment on if dog, cat or other animal saliva kills bacteria. In all of the research, the steps seem to say that after culturing the bacteria she should put multiple saliva samples on the bacteria in one dish along with a control. However, she wants to use a number of dogs and cats etc.. Is the experiement still valid if you put the saliva samples into different dishes? She has three different types of bacteria. She was thinking to grow each type (enough dishes for one for each animal sample) and then put the saliva in each separate dish. Would it be better to group the saliva samples as many as she can into each dish and repeat the experiment? I guess she is just confused on the proper procedure to get valid results and I can't be much help!!

Thanks.

Another question - would it be a better project to just test of dog saliva kills bacteria or would it be better to compare dog, cat, cow, horse, human etc....

Hi. I have merged your two topics into one so that experts who reply will see your questions all in the same thread.

Amy
Science Buddies

Hi KSolheid,
Welcome to the Ask an Expert forums. I understand the temptation to try to test many variables at once, but this can become very confusing and time consuming. I would recommend starting simple. For example, test one type of saliva and one type of bacteria with controls and multiple trials. If your daughter is able to complete that with time to spare, then you can always add other types of bacteria, and other types of saliva.

Once she has settled on a procedure, we can give more specific advice to help with issues that might come up along the way.

I hope that helps!

Chris

Hi KSolheid,

What an interesting project! I agree with Chris that it will be best to keep things simple. So, for example, you could start by testing only dog saliva on one type of bacteria. Using multiple dogs (with a separate dish for each) will be best, because this will show that the experiment was replicated - repeated more than once. In other words, your granddaughter will be able to show that it is (or isn't) dog saliva in general that had an effect, rather than the saliva of one particular dog. You should also make sure to have controls: dishes without dog saliva, in which the bacteria are allowed to grow. That will give you a comparison, so you can see whether the bacteria grew better or worse with the dog saliva compared to what would have happened with no saliva.

Please keep us posted with your granddaughter's progress!

Best wishes,
Heather

I think it would be fine to put the samples in separate dishes. Your protocol was probably trying to: (a) save you money by using fewer dishes and (b) control so that the bacteria are all the same, as they most likely would be if they grew in the same dish. But if you grow all the dishes the same way, you should be just fine!

Hi, What would be an example of a hypothesis if you are comparing dog, cat, and human saliva. My son is in 7th grade and plans to test the amount of bacterial growth of each of 9 specimens (which will have more bacterial growth?) - with 9 petri dishes in all - 3 different dogs, 3 different cats, and 3 different humans. Also, he understands that there should be a control, but how do we have a control - an agar petri dish that has not been swabbed with anything?

Hello kabota:

Welcome to Science Buddies. I have provided you with a link that explains how we generate hypotheses.

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

Also, you are on the right track with the control. You would use an agar petri dish that has not been swabbed with saliva.

Please let us know if you have any more questions.

Thanks,
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