Science Buddies: "Ask an Expert"

I am doing the project similar to 'How Do Food Preservatives Affect the Growth of Microorganisms?' After I take samples on days 1,3, 5 and 7 of the four diff solutions with varying salt concentrations, what do I do with the agar plates - do I look at them after the 7th day? Are they supposed to be stored in a dark place or anywhere special? I have a low end microscope at home - should I use that or just observe and record the changes that I see?

Thanks

Hi there,
Are you talking about this project: https://www.sciencebuddies.org/science- ... p016.shtml?

Think about if time is going to be one of your variables. If so, then you may want to look at them after the 7th day. However, if you do not want to see how long the bacteria survive, then you do not have to. However, it is up to you what you decide to do.

After finishing with the agar plates, you should dispose of them properly. Look under "Disposal" here: https://www.sciencebuddies.org/science- ... Agar.shtml
Or, just read the blurb under the procedure of the project you are doing.

I'm not really quite sure what you mean by your question, "Are they supposed to be stored in a dark place or anywhere special?" Do you want to store the bacteria permanently, or do you want to incubate?

For observing the bacteria, you can do both.

Hope this helps,
blueswim

Yes, that is the project that I am talking about. I don't want to store the agar plates, I want to make sure that while they are incubating, I have them in the right place and temp (room temp).

Also, I guess if time is a variable - I would look at them all after the 7th day. Otherwise, do I look at them one day after I inoculate the petri dish?? How long do I wait to let the bacteria grow? I'm confused.

Thanks.

PLEASE HELP! Tomorrow is day 7 and I am not sure how I am supposed to interpret the samples that I have taken. Do i look at them under a microscope? Should I research what type of bacteria is growing (I looked at the "interpreting plates" link on the experiment page, but I am still confused)? The control definitely has the most stuff growing, so it shows that the salt concentrations are effective, but what else should I observe?

I am starting another trial (tomorrow will be day 1) to back up this trial but I don't know how to interpret the plates...

Hi ,

I do apologize for the delay in responding to your question. This is a great project. To interpret the plates, you can try counting the individual colonies on the control and preservative plates. If the colonies are not visible and are all grown together, you can report the results at “too numerous to count,” or “TNTC.” If you cannot distinguish visible colonies, you can estimate the percentage of the surface of the plate that is covered in growth. Try your best to quantitate the results.

You should also describe the color, form, elevation, and margin of the colonies using the terminology in the following guide:

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

For presenting your results, you can take photographs so the science fair judges can “see” your results.

Since you will be starting a new trial tomorrow, you should perform the experiment the same way and make observations on the microbial growth every other day. Your Petri dishes should be sealed with tape and kept in sealed plastic bags since you are working with unknown organisms.

Please let me know if you need more information.


Donna Hardy

Thank you so much for your help. I have just two more questions - 1 - the point of my project is that I am trying to show the best concentration of salt as a preservative. My experiment is showing that it is the higher concentrations - but now I have to apply my findings to the real world. I know that sodium is not healthy in large quantities and food doesn't taste good when it's too salty, so even though the higher concentrations are more effective, I am concluding that salt wouldn't be used alone as a food preservative in most cases (I think). Does that sound right? And 2- I would love to come up with a title that is not boring, do you have any suggestions?

Hi,

You have a thoughtful concern about your project. Your experiment is an investigation about the concentration of salt that will inhibit the growth of bacteria and it sounds like you have a well designed experiment with quantitative result done in duplicate. The health problem associated with consuming too much sodium is outside of the scope of your specific project, but you might want to mention the problems with using too much salt in your conclusion section and perhaps suggest that a potential future project would be to investigate methods for removing salt from preserved foods. That would demonstrate to the science fair judges that you are thinking ahead.

A question usually makes a good title for a science project. A couple of possibilities are:

1. Does sodium chloride inhibit bacteria?
2. Will salt kill bacteria?
3. Salt vs. bacteria
4. Can salt prevent microbial growth?
5. Does sodium chloride affect bacterial growth?

In your project write up, be sure to use “sodium chloride” instead of “salt’ most of the time. In chemistry, “salt” can refer to any type of ionic compound. In your project, you are a salt that is composed of two elements, sodium and chloride.

Donna Hardy

Thanks I will think about those questions as titles. I am recording data on the agar plates. Some of the plates definitely have round bacterial colonies that I can count. Some of them, though, have most of the plate surface covered in a large continuous opaque blob - I don't see any circles in it (it almost looks like little specs of yellow chicken broth might be in it). Would that be one big bacterial colony or is it just the sample on the agar with nothing growing in it? One of these the control so I am surprised. Maybe I didn't streak it right or something?

Hi,

It’s good that you can count some of the plates. For the plates covered with a large continuous blob, it sounds like you are looking at either no growth or “too numerous to count” Agar with no growth might have little specks of undissolved particles embedded in it. Did the blob plates change in appearance from the beginning of the incubation time? If it’s the same, then it’s probably no growth. It might be best to get a second opinion from someone who is familiar with bacterial growth. Of perhaps you could post a picture or two on this website.

Donna Hardy

Hi
Thank you for your answers. I attached two pictures of agar plates in a pdf file - one has nice spots that look like colonies and the other one shows a big blob. I would try to show you the blobs with the specs but my camera isn't that good and I bet you can't see it. So what do you think of these. Also, when I destroy these, my mom was going to help me and put them in a bleach solution. Do I have to do that since I don't know what I'm growing??

Hi,

Your pictures are great! You are right; the one plate shows individual colonies and the second plate shows one big blob colony, so you can't count the number of colonies on this plate. I cannot see the specks you have mentioned. You can describe the growth using the guide from the Science Buddies website using the official terminology. And you can definitely use these photographs to present your data.

https://www.sciencebuddies.org/science-f ... ates.shtml

The colonies look a lot like a Bacillus species to me. This is a large group of mostly non-pathogenic aerobic spore forming bacteria, however, since the identity of the organisms are unknown and you don't have any way to do further identification, you will have to soak the entire plate in bleach to kill everything before you discard it.

You have enough time to do a really good job in writing up this project. Here is information from the Science Buddies website for preparing you board.

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

Please let me know if you have any questions on this phase of the project.


Donna Hardy