Science Buddies: "Ask an Expert"

Hi. I'm doing a project that tests the uniqueness of bacteria in different coordinates of the same source. For example, I'll be swabbing opposite sides of the same leaf and comparing them once they've grown on the petri dish. I was wondering if there were any actualy tests that I could perform once they cultures are grown (takes 1 week, yes?), that would get me some data applicable to the project idea? I'm looking to get some data that goes beyond just appearance. I'm hoping for one that can be done in 1-2 days, as I'm pretty pressed for time. My resources are also somewhat limited. As long as the requirements for the test are some easily purchasable chemicals or tools, then that'd be fine. Thanks!

Hi,

Do you plan to do swabs on something else besides leaves? What is your hypothesis? What question are you trying to answer by your experiments?
I'm assuming you are using nutrient agar plates because that is what most students use, so you will get some fungi--molds and yeasts--growing on the agar as well as some bacteria. Try to keep the plates at 78-85F (26-30C). If they are too cold, the microorganisms won’t grow very well.

You could ask the question: "Are there more bacteria or fungi on the upper or lower surface of a leaf? The upper surface may be exposed to sunlight and the UV in sunlight is harmful to many types of bacteria. Thus you might predict that you would find more species of bacteria on the underside of a leaf than on the top. Fungi are more resistant to sunlight and might not show this effect.

You would have to do swabs from several species of plant because leaves vary a lot in structure and microbial colonization. If you have a limited number of plates, you can divide the agar area in half by drawing a line across the center on the bottom of the Petri dish with a Sharpie. Then you can do two swabs on it--one from the top surface of a leaf and one from the bottom and compare them directly by taking a photo.

When you do swabs on nutrient agar there is a chance of growing a dangerous human pathogen, so after you have swabbed them, seal the lid to the bottom with tape and DO NOT open them. You can take pictures through the clear plastic lid.

After your experiment is all done, autoclave the plates if you can, and if not, fill a pail with 10% Clorox and put the plates in the liquid. Leave them there for a couple of days. The Clorox will seep inside and kill the bacteria. Then you can dispose of them in the biohazard waste bin at your school.

Identifying species of bacteria is difficult and requires a lab and special chemicals and methods. You can often distinguish bacterial colonies from fungal colonies by their appearance, and that would be one test you could do. Check out this reference that shows you what various types of bacteria look like growing on agar: https://www.sciencebuddies.org/science- ... ates.shtml

If you have more questions, we will be happy to help.

Best wishes,

Sybee

The leaf is just an example of how it will be from a slghtly different place wthin the same source. I'll do household objects, a car, the street, ect, and take two swabs from each, and compare if the dish that is of the same source will exhibit any notable differences. That's the jist of it. Unfortunately, it's too late for me to change a project idea.

My hypothesis states that the swabs from the same source will not exhibit any notable differences. I'm basically going in not expecting anything particulalry interesting but acknowladging that the hypothesis could be completely wrong.

So are the tests pretty much out my reach, as far my resources/time go?

Even if that is so, I'd really appreciate if you could give me some advice or guidelines for analyzing the cultures?

I kinda like the leaf idea. At least there's something scientific to it. You have the potential to distinguish fungi from bacterial colonies and connect this with conditions on the underside of the leaf that are different from the upper. Plus, you might find some really cool bugs that you would never see on ordinary surfaces in human dwellings.

If you must do ordinary things then at least maybe break it down into those parts of the object that humans specifically touch like door or refrigerator handles and compare the bacteria and fungi on them to swabs from the parts that humans never touch. If you have a pet, why don't you make a swab of their fur, the pads of their feet and their mouth for comparison.

As i said before, SEAL the plates and do not open them after you do the swabs! There are some microbes out there that can make you very sick and you don't want to encourage them by giving them lots of nutrient agar to eat, then expose yourself to them by opening the Petri dish. Try to find a warm, dark place to leave the plates where no one will disturb them. Human bacteria grow best at temperatures above 90F, but fungi like it cooler. Check the plates every day and write down what you see in your lab notebook. Take pictures too.

There's no tests you can run without a lab. I would just take lots of close-up pictures and try to do what identification you can by color and form. Read the information I gave you on the link. It applies to bacteria and fungi from any source.

Good luck and do let us know how it turns out.

Best wishes,

Sybee

Ok. Thanks a bunch for your help!

Sounds like a really interesting project!
One thing as a biology teacher I want to remind you is that along with keeping the plates sealed throughout your experiment - when you discard them, be sure to follow the safety requirements for this.
We always soaked our plates in a 10% bleach solution - do not open the plates.
the bleach should kill what is living there -
then you may discard.

good luck.

Oops I actually do have 1 question- I've now gotten all the swabs in petri dishes, and have just placed them all in a ziploc bag. The guide I'm looking at says to "open the bags" daily but it doesn't specify how long. Basically the question is-for how long should I open the bags?


Also, I've set the plates all right next to each other but they're still all in different bags. Is that ok?

There is no reason to put the dishes inside bags IF you tape the lids to the bases so they can't come off. Did you do that? The bacteria and fungi that you are looking to grow on these plates need oxygen and if you put them inside a ziplock bag they may not get enough. I would advise you to take the petri dishes out of the bags and leave them out with the lids taped down so they cannot come off. Did you write on the edge of the BOTTOM of each dish with a Sharpie where the swab was from?

Where are you keeping the dishes? Are you keeping them in a warm [85-90F]. dark place? Many of these bacteria need warmth to grow so you if you put them in a cool place, they will take much longer to form colonies. I would put all the dishes together inside a box in a warm, dark place where they can get air and not be disturbed by children or pets. There is no risk of anything coming out of the dishes as long as the lid is taped down.

You need to let the plates grow for several days to as long as a week. Check them every day and take photographs of each plate.

Let us know what the results are.

Sybee

I did in fact tape them shut and label them. So I took them out of the bags. I'm keeping them in a lower compartment in a large desk. Since the compartment is directly below the desk, they probably shouldn't get much light exposure. As for temperature, it is about room temperature, so I dunno how detremental that would be. There's no pets or young children in the household, so that isn't a burden.

And once again, thanks for helping, I really do appreciate it!

You are welcome!

If your room temp is 75F or above the microbes will grow ok--just slower than at 90F. Look at the plates every day and record any changes that you see, the appearance of colored colonies, fuzzy growths or anything that looks out of the ordinary.

Document your experiment with photographs of all dishes taken on specific days. Do this several times during the growing period so you have a record of the changes. Bacteria and especially fungi can produce chemicals that can diffuse through the agar to inhibit the growth of other microbes nearby. This is a sort of chemical competition and you may see it happening in mixed cultures such as yours.

Do let us know how it turns out.

Best wishes,

Sybee

This may be my last question. It has been roughly 24 hours since I cultured the swabs, and I don't see much of anything on the plates, spare a few dots on 1 or 2 of them. Is this normal? If so, when should I be expecting to get some results? The temp is about 70 degrees F, and the area is quite dark, and for each swab, I rubbed on the dish for about 5-10 seconds.

Hi,

70F is a bit too chilly. I told you it was necessary to have a temperature of 85-90F, otherwise it might take a long time for some of the bacteria to grow.

For growing bacteria most science fair students use a low-wattage lamp to warm up the space where the dishes are. If you have the petri dishes in a closed box, you can put it next to the lamp and the heat from the bulb will warm it enough to speed up the growth.

You would not see much growth in 24 hours unless you had the plates at 98.6F [37C, human body temperature]. Do as i suggested. Keep the plates warm and check them every day. Photograph them and record what you see. It will probably take several days before you see any larger colonies and i would let them go for 7 days at least before terminating the study. There may be some unusual microbe that is slow growing but very important.

Keep us posted on your progress.

Sybee

In our biology classroom, I had to plan on lab time over the course of several days and others' schedules - so I deliberately left our petri dishes out in the classroom at room temperature which was about 72 - 75 degrees F - it took about 3 days for us to see growth.
The slower rate of growth was actually a good thing - we counted colonies on days students had this class -
It also allowed us to use this data on a graph - and we could analyze which plates / conditions where bacteria growth proceeded the fastest/ slowest.

In an incubator - (another year) our growth was as predicted - quicker - but did not allow for us to see the change in growth over time, since most plates had too many bacteria colonies to count.

Good luck!