how would i measure bacteria growth in a petri dish?

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i am in 6th grade and for my science project i am seeing whether a dog's mouth is cleaner then a human's. so far i have taken a swab of saliva from both a human's mouth and a dog's i then put them in two different petri dishes (with agur) but i don't know how i would measure the bacteria to get i good idea on how much more bacteria grew then the other. also i have left the dishes out on my kitchen counter but nothing has grown yet and it has been a whole three days! and i need the results in two days! i'm very worried!!!

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@bacteriagirl97,

do you have a microscope?

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@bacteriagirl97,

You look at the petri dish and you count colonies. Do you have a teacher? Why not ask the teacher? Stop worrying and just ask. Or check your text.

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@ossobuco,

She should see the colonies (without a microscope) by now right? Maybe she got a flawed sample of bacteria or the agur had expired or something along those lines....

Either way, consult your teacher like Ms Ossobuco wisely suggested.

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@tsarstepan,

Agar plating is routine, just do it. No microscope needed as colonies show up in x number of hours, appear and vary (though I don't remember them, I once could recognize many). There is also blood agar, but I don't remember the details - it had to do with hydrolysis, or maybe I made that up.

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@ossobuco,

I was under the impression that for whatever reason, nothing is growing in her petri dishes. I assumed some part of her process or equipment is flawed.

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re our poster, 48 hours would give some results.

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@tsarstepan,

Ooh.

I have no idea from here why swabs (swabs???? a loop, maybe) would not have growth in a petri dish. I'd be worried.

Or did it sit out in the cold?

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Quote:

also i have left the dishes out on my kitchen counter but nothing has grown yet and it has been a whole three days!

Let grow in undisturbed warm location, ideally in an environment around 100° F (37° C) - not in sunlight or on a heating register.

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When the bacteria grow, each bacteria will become a bacteria colony that can be seen as a small spot on the petri dish.

http://www.freesciencefairproject.com/biology/images/bacteria_colonies/Bacteriacount.jpg

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@dadpad,

37 degrees is routine.
Three days and nothing? I'm questioning the access - a loop or swab. What exactly did you swoop at?

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Mostly I'm being nice. Patterns would be up in 24 hours at the least.

I guess I want to know about the procedure for the plating.

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@ossobuco,

Bacterial girl, how did you put the specimen on the plate? I am guessing you weren't told quite how to do that or maybe missed the instruction. I'm not accusing you, just interested. Maybe we can figure how to do it better.

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@bacteriagirl97,

bacteriagirl97 wrote:

i am in 6th grade and for my science project i am seeing whether a dog's mouth is cleaner then a human's. so far i have taken a swab of saliva from both a human's mouth and a dog's i then put them in two different petri dishes (with agur) but i don't know how i would measure the bacteria to get i good idea on how much more bacteria grew then the other. also i have left the dishes out on my kitchen counter but nothing has grown yet and it has been a whole three days! and i need the results in two days! i'm very worried!!!

Actually, you do have results. The result is that nothing has grown. That is a valid scientific result. One of the best lessons you might take from this class is not to anticipate what you expect your experiment to produce. Your teacher might appreciate that.

You should document your procedure carefully to show that you followed instructions and then you should report your results accurately. That's the scientific method.

If the bulk of similar experiments (from classmates or other sources) produce radically different results, then you might question the conditions of your experiment. For example, maybe your little sister or brother came along and sprayed Lysol into your experiment as a joke. But these are all different issues from reporting your results.

You might also report your results on this batch, and then repeat the experiment several other times with more controlled conditions. And then report those results accurately.

Have fun.

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@tsarstepan,

wed do an agar with a strained beef broth (no salt). You can photograph colonies with a digital camera . Record the time and have a visual scale next to the colonies and take pix every several hours. However , you have some other problem. You should keep your colonies covered and in a constant temp of say 90-105 degrees F. Your bacteria can live in many media and temps (theyre facultative) but it takes a while for a good start when you only keep it at room temp.

1 Reply

@rosborne979,

Quote:

Actually, you do have results.

not really. A standard method to reproduce optimal bacteria growth was NOT followed somewhere or else the mouths of people and animals are aseptic (Thats hardly more than a victorian aged conclusion).
Still, discussing the disparity between bacteegirls method /result and the expected result would help to document what went wrong. (However, I dont think bacteegirl has a meticulous control of her environmental data)

1 Reply

@farmerman,

farmerman wrote:

Quote:
Actually, you do have results.
not really. A standard method to reproduce optimal bacteria growth was NOT followed somewhere or else the mouths of people and animals are aseptic (Thats hardly more than a victorian aged conclusion).
Still, discussing the disparity between bacteegirls method /result and the expected result would help to document what went wrong. (However, I dont think bacteegirl has a meticulous control of her environmental data)

Obviously something went wrong with the experimental controls, or the way it was done. But I think it's an important lesson for little scientists to learn to report their data and not anticipate their results (as a general lesson).

The only reason we suspect that this experiment wasn't done correctly, or was damaged in some way is that it's been done before. But in the strictest sense, this experiment as an isolated incident, *does* have results.

1 Reply

@rosborne979,

Yeh youre right, this experiment is actually "new" to her class and that is worth recording all the disparate data from all the student scientists involved.

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@ossobuco,

i used a cotton swab (you know the kind used for makeup and stuff) then a made sure it was satrarated with spit and then i just rubbed it on to the agur

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@rosborne979,

there should be results though it would be weird if it turns out there is no bacteria in a dog's mouth!

0 Replies

how would i measure bacteria growth in a petri dish?

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