Science Buddies: "Ask an Expert"

I completed my experiment, collecting samples of bacteria at fast food restaurants in the bathroom sink, on a table and on a floor. The bacteria grew in petri dishes with nutrient agar, and I saw lots of growth! I thought I would simply be able to count the colonies to tell which restaurant is the cleanest. However, in some dishes the colonies are very small and numerous of the same bacteria, making a large number, but not filling the dish. Others have small numbers of larger colonies of various types of bacteria, but take up most of the dish.

What is the best way to interpret these results? Does the largest number of small colonies mean it's the "dirtiest" or does a fuller dish with many different types of colonies mean it is "dirtiest"?

Any help is appreciated!!

Hello,

If you are counting the bacteria in general then in that case you would just count the colonies and whichever plate has more would be representative of being the “dirtiest” .Example: If you have three plates and find that the largest number of colonies is on the second plate then that area where you collected your sample from will be the dirtiest. Understand? However if you are counting how many bacteria of a certain kind there are then that is a different case. If that is what you are doing I would say to just collect the number of species there are and how many colonies per each species you have .Then whichever plate has the most of that species,then that would be the "dirtiest".But that depends if you are investigation how much bacteria there is in an area as a whole or if there is one certain kind of bacteria and where it's more prominent.

I hope that will help to guide you and good luck!

Thanks science diplomat for the reply.

I am just struggling with calling a dish with over a hundred very small dots "dirtier" than a dish with less, larger colonies that almost fill the dish. Does that make sense? But if that is the correct way to count the bacteria, then that is what I will do.

anyone else with input??? please!!

You are welcome I wish I knew a bit more to give you a more thorough explanation but I have had limited experience in molecular biology.I see what you are saying.I seached for a similar question that might give you some more input,it's not too different from my own answer but maybe it will clarify the situation.Here is the link:http://www.newton.dep.anl.gov/askasci/b ... o99891.htm

Hi,
What might be happening with the larger colonies is that there could be more bacterial colonies that have multiplied so quickly that they have merged with nearby bacterial colonies to form these "blobs" that you now see. In addition, if you want a more accurate way of measuring the bacterial colonies, you can check out this website:

http://www.ehow.com/how_5008505_count-b ... -dish.html

Hope this helps!

Microbiologists typically count number of colonies, or "colony-forming units."

In that case, the question you're asking is, how many bacteria were in my original sample? You couldn't see them when you first took the sample, so you set them on the agar and give them time to multiply. Each colony represents a bunch of bacteria that came from one original bacterium (or colony-forming unit).

It's counterintuitive, but the size of the colony isn't the key factor here. As a previous expert noted, the bigger colonies are just the ones that grew faster. There are a couple reasons for this. The big spots may be a kind of bacteria that grows really fast. Also, if there are only a few bacteria on a plate, then they'll have all the nutrients they need and can divide many times, making a big colony. But if there are lots of bacteria to start with, they'll have to share the same amount of nutrients--thus they may grow smaller colonies just because they're not getting as much to eat and dividing less.

If you have a ton of colonies on a single plate, it can be tough to count them all. Microbiologists typically lay or draw a grid on the bottom of the plate, then you can count the number of colonies in each square without losing count over the whole plate. For example see this picture: http://water.usgs.gov/owq/FieldManual/C ... g7.1-4.gif

If you are seeing different kinds of colonies, that would certainly be a good thing to note. It may indicate the diversity of your sample.

---Amber

Thanks for the replies!

When I planned my experiment, I just thought I would count the colonies of bacteria to determine which restaurant is dirtiest. So you think that to figure out the dirtiest I should count the different types of bacteria on each dish? Or should I count the total number of colonies per dish (all types combined)?

Thanks for the help!

Each count will answer different questions.

Counting #colonies="How many bacteria (of any type) were on the floor?" I think this is closest to the "how dirty?" question.

Counting the types="How many different kinds of bacteria were on the floor?"