Expert: Walter Hintz - 4/26/2007

Question
Thank you Walter for your help with my previous question.  You said something about noting colonies etc.  How should we go about doing this?  Noting different colors, the numbers?  Thanks so much for you help.  This has been very interesting.  I wish we knew what types of bacteria we were looking at.

Answer
Each species of bacterium has some observable characteristics regarding the colony.  here are some questions to ask.
What color is the colony? White, off white, yellowish, pink etc.
What shape is the colony. Circular with smooth edges. More spread out with rough irregular edges.
How big is it in relation to the other colonies on the plate.
Does the colony look creamy and smooth or rough.
Is it kind of mucus like. To find this out you can use a toothpick. Stick it in the colony.  If you do this be sure to touch the colony only with the toothpick and then drop the toothpick into alcohol.
To make a quantitative observation make a count of the different colonies.  If there are a lot this is difficult.  In that case you can make a grid. Using a ruler mark of a big square of about 10 centimeters.  Then divide the big square into square centimeters 1 cm x 1 cm. You now have a grid. Determine the total sq cm of the petri dish  ( 2 pi r) Place the petri dish over the grid.  Count the colonies in a square centimeter chosen at random.  Count 5 or 6 of these and find the average per square cm and multiply it by the total surface of the dish.