Expert: Walter Hintz - 1/23/2006

Question
My son is testing whether dogs or cats have more bacteria in their mouth for his 4th grade science fair project.  Last Wed we had the vet swab our cats and dogs mouths and put the sample in the petri dishes with agar.  Is everything in that dish that is growing bacteria?  They are so many spots in there that we couldn't possibly count them.  Do you have any suggestions at this point on how we can get a good measure of the amount of bacteria in the dishes?

Answer
Hi Pam:What grew are either bacteria colonies or fungus colonies. Be sure you or your son does not touch any of the bacterial colonies that are on the agar. Also be sure you tape the dishes shut before he shows them to the class.
 Each spot is a separate colony that grew fron one bacteria cell.  There are a couple of ways to compare the dishes.  You can do a sampling count. As you say there is no way to count thenm all. On a piece of paper make a grid of one/half inch squares.  Make it larger than the area of the bottom of the petri dish. Set the dish on the grid and count the number of squares the dish covers. Since the dish is round you will have to do some estimating  Chose five squares at random and count the colonies in each one. Add them up and divide by five to get an average.  Multiply that number times the number of squares the dish covers. You will get a fairly accurate number of total colonies.  Which has more colonies? The cat or dog dish.
 You  can also compare what is in the mouths because all bacterial colonies are nit the same.  They differ in size, shape, color and consistancy.
  My quess is that ther ewill be no difference in the mouth of a cat and a dog.  If this is the case that is al right.  Infromation has been gained and that is what science is all about.
 Please let know your results.