How to Count Bacteria Colonies in a Petri Dish

Counting bacteria may seem like a task without purpose, but it is valuable in that it allows scientists and doctors to determine how fast a bacteria is replicating and how potentially dangerous the bacteria is. Counting bacteria can be done directly with a microscope, but a more common method of counting is the serial dilution method.

Things You'll Need

• Agar plate (petri dish)
• Swabs
• Tube of 1 ml saline solution
• Tube of broth (sterile)

Instructions

Serial Dilution Method

1. Put a swab in the saline tube and squeeze out any extra saline against the side of the tube.

2. Rub the swab against the surface of anything you want to test for bacteria.

3. Dilute the bacteria sample by swishing the swab in the tube of sterile broth.

4. Remove the swab from the broth and, with the tube capped, shake the tube to make sure the bacteria get mixed throughout the broth. This makes a 1:10 dilution.

5. Uncap the broth tube and dip a fresh swab into the broth, squeezing out any extra broth from the swab against the side of the tube.

6. Rub the swab against the surface of a petri dish.

7. Put the dish in an incubator or any warm place and wait up to two days for bacteria colonies to grow.

8. Manually count the number of bacterial colonies you see in the dish.

9. To estimate the number of bacteria in the dish (and thus on the surface you tested), divide the number of colonies you counted by the dilution factor (10).

Tips & Warnings

• To make sure your estimate is accurate, you will want to repeat the process above using 1:100 and 1:1000 dilutions, as well.

• Some bacteria are very dangerous to your health. Always wear gloves while working with your samples.

Resources

• methods for counting bacteria