Allow the plates to incubate until colonies are easy to count but still easy to distinguish. Typically overnight incubation is sufficient for E. coli colonies to appear.
The below video shows how to manually count colonies from Compact Dry plates using an eCount colony counter.
Devices for counting colonies
A marker and good lighting conditions is in principle sufficient for counting colonies. But often you make counting errors when keeping the numbers in your head. It is highly recommended to let a device remember the count for you, e.g. by using an eCount or a touch-sensitive device like Scan 50 pro.
Automated colony counters are also available, but they are usually relatively expensive.
Step by step
1
Get the plates from the incubator
Retrieve one bracket of plates at a time. It is easier to detect colonies before condensate emerges at room temperature.
2
Find the plate with 25–250 colonies
Take a glance and pick the plate the one that likely contains 25-250 colonies.
3
Count colonies
Flip the plate and use a marker to count the number of colonies.
Tilt the plates at an angle to ensure you detected all the colonies.
4
Record colonies
Flip the plate and record the number of colonies on the plate.
The result is acceptable if the result is within 25–250 colonies. If not, count the adjacent dilution.
5
Calculate CFU/mL × Dil.
Determine the concentration of the sample by multiplying CFU/mL with the corresponding dilution. Only include the information from the suitable plates. The table below is a representative example when plating 1 mL. Multiply by 10 if you are only plating 0.1 mL per plate.
ID
Dilution factor
CFU
CFU/mL × Dil.
-8#1
10-8
125
1.25 × 1010
-8#2
10-8
107
1.07 × 1010
-8#3
10-8
113
1.13 × 1010
6
Add the CFU/mL values to the Access table
Transfer the values to the three BactoBox measurements in the column CFU/mL as shown with yellow highlight below. Access will use the individual results to calculate arithmetic mean and standard deviation. It doesn't matter which of the three BactoBox® measurements you add #1, #2, and #3 to.
Summary
You have now counted the number of colonies and transferred the results to Access. Next step is to Cross-check data.
