How many colonies on solid medium represent 100000 cfu/ml

To determine the number of colonies on solid medium that represent 100,000 colony-forming units per milliliter (cfu/ml), you need to perform a colony count on a plate of the medium.

Here's a step-by-step guide on how to do it:

1. Prepare the solid medium (such as agar) according to the appropriate formulation for the bacteria or organism you are studying or working with.
2. Take a known volume of your bacterial suspension, which contains the desired concentration of 100,000 cfu/ml.
3. Serially dilute the bacterial suspension by transferring 1 ml of the suspension into 9 ml of sterile diluent (e.g., saline solution or sterile water). This is the first dilution.
4. Repeat the dilution process by transferring 1 ml from the first dilution tube to a fresh tube containing 9 ml of diluent. This is the second dilution.
5. Continue this serial dilution process until you reach a dilution factor that allows for the countable number of colonies on a plate (usually between 30 and 300 colonies per plate is ideal).
6. Take 0.1 ml from the appropriate dilution tube and plate it on the solid medium using a sterile spreader or pipette.
7. Spread the bacterial suspension evenly over the surface of the plate.
8. Incubate the plate at the appropriate temperature and conditions for the organism you are studying or working with (e.g., 37 degrees Celsius for bacteria) for a suitable amount of time (usually 24 to 48 hours).
9. Count the number of colonies that have formed on the plate. It's important to count only those colonies that are distinguishable and not merged together. If there are too many colonies, you may need to perform another dilution to achieve a countable range.
10. Once you have the count, calculate the colony-forming units per milliliter (cfu/ml) using the dilution factor and volume used for plating. The formula to calculate the cfu/ml is:

cfu/ml = (colonies counted / volume plated) × (1 / dilution factor)

For example, if you counted 50 colonies on a plate from a 1:100 dilution (0.1 ml plated), the calculation would be:

cfu/ml = (50 colonies / 0.1 ml) × (1 / 100)

cfu/ml = 50,000 cfu/ml

By following these steps, you can perform a colony count on solid medium and determine the dilution that will result in 100,000 cfu/ml.