How Can One Calculate CFU?

CFUs are used to measure bacterial growth and are often referred to as “colony-forming units”. A CFU represents one cell that grows into an independent bacterium after being placed on an appropriate medium. For example, suppose you wanted to know if there was enough S. aureus present to cause infection, you could take a swab from your wound scabs, plating out what’s left onto agar plates containing antibiotics, such as methicillin. If no colonies were found after incubation, then you would conclude that the amount of S. aureus in your sample was not high enough to infect you.

The term colony refers to small clumps of bacteria that grow on an agar plate after being inoculated with samples containing microorganisms. A single cell does not form colonies, but if you put many different kinds of cells into one dish, they will begin to cluster together by themselves to form a colony. This happens because each type of cell produces a kind of glue that helps them stick together so that they cannot easily separate from one another. The result is usually a flat surface covered with tiny dots called “clusters”. When these clusters become large enough, they are visible under the microscope. Read on to know how to calculate CFU.

What is CFC?

CFC stands for Colony Forming Cell which means it gives out a number of colonies when grown on agar medium containing specific nutrients called growth factors. It forms one single colony with unique morphology i.e. shape and size. All strains have their own preferred media conditions that give them the best growth rate. Different types of bacteria may grow at different speeds depending upon several parameters including temperature, pH, type of culture vessel, nutrient source, etc.

How do you calculate CFU?

To find out how many CFUs there are from any given sample simply take note of the following steps:

– Take your samples into sterile Petri dishes.

– Add 10 µl of each inoculum directly onto the surface of an appropriate plate. If necessary dilute further 1:10 before adding.

– Incubate plates overnight at 37°C.

– Count the number of colonies after incubation.

– Calculate CFU from this figure according to the formula below:

CFU Number of Colonies × Dilution Factor x Volume Of Sample Added To Plate / 100 mL.


If you see 2 colonies, then 10µL was added. So multiply 2×1020. 20÷100ml0.02 so 0.02cfu /ml is obtained. Similarly if you get 5 colonies then divide 5by100.05cfe/.01 ml.


Miles & Misra Method

In this method, we take 100 mL of culture in a 250 ml flask and add 10% Tryptone broth and incubate it at 37°C with shaking on rotary shaker till OD600 reaches 0.5. After that we take 1ml sample from each tube and pour into 9 cm petri dish containing a preweighed sterile filter paper. We allow them to air dry for 3 hrs so that they become completely dried out. Then we weigh both sides separately and subtract the weight difference between two sides, which gives us a total number of colonies formed. This method has less error than other methods because not only counting surface area but also considering the amount of media used. It’s an easy method to do.

The Miles & Misra formula is given below:

CFUs Total Number of Colonies × Dilution Factor ÷ Area of Petri dish x Volume of Culture Added