Introduction:

Colony-forming units (CFU) is a useful measure in microbiology, helping researchers and professionals in determining the viable number of microorganisms present in a liquid sample. The representation of these microorganisms is in CFUs per milliliter (CFU/mL). Accurate determination of CFU/mL is vital for various microbiological applications, such as bacterial growth monitoring, antimicrobial testing, and standardizing laboratory procedures. This article will guide you step-by-step on how to calculate CFU/mL.

Step 1: Prepare the Dilution Series

1. Obtain the original liquid sample containing the microorganisms.

2. Create a serial dilution from the original sample. Typically, this involves adding specific volumes of the original sample into several tubes or wells containing a known diluent like sterile water, saline solution, or buffer solution.

3. Mix each dilution well to ensure even distribution of microorganisms.

Step 2: Plate the Dilutions

1. Label Petri dishes or other suitable surfaces with appropriate identification and dilution information.

2. Transfer an aliquot (usually 0.1 mL or 1 mL) from well-mixed diluted samples onto individual labeled plates.

3. Spread the aliquots evenly on the plates using a sterile spreader or plate spinner.

4. Incubate the plates at optimal growth temperature and time for the target microorganisms.

Step 3: Count Colonies

1. After incubation, retrieve the plates and observe them for visible microbial colonies.

2. Select a plate with an easily countable number of colonies within an acceptable range (usually 30-300 colonies).

3. Count colonies on this plate manually using a colony counter or by recording them individually on counting tools.

Step 4: Calculate CFU/mL

1. Divide the total number of colonies counted by the aliquot volume plated (in mL). This gives you CFU/mL for that specific dilution.
CFU/mL (dilution) = Number of colonies counted / Aliquot volume plated

2. To obtain the original sample’s CFU/mL, multiply the calculated value by the dilution factor corresponding to the chosen plate.
CFU/mL (original sample) = CFU/mL (dilution) x Dilution factor

Example:

Suppose you plated a 0.1 mL aliquot from a 1:1000 dilution (10^-3), and the chosen plate contained 150 colonies. First, divide the number of colonies by aliquot volume:

(150 colonies / 0.1 mL) = 1500 CFU/mL for the specific dilution.

Next, multiply this value by the dilution factor to find the original sample’s CFU/mL:

1500 CFU/mL x 1000 (dilution factor) = 1,500,000 CFU/mL.

Conclusion:

Calculating CFU/mL is a crucial aspect of microbiological studies, and understanding this process is essential for anyone working with microorganisms. By following these steps and using proper aseptic techniques, you can successfully calculate CFU/mL to perform tasks like comparing bacterial growth under different conditions or evaluating antimicrobial effectiveness.