Serial Dilution Method for Estimating Viable Count of Bacteria

Serial Dilution Method for Estimating Viable Count of Bacteria

Serial Dilution Method for Estimating Viable Count of Bacteria: Estimating the viable count of bacteria in a sample is an essential aspect of microbiology. This number, also known as the colony-forming unit (CFU), gives an idea about the number of viable (or living) bacteria in a sample.

One of the most common techniques used to calculate this count is the serial dilution method. This method involves a series of step-wise dilutions, followed by plating and colony counting.

Serial Dilution Method for Estimating Viable Count of Bacteria: A Step-by-Step Guide
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Materials Required for Serial Dilution Method for Estimating Viable Count of Bacteria

  1. Bacterial culture (sample)
  2. Diluent (sterile saline or broth)
  3. Sterile pipettes and pipette tips
  4. Sterile test tubes
  5. Agar plates
  6. Incubator
  7. Colony counter
  8. Safety goggles, lab coat, and gloves

Steps for Estimating Viable Count of Bacteria using Serial Dilutions Method

Step 1: Plan Your Dilution

First, determine how many dilutions you’ll need. This number often depends on the estimated concentration of your initial sample. For a highly concentrated sample, you might need more dilutions. A common series might include dilutions of 1:10, 1:100, 1:1000, 1:10,000, and so on.

Step 2: Prepare the Diluent

Prepare enough diluent (usually a sterile saline solution or nutrient broth) to use in each of your dilution steps. The diluent should be stored in a sterile container to prevent contamination.

Step 3: Make the First Dilution

Transfer a specific volume of your original bacterial culture into a test tube containing the diluent. For a 1:10 dilution, you could add 1 mL of your culture to 9 mL of diluent. Ensure thorough mixing to create a uniform solution.

Step 4: Continue the Dilution Series

Repeat the process described in Step 3 for each subsequent dilution. Always use a new, sterile pipette to avoid cross-contamination.

Step 5: Plate the Dilutions

After making the dilutions, spread a known volume (e.g., 0.1 mL) of each dilution on an agar plate. Ensure to change pipette tips between each plating to prevent cross-contamination.

Step 6: Incubate the Plates

Incubate the plates at an appropriate temperature for your bacteria (usually 37°C for human pathogens) to allow colonies to form. The incubation period might vary, usually between 24-48 hours (Plate Exposure Method).

Step 7: Count the Colonies

After incubation, count the colonies on each plate. Choose a plate that has between 30 and 300 colonies to ensure accurate counting. Plates with fewer than 30 colonies may not give a reliable estimate, while plates with more than 300 colonies may be too crowded for accurate counting.

Calculating the Viable Count of Bacteria by Serial Dilution Method

After obtaining the colony count, you can calculate the viable count of bacteria in your original sample using the following formula:

Viable count (CFU/mL) = Number of colonies x dilution factor ÷ volume of culture plated

Here’s an example of how to perform serial dilutions and subsequent bacterial colony counts by Serial Dilution Method:

Table 1: Dilution Series

Dilution StepVolume of Bacterial Culture (ml)Volume of Diluent (ml)Final Volume (ml)Dilution Factor

Table 2: Colony Count and Viable Count Calculation

Dilution StepNumber of Colonies (After Incubation)Viable Count (CFU/ml)
32502.5 x 10^6

In this example, for the 1:1000 dilution, if we plated 0.1 ml of the diluted culture and counted 250 colonies after incubation, the viable count would be calculated as:

Viable count = 250 colonies x (1/0.1 ml) x 1000 (reciprocal of the dilution factor) = 2.5 x 10^6 CFU/ml

This means there were approximately 2.5 million viable bacteria per milliliter in the original bacterial culture. For dilutions resulting in more than 300 colonies, the count is considered too numerous to count (TNTC) and results are deemed inconclusive. Always aim for a countable range between 30 and 300 colonies for accurate estimation.


While performing serial dilutions and viable count estimations, maintain sterility and avoid cross-contamination. Always wear appropriate personal protective equipment (lab coat, gloves, and safety goggles) and dispose of all biological waste appropriately.

Remember, while this guide provides a general method for performing serial dilutions and estimating viable bacterial counts, adjustments may be needed based on your specific experiment or bacterial strain. Always consult relevant research papers, protocols, or lab manuals for specific instructions.


Recording observations during the process of serial dilutions and viable count estimation of bacteria is an integral part of the experiment. Here are a few things you might observe and want to note down:

  1. Growth Patterns: Observe the pattern of bacterial growth on the agar plates. Are the colonies evenly distributed? Is there a visible difference in growth between the plates corresponding to different dilutions?
  2. Colony Morphology: Examine the morphology of the colonies. Are they all similar, indicating a pure culture, or are there variations suggesting a mixed culture? Note the size, shape, color, and texture of the colonies.
  3. Counting Variations: Are there significant variations in colony numbers between replicates of the same dilution? This could indicate an issue with your dilution or plating technique.
  4. Colony Numbers: If there are too few or too many colonies to count, note this down. It may suggest that you need to adjust your initial dilution factor.
  5. Contamination: Look for signs of contamination, such as unexpected growth in control plates or unusual-looking colonies.
  6. Dilution Consistency: Is there a consistent reduction in colony numbers with each dilution step? This would typically be expected in a serial dilution experiment.

Each of these observations can provide valuable insights into your experiment, and they can help you interpret your results, troubleshoot issues, and plan future experiments.

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