1. What is the colony count method?
- The colony count method is the most common microbiological method today, used to determine the number of living microorganisms in a test sample by culturing on agar medium and counting the colonies formed after the appropriate incubation time.
- Each colony is assumed to form from one living microbial cell or a cluster of cells, and results are usually expressed as CFU (Colony Forming Unit).
👉 What is CFU?
- CFU (Colony Forming Unit) is the unit expressing the number of living microorganisms capable of forming colonies in 1 g, 1 mL, or 1 unit
2. The principles of the colony count method
2.1 Basic principles
- Sample dilution: the original sample usually has a high microbial density, so it must be diluted several times so that the countable colonies fall in the appropriate range when plated.-
Selective/differential culturing: use a culture medium and incubation conditions (temperature, time) that allow only the target microorganisms to grow, creating recognizable c-
Safe counting range: count only plates with 30-300 CFU/plate (or 15-300 CFU/plate depending on the regulation) so the result has high statistical value.-
Accurate counting: count each colony individually, mark the counted colonies (on the bottom of the plate), or use an automatic counter, to avoid confusing contamination -
Calculating the result: use a formula to calculate the microbial density in the original sample (CFU/mL or CFU/g) from the counted colony number and the dilution factor.➡️ Each colony appearing on the plate is regarded as the growth result of one living microorganism in the original sample.
2.2 Conditions for applying the colony count principles
To ensure accurate results, the colony count method follows these important principles:
- Count only plates whose colony numbers fall in the allowed range 👉 Usually:
30 – 300 CFU/plate- The culture medium must be suitable for the target microorganism.
- Incubation conditions (temperature, time, atmosphere) must be strictly controlled.
- Handling must guarantee absolute sterility.
3. Common colony count methods today
3.1 Pour plate method
Principle: the diluted sample is placed in a petri dish, then molten agar medium (≈45°C) is poured in and mixed evenly.
* Characteristics
- Colonies grow both inside and on the surface of the agar
- Suitable for counting the total aerobic count
- It can affect heat-sensitive microorganisms
3.2 Spread plate method
- Principle: the sample is dropped directly onto the surface of the set agar medium and spread evenly with a sterile spreader.
* Characteristics
- Colonies grow only on the agar surface
- Easy to observe and count
- Suitable for aerobic microorganisms
3.3 Pre-poured plate method (ready-to-use plates)
Nowadays, many laboratories use
pre-poured culture plates such as Compact Dry, Easy Plate… following the same principle as the traditional method but:- No medium preparation needed.
- Fast, neat handling.
- Fewer handling errors.
- Suited to testing food, drinking water, cosmetics, and pharmaceuticals.
4. How to calculate the colony count result (CFU)
4.1. CFU formula
4.2 Illustrative example
- Counted: 120 colonies
- Plating volume: 1 mL
- Dilution factor: 10⁻³
👉 Result: 120 × 10³ = 1.2 × 10⁵ CFU/mL
5. Applications of the colony count method
The colony count method is widely applied in:
- Food microbiology testing
- Testing drinking and household water
- Cosmetics and pharmaceuticals
- Animal feed
- Production environments and hospitals
* Common analytes:
- Total aerobic bacteria (TPC)
- Coliforms
- E. coli
- Yeasts and molds
- Enterobacteriaceae
- Staphylococcus aureus
6. Advantages and limitations of the colony count method
Advantages
✔ High accuracy
✔ Reflects the number of living microorganisms
✔ Complies with ISO, AOAC, and TCVN standards
Limitations
✘ Long wait for results (24–72 hours)
✘ Cannot detect microorganisms that cannot be cultured
✘ Depends heavily on handling technique
The principles of the colony count method are an important foundation in microbiological testing, helping determine the exact density of living microorganisms in many different sample types.
With high reliability and wide standardization, this method remains the top choice in modern microbiology laboratories.
👉 Understanding the principles, application conditions, and how to calculate CFU helps businesses and laboratories ensure product quality, comply wi