Difference Between MPN and CFU

The main difference MPN and CFU is that MPN (Most Probable Number) estimates the concentration of microorganisms by growing them in a liquid broth whereas CFU (Colony Forming Unit) estimates the number of viable microorganisms by growing them in a solid agar. Furthermore, CFU is a more precise measurement than MPN.

MPN and CFU are two techniques used to estimate the concentration of bacteria in a sample. In MPN, the number of microorganisms is determined by counting the number of tubes with positive reactions. In CFU, colonies appear as spots either on a spread plate or pour plate. 

Key Areas Covered

1. What is MPN
     – Definition, Method, Facts
2. What is CFU
     – Definition, Method, Facts
3. What are the Similarities Between MPN and CFU
     – Outline of Common Features
4. What is the Difference Between MPN and CFU
     – Comparison of Key Differences

Key Terms

CFU, Colonies, Colony Forming Unit, Microorganisms, Medium, Most Probable Number, MPN, Positive Reactions in Tubes

What is MPN

MPN (Most Probable Number) is a method used to estimate the concentration of viable microorganisms in a sample by means of replicate liquid broth growth in ten-fold dilutions. It is mainly used to estimate the microbial count in the soil, water, agricultural products, etc. It is one of the common methods of quality testing of water to ensure the safety. The presence of fecal coliform in water is an indication of fecal contamination of the water source. Serially diluted water samples can be inoculated in lactose broth and are incubated. Coliform in the lactose medium produces acid and gas. The presence of acids give a color change in the medium or produced gas may be collected as bubbles inside the inverted Durham tubes. The number of total microorganisms is determined by the number of tubes with positive reactions.

Figure 1: MPN for Fecal Coliforms

What is CFU

CFU (Colony Forming Unit) is a measurement of the number of viable microorganisms in a sample, which is proportional to the number of colonies in or on a solid plate. A colony forming unit is an individual colony of bacteria formed by a single microorganism. CFU is generally measured as CFU per unit volume. A serial dilution of a sample is poured and spread evenly on agar plates and the plates are incubated for the colony growth. Then, the number of colonies can be counted. Since it measures the number of colonies on a solid medium, CFU is a more precise method than MPN during the determination of the concentration of microorganisms.

Figure 2: Colony Counting in CFU

Similarities Between MPN and CFU

• MPN and CFU are two measurements that estimate the concentration of microorganisms within a sample.
• They measure the viable count of microorganisms within a sample.
• Both use ten-fold diluted samples for the counting.

Difference Between MPN and CFU

Definition

MPN: A method to estimate the concentration of viable microorganisms in a sample by means of replicate liquid broth growth in ten-fold dilutions

CFU: A measurement of the number of viable microorganisms in a sample, which is proportional to the number of colonies in or on a solid plate

Type of Medium

MPN: Liquid Broth

CFU: Solid medium

Tubes/Plates

MPN: Tubes

CFU: Pour plates or spread plates

Counting

MPN: Number of positive colonies

CFU: Number of colonies on or in the plate

Precision

MPN: Less precise

CFU: More precise

Type of Microorganisms

MPN: Mostly used to determine the concentration of coliforms in water

CFU: Bacteria and fungi

Conclusion

MPN estimates the count of microorganisms in liquid broths while CFU estimates the number of colonies in the solid plates. Both MPN and CFU are methods to estimate the concentration of microorganisms in a sample. The main difference between MPN and CFU is the type of measurement obtained in each method.

Reference:

1. “Most Probable Number (MPN) Test: Principle, Procedure and Results -.” Microbe Online, 26 Mar. 2018,   Available Here 
2. Kung’u, Jackson. “Colony Forming Units (CFU).” Mold Bacteria Consulting Services, Available Here

Image Courtesy:

1. “Faecal coliforms (MPN method) (4370533922)” By SuSanA Secretariat – (CC BY 2.0) via Commons Wikimedia  
2. “Manual CFU counting” By Quentin Geissmann – Own work (CC BY-SA 3.0) via Commons Wikimedia