Understanding MPN and CFU
MPN (Most Probable Number) and CFU (Colony Forming Units) are both methods used to estimate the number of microorganisms in a sample. However, they differ in their methodology and the type of information they provide.
MPN Method
The MPN method is a statistical technique that relies on dilution series and the presence or absence of growth in a series of tubes.
- It involves diluting a sample multiple times and inoculating a series of tubes with each dilution.
- After incubation, the tubes are observed for growth.
- Based on the pattern of growth and no growth, the MPN is determined using statistical tables.
MPN is best suited for estimating the number of viable microorganisms in a sample, especially when the organisms are difficult to culture on solid media.
CFU Method
The CFU method involves plating a known volume of a sample onto a solid medium and counting the number of colonies that grow.
- Each colony is assumed to have originated from a single viable microorganism.
- The CFU count provides an estimate of the number of viable microorganisms in the original sample.
CFU is a direct count method that provides a more precise estimate of the number of microorganisms than MPN, especially when the organisms are easily culturable on solid media.
Key Differences
| Feature | MPN | CFU |
|---|---|---|
| Methodology | Dilution series and growth/no growth observation | Plating on solid media and colony counting |
| Type of information | Statistical estimate of viable microorganisms | Direct count of viable microorganisms |
| Accuracy | Less precise than CFU | More precise than MPN |
| Applicability | Difficult-to-culture organisms | Easily culturable organisms |
Example: Water Quality Testing
- MPN can be used to assess the fecal coliform contamination in water samples, as these bacteria are difficult to culture on solid media.
- CFU can be used to estimate the number of bacteria in a food sample, such as milk or cheese, where the bacteria are easily culturable.
Conclusion
Both MPN and CFU are valuable tools for estimating the number of microorganisms in a sample, but they differ in their methodology and the type of information they provide. The choice of method depends on the specific application and the characteristics of the microorganisms being studied.
