Microbial MPN Count: Multiple-Tube Fermentation Table
What Is the MPN Method?
The MPN (most probable number) method estimates the concentration of a target microorganism (such as coliforms) from the combination of positive tubes in multiple-tube fermentation, and suits samples with low counts or those needing selective enrichment. The sample is serially diluted in ten-fold steps, each dilution is inoculated into several tubes, and the pattern of positive tubes maps to a most probable number (MPN value). The MPN value is itself an estimate whose 95% confidence interval is usually wide, so interpret it together with the interval.
How to Read the MPN Table
Record the number of positive tubes at each dilution (for example, 3-1-0) and look up the corresponding most probable number and 95% confidence limits in the MPN table. GB 4789.3 provides an MPN table for three tubes at three dilutions (0.1, 0.01 and 0.001 g/mL), while some industries use a five-tube method (five tubes at three dilutions). The tool includes both tables and returns the MPN value and confidence interval directly from the positive-tube combination.
How to Report Results
Report the result as MPN/g (mL) or MPN/100 mL (for example, coliform standards based on 100 mL) and note the inoculum volume and dilutions; the MPN value must be converted according to the lowest dilution if the minimum inoculum is not 0.1. The tool converts automatically and gives the 95% confidence limits for comparison with the limit. State the table type (3-tube or 5-tube) and inoculum volume in the report to avoid misinterpretation, and export the report for audit traceability.
Precautions
Choose dilutions that produce a mixture of positive and negative tubes, since all-positive or all-negative patterns are unreliable and the dilutions should be adjusted and repeated. More parallel tubes per dilution (for example, five) give a more precise estimate. Because MPN is a statistical estimate with a wide 95% confidence interval, re-test or add samples when results approach the limit, ensure representative sampling, and keep strict aseptic technique so that contamination does not distort the positive-tube counts.