Dilution Theory / Supplementary Pages:

A Five Tube MPN Table

At the right is a 5-tube MPN table, taken from the Standard Methods for the Examination of Water and Wastewater, 15th edition (1980) and adapted for use to determine the most probable number of positive organisms per inoculum of the middle set of tubes. The same rules apply as previously stated in that the actual amount of sample inoculum decreases ten-fold with each succeeding set of tubes – which can be accomplished by (for example) making one ml inoculations from decimally-increasing dilutions as in the following example.

As an example problem: Suppose 5 tubes of an all-purpose medium are each inoculated with 1 ml of a 10^–2 dilution of a water sample, and 5 more are likewise inoculated from a 10^–3 dilution as are 5 more from a 10^–4 dilution. If the results show 5 positives for the first set of tubes, 3 for the second and 1 for the last, the 5-3-1 combination matches with the MPN value of 1.1 which, according to the table, means that there would be (on the average) approximately 1.1 positive organisms per ml of the 10^–3 dilution. Therefore, the number per ml of the original, undiluted water sample would be 1.1 X 10³. Remember that such a value is really a rough estimate. Lower and upper 95% confidence limits listed for each combination of positive results in Standard Methods can be different by ten-fold or more.

© 2000, John Lindquist