The most probable number (MPN) method is a statistical assay of cell numbers based on the theory of probability. The goal is to successively dilute a sample and determine the point at which subsequent dilutions receive no cells.
To determine the MPN, three sets of three or five tubes containing the same growth medium are prepared (figure 4.15). Each set receives a measured amount of a sample such as water, soil, or food. The amount added is determined, in part, by the expected bacterial concentration in that sample. What is important is that the second set receives 10-fold less than the first, and the third set 100-fold less. In other words, each set is inoculated with an amount 10-fold less than the previous set. After incubation, the presence or absence of growth in each tube in each set is noted; in some cases, growth along with a characteristic visible change such as gas production is noted. The results are then compared against an MPN table, which gives a statistical estimate of the cell concentration. The MPN method is most commonly used to determine the approximate number of coliforms in a water sample. Coliforms are lactose-fermenting, Gram-negative rods that typically reside in the intestine and thus serve as a bacterial indicator of fecal contamination. ■ coliforms, p. 793
Instead of measuring the number of cells, the cell mass can be determined. This can be done by measuring the turbidity, the total weight, or the precise amount of chemical constituents such as nitrogen. These all relate to the number of cells present.
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