Estimation of Bacterial Densities by Means of the "Most Probable Number"

THIS PAPER attempts to give a simple account of the concept of the most probable number (m.p.n.) of organisms in the dilution method. The concept is quite old, going back to McCrady (4) in 1915, and has been discussed by various writers from time to time, so that little of what I shall present is new. In addition, some advice is given on the planning of dilution series. The dilution method is a means for estimating, without any direct count, the density of organisms in a liquid. It is used principally for obtaining bacterial densities in water and milk. The method consists in taking samples from the liquid, incubating each sample in a suitable culture medium, and observing whether any growth of the organism has taken place. The estimation of density is based on an ingenious application of the theory of probability to certain assumptions. For a biologist, it is more important to be clear about these assumptions than about the details of the mathematics, which are rather intricate.