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

TLDR

The most probable number concept dates back to 1915 and has been repeatedly discussed, with the present work focusing on clarifying its underlying assumptions rather than the intricate mathematics. This paper aims to provide a straightforward explanation of the most probable number method and practical guidance for designing dilution series. The dilution method estimates bacterial density without direct counts by sampling, incubating in culture media, observing growth, and applying probability theory to infer organism density, with the paper outlining the procedure and planning advice.

Abstract

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.