The Relations Between Plate Counts and Direct Microscopic Counts of Escherichia coli During the Logarithmic Growth Period.

The relation between the plate count and the direct microscopic count of bacteria is of practical as well as theoretical importance. Particularly during the logarithmic growth period, where the constant rates are used for comparative purposes, it is desirable to know whether the number of cells as determined by plating always bear a constant relation to those enumerated microscopically, that is, whether the plate count is a reliable, although relative, measure of the total number of cells present. That the plate count is usually lower than the total count was noted years ago (Winterberg, 1898; Klein, 1900; Hehewerth, 1901), and this discrepancy has been the subject of a relatively few papers since that time. In general, are the differences between such counts, even during the logarithmic growth period with pure cultures, due entirely to clumping, or to the death of a certain proportion of bacteria, or to a combination of these factors? The more recent studies have brought various facts to light in respect to some of the relationships and have offered explanations in certain cases, without having clearly answered these questions. It is recognized, for example, that in old cultures many cells are not viable, but it is uncertain whether or not this is the case during the period of logarithmic reproduction in a suitable medium.