The Plate Count Method: An Attempt to Delineate the Bacterial Life in the Microhabitat of Soil

Modem microbiology has developed on the basis of methods for isolation and cultivation that were established by Pasteur and Koch in the nineteenth century. The plate method itself was developed by Koch [1], primarily to permit the isola­tion of pure cultures of bacteria, and then it was applied to the quantification of bacterial numbers in air, water, and soil. Since then, many soil microbiologists have used the method to study bacteria both qualitatively and quantitatively. However, from the early stage of soil microbiology, there have been many diffi­culties in the practice of the plate count method, e.g., the composition of the medium, the dispersion of soil samples, the duration of incubation. Various au­thors have made attempts to overcome these difficulties [2-8], but neither modi­fication nor combination of modifications has succeeded in making the method reliable and effective for soil microbiologists. There is no comprehensive expla­nation why direct counts of soil bacteria by microscopy are two to three orders of magnitude higher than plate counts [8-10]. It is often considered that the plate count method is useless in understanding bacterial life in soil or gives false information about soil bacteria. However, the difficulties with the method have rarely received systematic analyses. If these difficulties are systematically investi­gated, the plate count method can conceivably convey valuable information that might be useful for understanding microbial life in soil. 271

A great variety of bacteria live in soil and this habitat has a microstructure whose arrangement consists of organomineral particles of various sizes. It is still difficult to form a picture of the life of bacteria in the soil microstructure, e.g., their physiological state, in which microsites of soil they reside, how the micro­bial diversity is conserved.In this chapter we describe the bacterial life in the soil microstructure by reviewing our studies on problems of the plate count method, e.g., the use of a diluted medium, the fractionation of soil microbes by a washing procedure, the kinetics of the formation of bacterial colonies. Furthermore, we inquire why plate counts of soil bacteria are much less than their microscopic counts and what the difference means.