326 Method of estimating Bacterial Density 



and may be thrown into the characteristic statistical form of the estima- 

 tion of a population from a sample. Only in peculiarly favourable cases, 

 however, as will be seen more clearly below, could we rely upon an 

 a priori mathematical solution. 



2. The Plating Method 



The plate method of counting soil bacteria is an adaptation of the 

 plate counting technique, developed by Koch in 1881, applied to the 

 special conditions of soil bacteria. 



The process in general consists in making a suspension of a known 

 mass of soil in a known volume of salt solution, and in diluting this 

 suspension to a known degree. The bacterial numbers in this diluted 

 suspension are estimated by plating a known volume in a nutrient gel 

 medium and counting the colonies that develope on the plate. An 

 estimate of the bacterial numbers in the original soil is then made by a 

 simple calculation, the mass of soil taken and the degree of dilution being 

 known. 



There are great variations in the details of the method as employed 

 by various workers. These differences concern all the stages in the process 

 and also the nature of the gel medium used in plating. An idea of the 

 extent of this lack of standardisation may be gathered from a paper by 

 Z. N. Wyant(i6) in which a number of the variations in technique used 

 by different workers has been collected from the literature. 



As an example illustrating the process, however, the technique used 

 at Rothamsted and employed by Cutler in the bacterial count work 

 discussed below, will be described. 



Ten grams of the soil sample are placed in 250 gm. of sterile saline 

 solution and shaken for four minutes to obtain a suspension of the soil. 

 1 c.c. of this suspension is placed in 99 c.c. of sterile saline solution and 

 shaken for one minute to ensure a uniform distribution of the contained 

 organisms. 1 c.c. of this second dilution is placed in another 99 c.c. of 

 saline and shaken for one minute. 



Every cubic centimetre of this final dilution will then contain o-soWo 

 grams of the original soil sample. 



One c.c. of this dilution is then delivered into each of five petn dishes 

 and mixed with an agar medium. After incubation the bacterial colonies 

 on each plate are counted, and the mean of the five parallel counts taken. 

 From this the bacterial numbers per gram of soil are estimated. 



The bacterial numbers obtained by the plating method do not repre- 

 sent the total bacterial content of the soil. This is clear from the fact 



