CULTURE MEDIA FOR USE IN THE PLATE 

 METHOD OF COUNTING SOIL BACTERIA * 



H. JOEL CONN. 



SUMMARY. 



i. Two new culture media have been tested to determine their 

 merits when employed in the plate method of counting soil bacteria. 

 One is a soil-extract gelatin, the other an agar medium containing 

 no organic matter except the agar, dextrose and sodium asparaginate. 



2. The soil-extract gelatin is recommended primarily for use 

 when the plate method is employed as a preliminary procedure in 

 a qualitative study of soil bacteria. Its advantages are that the 

 colonies produced upon it by different types of bacteria are fairly 

 distinct in appearance, and rather more of the soil bacteria produce 

 colonies upon it than upon any other medium investigated. Its dis- 

 advantages are such that they do not render it less satisfactory for 

 qualitative purposes although they might make its use inadvisable 

 in quantitative work. 



3. The chief advantage of the asparaginate agar is that it con- 

 tains no substance of indefinite composition except the agar itself. 

 This ought to allow comparable results to be obtained by its use, 

 even though the work be done by different men and in different 

 laboratories. It is therefore especially adapted to quantitative 

 work. 



4. Four other media have been compared with these. They are 

 those recommended for use in soil bacteriological studies by Fischer, 

 by Lipman and Brown, by Temple, and by Brown. For qualitative 

 purposes they are all distinctly inferior to gelatin. For quantitative 

 work they are undesirable because they contain substances of 

 indefinite chemical composition. It has been found that no one 

 of the five agar media has a distinct advantage over any of the 

 others in the matter of the total counts obtained by their use. 



INTRODUCTION. 



Ever since the bacteriology of soil was first studied, one of the 

 most common lines of investigation has been to determine the 

 number of bacteria living in different soils. It was thought at first 

 that the number of bacteria present was proportional to the pro- 

 ductivity of the soil. Investigation, however, soon proved that the 

 rule held only in a very general way, and that exceptions were 



* Reprint of Technical Bulletin No. 38, November. 



[197] 



