200 Report of the Department of Bacteriology of the 



in qualitative work. Qualitative bacteriological analysis depends 

 upon the ease with which the colonies of different types of bacteria 

 can be distinguished from each other. The differences in appearance 

 of the colonies result from certain peculiarities possessed by the 

 bacteria themselves, such as their nutritive requirements or methods 

 of growth. The extent to which these peculiarities are impressed 

 upon the colonies depends upon the conditions of growth furnished 

 by the medium. Unfortunately many of the media in use for the 

 plate method are not favorable to the development of these differ- 

 ences in appearance. 



CHARACTERISTICS OF A SATISFACTORY CULTURE MEDIUM FOR 



THE PLATE METHOD 



These limitations of the plate method cannot be entirely over- 

 come; but the technique may be made more serviceable by using 

 a more satisfactory medium. There are at least three important 

 requirements that must be met by any medium before it can be 

 considered perfectly satisfactory for soil work: (1) It must allow the 

 growth of the greatest possible number of soil bacteria. (2) The 

 colonies produced upon it by different types of bacteria must be as 

 distinct as possible in appearance. This requirement, however, 

 need not be met if mere quantitative results are desired. (3) It 

 must be what bacteriologists often term a " synthetic " medium; i. e., 

 of definite chemical composition. This requirement applies especially 

 to quantitative work. 



As the plate method serves at least two distinctly different pur- 

 poses, it may be possible to use two different media, neither of 

 which meets all three requirements. One medium, designed primarily 

 for qualitative work, should fulfill the first and second requirements; 

 the other, intended for quantitative purposes only, should fulfill the 

 first and third. 



In the present investigation an agar medium and a gelatin medium 

 have been studied. Both have been tested as to their ability to 

 meet the first of these three requirements. The former has been 

 tested because, like all other gelatin media, it allows good distinc- 

 tions between the colonies of many kinds of bacteria, and thus 

 fulfills, in part at least, the second requirement. The latter was 

 tested because it contains no material of indefinite chemical com- 

 position except the agar itself, and thus nearly fulfills the third 

 requirement. 



REVIEW OF LITERATURE. 



Previous work along this line has had but one main object in 

 view — that of obtaining a medium allowing the greatest possible 

 number of soil bacteria to produce colonies. The other two require- 

 ments just mentioned have been largely overlooked. Some investi- 

 gators have used the ordinary media of general bacteriological work, 



