MEDIA 55 



necessary to prepare a number of kinds of media before 

 any useful study can be made of the various species of 

 bacteria which are commonly met with. 



The way in which the colonies develop in or upon the 

 different media and the form which they assume, as well 

 as the chemical action which the organisms have upon 

 the media, are matters of the greatest importance in 

 assisting us to distinguish one species from another. It 

 is essential that scrupulous care be taken in the prepara- 

 tion of the various kinds of media, and for each kind 

 some definite method should always be adopted and 

 adhered to, since slight modifications in their acidity and 

 composition or in their treatment in the sterilization 

 process often extensively influence the resulting growths 

 of bacteria upon them. 



The same species of organism may exhibit many 

 points of difference when grown upon two media, which 

 are very nearly but not quite alike in acidity and com- 

 position. The multiplicity of so-called different species 

 of bacteria, and the great difficulty there is in comparing 

 the work and conclusions of those who have carried on 

 investigations in the subject, is due in a measure to the 

 want of uniformity in the media which have been used. 



The following are among the most useful media 

 needed by the student Those required for the study of 

 special organisms present in soil, manure, or dairy pro- 

 ducts will be mentioned later. In the examination of 

 new or unrecognized species its growth upon a variety of 

 media must be studied and recorded. 



In the following exercises instructions are given for 

 the preparation of the kinds of media most frequently 

 needed for the growth and examination of common 

 bacteria. 



