14 MODERN DAIRY PRACTICE. 



but few dissimilarities in their outward appearance. There 

 are, e.g., a large number of species among the micrococci 

 which appear entirely similar by examination with a micro- 

 scope, but which show great differences in their effects and 

 general behavior in one and the same substratum, and thus 

 indicate beyond a doubt that they are in reality different 

 species. It has therefore been found necessary to seek a 

 more certain method of identification than the microscopic 

 examination, and such a one has also been found in the 

 culture method. 



b. Culture Method of Bacteria Investigation. Bacteria 

 can develop in several kinds of media, both liquid and 

 solid. In the former they form all kinds of flocculent and 

 sandy growths, while in the latter they grow into more or 

 less characteristic isolated masses, technically called colo- 

 nies. In the culture of bacteria for scientific objects French 

 bacteriologists previously used mainly liquid substrata and 

 German preferably solid ones. Both kinds are of late made 

 use of by all bacteriologists. The liquid nutritive media dis- 

 tinguish themselves by the fact that the conditions which 

 they offer to the bacteria resemble those found in nature, 

 and that all kinds of fermentations may be more easily 

 observed in them; the solid substrata, on the other hand, 

 are the more convenient ones to use when the question of 

 the specific growth of different species is studied. Among 

 the liquid substrata used in bacterial investigations may be 

 mentioned beef-broth, milk, malt-extract, sugar solutions 

 prepared in different ways, and extracts of various fruits; 

 among the solid nutrients, prepared potatoes and beef -tea, 

 to which pepton and all kinds of gelatinizing substances 

 have been added, as gelatine, agar-agar, etc. Coagulated 

 blood-serum and soaked wheat-bread are also often used. 



