NORMAL MILK BACTERIA IN LITMUS GELATIN. 277 



very characteristic (Fig. 41, c). A study of these colonies has 

 shown that there is a variety of bacteria which produce such 

 colonies, and several species would be included under this 

 head of neutral colonies (see Fig. 18). It is, however, im- 

 practicable to differentiate them from each other upon the 

 gelatin plates, and it is, therefore, necessary to group them 

 together under one type as neutral forms. 



Yellow Colonies. In some localities it is almost universal 

 to find, upon the surface of the gelatin and just beneath the 

 surface, some bright yellow, or occasionally orange, colonies, 

 which may become of considerable size, and which may or 

 may not show the development of an acid. These are com- 

 monly a species of Micrococci or Sarcina. They are so com- 

 mon that, at least in some localities, they must be regarded 

 as almost universally present in normal milk. They can 

 usually be distinguished at a glance by the bright yellow color 

 with or without an acid reaction. 



A further differentiation of the colonies to be found upon 

 gelatin is perfectly possible. Indeed, the colonies of different 

 species of bacteria growing upon this litmus milk sugar 

 gelatin are commonly very sharply marked off from each 

 other and easily recognizable. Probably forty to fifty differ- 

 ent species of common milk bacteria can, by careful study, 

 be distinguished from each other without much difficulty. 

 To distinguish them all with accuracy requires long experi- 

 ence, and can only be done after the observer has studied the 

 different colonies for a considerable time and has learned to 

 recognize them. Moreover, it is never possible, with absolute 

 accuracy, to distinguish all of the colonies from each other, 

 and at best the results will be approximate rather than strictly 

 accurate. So far as concerns the chief types, however, the 

 differentiation here described is quite accurate, and it is only 

 in regard to the forms present in smaller numbers that the 

 method described is uncertain. 



