90 METHODS USED IN THE STUDY OF BACTERIA 



solid, and also because many bacteria digest the gelatin and 

 the colonies run together. It was, therefore, a considerable 

 step in advance when agar-agar replaced gelatin. The high 



FIG. 36. Plate culture. Light areas represent colonies from which pure cul- 

 tures can be obtained. After Lipman. 



melting point of this material makes it possible to use it at 

 any temperature suitable for the growth of bacteria. It has, 

 however, certain disadvantages, among which should be men- 

 tioned the fact that certain bacteria grow much less character- 

 istically on it than on gelatin, and also because the water of 



