ADVANTAGES OF A CARBOHYDEATE MEDIUM 



487 



indicated that the amount of carbohydrate may be reduced to 

 0.1 per cent without impairing the value of the medium. 



Another advantage of a medium containing a carbohydrate 

 over plain agar is that the former is of considerable value in 

 differentiating the types of organisms on the plates. The colo- 

 nies of acid-producing bacteria on sugar agar are surrounded by 



^1 YepiOCOCCU5 



Fig. 3. Same as figure 1 except a culture of a streptococcus was used. 



a precipitate of protein matter, thus giving the typical hazy 

 edge which is so characteristic of organisms of the B. lactis- 

 acidi group. On agar made without clarification with egg albu- 

 men, and which is consequently more opaque, the peptonizing 

 bacteria may be quite readily identified by means of the clear 



