58 



GENERAL BACTERIOLOGY 



FIG. 18. Method of making Roll-cultures. (Abbott). 



fied in a thin, even layer on the inner surface of the test-tubes. This 

 is best accomplished by means of a piece of ice placed in a dish on 

 a piece of cloth, by which it can be kept in the desired position 



(Fig. 18). A hori- 

 zontal groove is melt- 

 ed in the ice by means 

 of a test-tube filled 

 with hot water. In 

 this groove the test- 

 tubes, inoculated as 

 in case of plate cul- 

 tures, are rapidly 

 whirled until the me- 

 dium is thoroughly 

 set. Both agar and 

 gelatin can be used, 

 although gelatin cannot be used successfully with those species which 

 liquefy this medium. In the case of agar the tubes should be placed 

 in a horizontal position a few hours (over night) until the medium 

 has become attached to the tube : afterwards they can be stored in 

 the usual receptacles for tube cultures. 



REFERENCES. A. 137 ; H. 69 ; M. & R. 56 ; McF. 206. 

 SPECIAL DIRECTIONS, a. Melt a tube of gelatin and without inoc- 

 ulating it practice making a roll-culture as described above. Avoid 

 tipping the tube enough to get medium on cotton plug. Remelt and 

 roll again and again until the knack is acquired. 



~b. Make two roll-cultures in gelatin of E. coli (13) , using a water- 

 blank instead of gelatin tube No. 1. 



c. Make two agar cultures of B. siibtilis in same way. 



d. Incubate 6. in cool chamber, and c. at 28 C. 



EXERCISE 33. STUDY OF PLATE CULTURES. 



MACROSCOPIC. As the colonies appear, note: a. form, &. size, 

 c. surface elevation, d. consistency, e. color. Both the surface and 

 deep colonies should be described, as they are frequently very differ- 

 ent. Drawings should always be made wherever they will be of 

 value; study should be continued as long as changes are noticed. 

 (See Chapter III.) 



MICROSCOPIC. The colonies appearing on the plates are to be 

 studied under a low power of the microscope. Use a -f- in. (16 mm.) 



