212 



Cultures, and their Study 



tin should not be soft and semi-fluid at the time the puncture is 

 made, or the bacteria diffuse themselves and the typical appearance 

 of the growth may be masked. On the other hand, if the gelatin 

 be old, dry, or retracted, it is very apt to crack after the culture has 

 been made and thus entirely destroy the characteristics of the 

 growth. The wire used in the operation should be perfectly straight, 

 and the puncture should be made from the center of the surface 

 directly down to the bottom of the tube and then withdrawn, so 

 that a simple puncture is made. The appearances presented as the 

 growth progresses are subject to striking variations according to 

 the liquefying or non-liquefying tendency of the micro-organisms. 



U 



•A 



Fig- 52. — Types of growth in stab cultures. A, Non-liquefying: i, Filiform 

 (B. coli);"2, beaded (Str. pyogenes); 3, echinate (Bact. acidi-lactici) ; 4, villous 

 (Bact. murisepticum); 5, arborescent (B. mycoides). B, Liquefying: 6, Crateri- 

 form (B. vulgare, 24 hours); 7, napiform (B. subtilis, 48 hours); 8, infundibuli- 

 form (B. prodigiosus); 9, saccate (Msp. finkleri); 10, stratiform (Ps. fluorescens) 

 (Frost). 



Various types of gelatin cultures are shown in the accompanying 

 diagrams, and it is rather important that the student should 

 familiarize himself with the terms by which these different growths 

 are described, in order that uniformity of description may 

 be maintained. Gelatin cultures may not be kept in the incubating 

 oven, as the medium liquefies at such temperatures. On the other 

 hand, they must not be kept where the temperature is too low, else 

 the bacterial growth may be retarded. The temperature of a com- 



