210 



Cultures, and their Study 



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 down to the bottom of the tube and then withdrawn, so that a 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 lique- 

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

 types of gelatin cultures are shown in the accompanying diagrams, 



Fig. 59. 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). 



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 comfortably heated room, 

 not subject to excessive variations, such as are caused by steam 

 heat and the burning of gas, etc., is about the most appropriate. 

 Like the" colonies, the cultures must be carefully examined from day 

 to day, as it not infrequently happens that a growth which shows no 



