72 



CULTIVATION OF AEROBIC MICRO-ORGANISMS 



C. The sowing of stab cultures. The method of sowing a stab culture in 

 gelatin will be described. 



1. Proceed as under A, substituting a tube of sterile gelatin for the broth tube. 



2. Hold the two tubes in the left hand thus : place the culture from which 



the material is to be taken in the hollow of 

 the hand, supporting it between the thumb and 

 first finger, and keeping it as nearly horizontal 

 as possible. Place the gelatin tube between 

 the first and second fingers, so that it is held 

 firmly between the dorsal surface of the index 

 finger and the palmar surface of the second, 

 with the mouth pointing vertically downwards . 



3. Hold the straight platinum wire in the 

 palm of the right hand, leaving the thumb 

 and index finger free. Sterilize the needle. 



4 and 5. As in A. 



6. With the thumb and first finger of the 

 right hand remove the plug from the gelatin 

 tube. Hold the already charged platinum 

 wire vertically below the mouth of the tube, 

 pass it into the tube until it touches the 

 surface of the gelatin, let the gelatin tube 

 fall by its own weight on to the wire until 

 the latter touches the bottom, then withdraw 

 it sharply (fig. 58). 



7, 8, 9. Complete the operation as in A. 



Notes. It is difficult by forcing the needle into the gelatin to get a straight stab 

 which reaches to the bottom without touching the sides. A satisfactory stab will 

 be more easily secured by allowing the gelatin to impale itself on the needle. The 

 tube must therefore be held vertically, and not obliquely. 



Gelatin which has been made some time is often cracked : in that case stand 

 the tube in a water bath until the medium is liquefied, then let it set, and the gelatin 

 will be found to be quite homogeneous again. 



FIG. 58. Method of sowing a stab 

 culture. 



SECTION III. CONDITIONS ESSENTIAL TO SATISFACTORY GROWTH. 



To ensure growth taking place after the medium has been sown, the follow- 

 ing conditions must be fulfilled : 



(a) The cultures must be freely exposed to the air but at the same time 

 be protected from dust. This condition is readily satisfied by the use of the 

 ordinary wool plug, paper cap, etc. 



(6) The temperature must be kept constant. 



(c) As far as possible light must be excluded. 



The two latter conditions are met by keeping the cultures in an incubator 

 (Chap. III.). 



Some micro-organisms grow only at temperatures above 30 C., generally 37 

 or 38 C., while others only grow well at temperatures below 30 C. Gelatin 

 cultures of course must not be exposed to a temperature above 20-22 C. 



In the laboratory it is useful to have three incubators : 



1. One in which the temperature is maintained at 20-22 C. (the cool or gelatin 

 incubator). 



2. A second in which the temperature is 37-38 C. (the warm incubator). 



3. A third in which the temperature can be altered to meet special cases. Such 

 an incubator is required sometimes for cultures which need a temperature above 

 38 C. (39-41 C.), and at other times for growing organisms at temperatures 

 between 20 and 37 C. 



