53 



GENERAL BACTERIOLOGY 



EXERCISE 30. GELATIN PLATE CULTURES. 



Explanatory. Plate cultures are only possible with the lique- 

 fiable solid media, gelatin and agar. In making them the bacteria 

 are mixed with the medium while it is in a fluid state and spread out 

 on a horizontal surface to cool. The dilution is such that the indi- 

 viduals are separated from each other by several millimeters. In 

 the solidified medium the organisms are fixed and their growths 

 result in the formation of ' ' colonies. ' ' These vary in size and ap- 

 pearance according to the peculiarities of the organism and the age 

 of the culture, but are of the greatest service in the study and identi- 

 fication of the various species. These cultures are prepared as fol- 

 lows: 



General. Directions. Three gelatin tubes are marked Nos. 1, 2 

 and 3 and melted by placing them in a water bath at a temperature 

 of 42° C. For this purpose a small cup of 

 water placed on a tripod can be used (Fig. 

 13). They are inoculated by introducing the 

 material to be studied into tube No. 1. The 

 quantity of this material varies. The amount 

 clinging to the platinum needle will be suffi- 

 cient if a pure culture be used, while in other 

 cases several loops or even drops are necessary. 

 The inoculated material is thoroughly mixed 

 with the gelatin in No. 1. This is done by 

 rolling the tube gently between the palms of 

 PiQ_ 13, Method of melt- ^^^ hands, instead of shaking, so as to prevent 

 ing gelatin. the introduction of air bubbles. With a ster- 



ile loop two loopfuls of fluid gelatin are now transferred from No. 1 

 to No. 2, and mixed. For method of handling tubes see Fig. 14. 



In like manner 

 three or more 

 loops from No. 2 

 are carried over 

 to No. 3, which 

 in turn is well 

 mixed. The con- 

 tents of each of 

 the tubes are 

 now poured into 



FiQ. 14. Another method of holding test-tubes. , + "1 



Petri dishes. The process of pouring is performed as follows : The 



