32 



General Bacteriology. 



EXERCrSE XXVI. GELATIN PLATE CULTURES. 



EXPLANATORY. Plate cultures are only possible with the liqueflable solid media, 

 gelatin and agar. In making them the bacteria are mixed with the medium while it is 

 in a fluid state in such quantities that the individuals are separated from each other by 

 several millimeters when it is spread out on a horizontal surface to cool. As the medium 

 solidifies, the organisms become fixed and their growth results in the formation of "colo- 

 nies." These vary in size and appearance according to the peculiarities of the organism 

 and the age of the culture, but are of the greatest service in the study and identification 

 of the various species. These cultures are prepared as follows: 



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. 9). They are 

 inoculated by introducing the material to be studied into tube 

 No. 1. The quantity of this material varies. The amount cling- 

 ing to the platinum needle will be sufficient if a pure culture is 

 used, while in other cases several loops or even drops are neces- 

 sary. The inoculated material is thoroughly mixed with the 

 gelatin in No. 1. This is done by rolling the tube gently be- 

 tween the palms of the hands, instead of shaking, so as to pre- 

 vent the introduction of air bubbles. With a sterile loop three 

 loopfuls of fluid gelatin are now transferred from No. 1 to No. 2, 

 and mixed. For method of handling tubes see Fig. 7. In like 

 manner three or more loops from No. 2 are carried over to No. 

 3, which in turn is well mixed. The cpntents of the tubes Nos. 

 1-3 are now poured into separate sterile Petri dishes. 

 The process of pouring is performed as follows: The 

 Petri dish is placed on the desk; the gelatin tube is 

 taken in the right hand, the cotton plug removed with 

 the left hand; the mouth of the tube sterilized by 

 flaming it once or twice, and when the glass is cool FIG. 10. Method of pouring plates, 

 the gelatin is poured into the lower half of the dish while the cover is slightly raised 

 (Fig. 10), but not inverted or laid on the table. The cover of the dish is then replaced, 

 the test-tube filled with a solution of corrosive sublimate, and the cotton plug returned. 

 The gelatin is spread over the entire bottom of the dish by tipping it from side to side. 

 It is then allowed to harden by placing the dish on the cooling apparatus or leaving it 

 on horizontal surface at room temperature. A simple, inexpensive and effective cooling 



apparatus is a piece of soapstone, such as is sold at 

 hardware stores (Fig. 11). In winter this can be cooled 

 by hanging it out of doors, at other seasons by im- 

 mersing it in cold water. The three Petri dishes thus 

 prepared should be properly labeled and placed un- 

 der conditions where the gelatin will remain solid and 

 yet growth takes place. The temperature of the 

 laboratory should not be allowed to exceed 23 C. or 

 gelatin cultures are in danger of melting while under examination. Within a few days 

 colonies will make their appearance, in varying numbers, depending upon the dilution 

 used. 



FIG. 9. Method of melting gela- 

 tin. 



FIG. 11. Soapstone used for solidifying gela- 

 tin in Petri dishes. 



