32 General Bacteriology. 
EXERCISE XXVI. GELATIN PLATE CULTURES. 
EXPLANATORY. Plate cultures are only possible with the liquefiable 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 
q 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, 
L sore. and mixed. For method of handling tubes see Fig. 7. In like 
Fic. 9. Method of melting gelas manner three or more loops from No. 2 are carried over to No. 
tin. 3, which in turn is well mixed. The contents of the tubes Nos. 
1-8 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 Fic. 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 
hai ais. Geapeiosc cued Rare ats a yet growth takes place. The temperature of the 
tin in Petri dishes. 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 nunumbers, depending upon the dilution 
used. 
