. A laboratory guide in elementary bacteriology. 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- ; These vary in size and appearance according to
. A laboratory guide in elementary bacteriology. 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- ; 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° 0. 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 tjioroughly 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 contents 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 lef
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