OCCURRENCE OF BACTERIA 



397 



In preparing agar slants, the test tubes must be plugged with 

 sterile cotton and sterilized thoroughly. Then, the plug is re- 

 moved, the sterile agar quickly poured in, and the plug put back 

 in the mouth of the test tube. The test tubes are usually set 

 obliquely for the agar to cool, so that, when firm, the surface of 

 the media is slanting. Hence the term agar slant. This method 

 gives more surface for the growth of bacteria. 



In order to introduce bacteria for cultivation into a Petri dish, 

 the dish is uncovered and exposed to the air. After the expo- 

 sure, the dish is kept closed. Bacteria may also be introduced 

 with water, food, dust, or other foreign material. In introducing 

 bacteria into the agar slant, the cotton plug is removed carefully 

 so as not to get bacteria on it. The agar slant is then streaked 

 or stuck with a needle infected with bacteria. 



Occurrence of bacteria. Bacteria are so extremely minute that 

 they float in the air with the particles of dust. It is almost im- 

 possible to find any air that does not contain them. Since this 



One way of growing colonies of bacteria is on an agar 

 slant preparation. The surface of the agar is streaked with 

 the material containing the bacteria. The long slant gives 

 considerable surface on which growth may take place. 



" W 



is the case, it is quite impossible for any material exposed to the 

 air, for even a short time, to escape contamination from bacteria. 



