BACTERIA 135 



three days. Fortunately this rate is never maintained because the 

 food supply soon becomes exhausted, or their own excreted waste 

 matters check their rapid growth. The tuberculosis bacterium 

 divides every thirty minutes; compute the possible number pro- 

 duced per day. 



Occurence. Bacteria are found almost everywhere in air, water, 

 soil, food, inside plant and animal bodies whether dead or alive, 

 wherever they can find food and suitable living conditions. It is 

 fortunate that most of this host of one-celled neighbors are either 

 harmless or useful. 



The study of bacteria is called bacteriology. It is a science in 

 itself. The methods used in its study are interesting. 



Sterilization. In the first place all dishes and apparatus used 

 are sterilized ; that is, they are heated or treated with chemicals so 

 as to kill any bacteria that might come from the air or water. 



Making the "Medium." Then a "culture medium" is made 

 from some jelly-like substances such as gelatin or agar, with which 

 beef extract or some similar food is mixed and often peptone and 

 soda are added to make it easier for the bacteria to get their 

 nourishment. 



Inoculation. This culture medium is put in sterile dishes and 

 again sterilized several times by heat to kill any bacteria that 

 might be present; the dishes are plugged with sterilized cotton 

 which will keep other bacteria from getting in. Now we are ready 

 for the next step, called exposure, or inoadation of the cultures. 

 This is done by pouring upon the surface of the culture, a small 

 amount of the milk or water to be tested, or by exposure to the air 

 in the room where the bacteria are to be studied. Touching with 

 the fingers, exposure to dust, and various other means will permit 

 access of bacteria if any be present. 



Growth of Cultures. After exposure, the dishes are again covered 

 and set in a warm place for a few hours. We know that the culture 

 was sterile, i.e., had no bacteria in it, and we know that conditions 

 favorable to growth are provided. As a result if any bacteria have 

 been brought in contact with the culture they soon multiply so 

 greatly that a spot or colony develops on the gelatin. 



