EXERCISE 67 



A STUDY OF BACTERIA 



Statement. Bacteria are microscopic single-celled plants living either singly or in groups. Under 

 natural conditions they are present on almost every object and are exceedingly abundant in the air, 

 soil, and water. Most bacteria are harmless, many are beneficial, while some cause disease. Tuber- 

 culosis, typhoid fever, and cholera, the wilt of melons and cucumbers, and the crown gall common to 

 apples, alfalfa, and many other plants are common bacterial diseases. 



Object. To study bacteria and observe the effects of bacterial action. 



Materials. Four small bottles ; boiling water ; cotton; Bulgar tablets ; microscope; cover glass and 

 slide. 



Directions. Secure four small bottles or tumblers holding approximately half a pint. These should 

 be thoroughly cleaned by placing them in water and bringing it to the boiling point for a few minutes. 

 Fill each half full with sweet milk (not pasteurized). Buy some Bulgar tablets or tube cultures at the 

 drug store ; these contain the living bacteria, Bacillus bulgaricus. Place half a tablet in each of two 



tumblers of milk. .Do not put anything in the other samples. 

 Insert in the mouth of each bottle a firm cotton plug made by 

 rolling the cotton into a roll. Place one of the tumblers contain- 

 ing the Bulgar tablet, and one of the tumblers which was not 

 inoculated with a tablet, in a refrigerator. The other two 

 a b b tumblers should be placed in a room where the temperature is 



Fig. 104. A group of bacteria and two com- about 8o° F. At the end of twelve, twenty-four, and thirty-six 

 smut spores hours compare the two samples of milk containing the bacteria 



All are magnified S°Ottot*. showing relative w j th the other twQ untreated sam pl es aS to their odor and taste 



and record any difference that may be apparent. Describe 

 what you have discovered. Note the appearance of the milk in each sample. Record any difference 

 in the physical and chemical state of the samples. Return the bottles to their respective places and 

 reexamine samples at the end of forty-eight and seventy-two hours. Notice the differences between 

 the souring of the various samples, the characteristic odor, and the physical changes in the milk. 

 Examine a drop of sour milk under the highest power of your microscope. Can you see the 

 bacteria ? Describe what you see. In which sample are the bacteria most numerous ? 



Questions. What was the effect of bacteria upon milk ? How did the temperature at which the 

 milk was held affect the action of bacteria? At what temperatures do bacteria grow most rapidly? 

 What temperature is required to kill them ? What are the common ways of holding bacteria in check 

 or killing them ? Why do we store perishable products in the ice box ? Explain how low tempera- 

 ture retards the souring of milk. How does it retard the spoiling of meat and eggs ? How does boiling 

 and sealing while hot prevent spoiling in canned fruits and vegetables? Why was the cotton stuffed 

 in the mouth of the bottles in the experiment above? Will dirty milk keep as well as clean milk? 

 Will milk keep as well when placed in dirty vessels as in clean ones? What is meant by sterilization 

 and how may it be accomplished ? What is the effect of sterilization ? What is meant by pasteurization 

 and how may it be accomplished? What product is most generally pasteurized? Name some of the 

 most common disinfectants. Why do' we spray fruit trees? Name some common diseases of plants 

 which are due to bacteria and describe the best method of preventing them. What are some of the 

 common diseases of farm animals caused by bacteria? 



References. Conn, H. W. Agricultural Bacteriology. P. Blakiston's Son and Company. Lipman, J. G. 

 Bacteria in Relation to Country Life. The Macmillan Company. 



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