CHAPTER IV 



BACTERIA 



The world is everywhere pervaded by bacteria ; minute 

 microbes are carried by the atmosphere, they float in 

 the water and occupy the soil. When we or indeed 

 any other animal take our walks abroad, we create and 

 are surrounded by an invisible halo of bacteria ; some 

 are stirred up by the dust of our going, others shaken 

 from our clothes, and others are even distributed by our 

 breath. They are scattered over the food which we 

 eat ; the water which we drink is full of them, and as 

 a matter of fact it would seem detestably vapid if it was 

 germ-free. 



These germs form a very mixed multitude : some are 

 distinctly useful and even necessary forms, others are 

 harmless, and a few are malignant and fatal disease- 

 bacteria. 



In order to study any particular variety, it is necessary 

 to keep it from being contaminated by some of the 

 numerous wild forms. The methods in use require very 

 careful manipulation ; the germ is grown upon gelatine, 

 on peptone broth, or on agar-agar (a seaweed prepara- 

 tion), and atmospheric germs are carefully excluded 

 from the test-tube or glass shells in which the ^' culture " 

 is preserved, usually by a cork of cotton-wool. 



Dr. Kienitz Gerloff ^ made an interesting experiment 

 with one of his sterilised or germ-free agar-agar saucers. 

 He persuaded a healthy young maiden {gesundes junges 

 Mddchen) to imprint a hearty kiss upon this unfeeling 



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