CHAPTER III. 



Methods of Breeding ; Sterilization ; Liquid Culture Media ; Preparation of 

 Beef-bouillon ; Potato Cultures ; Beef-bouillon Gelatin and Agar-agar ; 

 Uses of Food Media for Obtaining and Maintaining Pure Cultures ; Plate 

 Cultures ; Petri Dish Cultures ; Esmarch's Roll-tube Cultures ; Pure Cul- 

 tures ; Culture of Anaerobic Bacteria ; Incubators ; Thermo-regulators 

 and Safety Burners. 



METHODS OF BREEDING. 



Were the microscopical examination of the bacteria as they 

 occur in their natural state the only means at our disposal for 

 studying- them, our knowledge of bacteriology would never get 

 beyond certain very narrow limits. "We might know something of 

 the wide-spread existence of these micro-organisms, we might ob- 

 serve their frequent presence in connection with certain forms of 

 disease, and we might perhaps even be able in some cases to prove 

 the regular presence of the same species (as judged by form and 

 appearance), and then, by jumping at a conclusion, maintain it to 

 be the source of the pathological conditions in question. But this 

 would be all, and even this little would stand on weak feet. It is 

 always unsatisfactory to form a judgment from the mere appear- 

 ance of the smallest living organisms, whose forms are the simplest 

 imaginable, and experience has shown to what great errors one 

 may be led by so doing. Bacteria, which seemed to agree fully as 

 to appearance, turned out on more careful examination to be alto- 

 gether different species, which had nothing in common but their 

 similarity of form. 



The disadvantage of this way of proceeding was soon recog- 

 nized, and attempts were therefore made to render the bacteria 

 as independent of their natural conditions as possible, in particular 

 to sever the parasitic kinds from the organisms whose parasites 

 they are, to bring them into conditions which allow of more easy 

 examination — in a word, to breed them artificially. 



In this we have succeeded with a great number of species, with 

 some easily, with others after much difficulty. 



It will at once be apparent what an immense stride was made 

 in our knowledge of the bacteria by this success. We were now 



