72 TEXT-BOOK OF BACTERIOLOGY. 



care must be taken to exercise the greatest cleanliness and nicety 

 in all the operations with the culture media. 



The cotton plug should be taken out as seldom and for as short 

 a time as possible; the mouth of the vessel, when opened, should 

 not be held directly upward, for fear of receiving germs from the 

 air, and every instrument which is employed must be thoroughl}' 

 sterilized. 



All our surroundings swarm with micro-organisms, and a single 

 one falling in the wrong place may sufBce to spoil evtrythi;ig. 

 The reproductive powers of the bacteria are so unlimited tlj;it in a 

 very short time they can multiply almost immeasurably, and expel 

 all the former inhabitants from their field of development. In this 

 manner it is easily possible to obtain "transformations" of one 

 species into another — for instance, the harmless hay bacilli into the 

 virulent anthrax bacilli, and vice versa. 



Among the bacteria, as among other creatures of the organic 

 world, there is a " struggle for existence." 



If two germs of different species and different origin alight 

 upon the same nourishing field, both will develop side by side for a 

 time in peace. But, from some cause or other, the conditions are 

 more favorable to one species than the other, it soon gains the 

 mastery, and often enough completely exterminates the other*. 



Thus it is seen what dangers for the pure cultures this exercise 

 of the right of the strongest may bring with it. One foreign germ 

 is able in a short time to alter and destroy such a culture, in either 

 case to rob it of all the advantages which constitute its real value; 

 and those who talk of "tolerably" or "almost" pure culture show 

 clearly that they have as j^et comprehended but little ' of the true 

 rules and principles of bacteriology. 



II. LIQUID CULTURE MEDIA— PREPARATION OP BEEF- 

 BOUILLON. 



The bacteria are not particularly dainty. An organic mass con- 

 taining nitrogen and carbon, especially if it has a slight alkaline 

 reaction, together with favorable atmospheric and thermal condi- 

 tions, is fully sufficient for most of them. Others are harder to 

 satisfy; particularly among the pathogenic species a great many 

 are found whose taste is much more circumscribed. 



Efforts have been made to discover a food solution whose com- 

 position might fulfil most, if not all, requirements." Pasteur and 

 Cohn made experiments in this line, and have given directions for 

 making two artificial food solutions which are now but little used, 

 but which, being of interest in the history of bacteriology, should 



