CHAPTER II. 



PBEPAEATION OF NUTRIENT MEDIA. 



In the cultures of bacteria, described in the preceding 

 chapter, the store of nutrient material necessary to 

 them becomes gradually used up by the activity of 

 these organisms, and little by little development and 

 reproduction comes to a standstill. Some of the 

 bacteria die from want of nourishment ; whilst others, 

 in a manner to be considered later, form permanent 

 forms, or "spores," which are able to remain quiescent 

 for a long time until favourable conditions for growth 

 reappear. In order to continue the propagation of 

 bacteria, it is necessary to convey them from time to 

 time into new nutrient media. Of course a substance 

 similar to that in which the bacteria were first culti- 

 vated is suitable for their further cultivation ; never- 

 theless, as a rule, other substances are chosen, which 

 possess in many respects considerable advantages over 

 the former ones. These substances are universally 

 designated as nutrient media, and are divided into 

 solid and fluid media. The preparation of these is not 

 difficult, but it requires great care and accuracy in 

 carrying out directions in order to ensure success. 



