CHAPTER V. 



PREPARATION OF NUTRIENT MEDIA AND 

 METHODS OF CULTIVATION. 



To cultivate micro-organisms artificially, and, in the 

 case of the pathogenic bacteria, to fulfil the second 

 of Koch's postulates, they must be supplied with 

 nutrient material free from pre-existing micro- 

 organisms. Hitherto various kinds of nutrient 

 liquids have been employed, and in many cases they 

 still continue to be used with advantage, but as a 

 general rule they have been in a great measure 

 supplanted by the methods of cultivation on sterile 

 solid media about to be described. The advantages 

 of the latter method are obvious. In the first place, 

 in the case of liquid media, in spite of elaborate 

 precautions and the expenditure of much labour and 

 time, it was almost impossible or extremely difficult 

 to obtain a pure culture. If a drop of liquid con- 

 taining several kinds of bacteria be introduced into 

 a liquid medium, we have a mixed cultivation from 

 the very first. If in the struggle for existence some 

 bacteria were unable to develop in the presence ol 

 others, or a change of temperature and soil allowed 



