CHAPTER IX. 



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 for general use 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 

 methods are numerous. 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. When a drop of liquid containing several kinds 

 of bacteria is 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 of others, or 

 a change of temperature and soil allowed one form to predominate 

 over another, then we might be led to the conclusion that many 

 bacteria were but developmental forms of one and the same micro- 

 organism ; while possibly the contamination of such cultures might 

 lead to the belief in the transformation of a harmless into a patho- 

 genic bacterium. The secret of the success of Koch's methods greatly 

 depends upon the possibility, in the case of starting with a mixture 

 of micro-organisms, of being able to isolate them completely one 

 from another, and to obtain an absolutely pure growth of each 

 cultivable species. When sterile nutrient gelatine has been liquefied 

 in a tube and inoculated with a mixture of bacteria in such a way 

 that the individual micro-organisms are distributed throughout it, 

 and the liquid is poured out on a plate of glass and allowed to solidify, 

 the individual bacteria, instead of moving about freely as in a liquid 

 medium, are fixed in one spot, where they develop individuals of 



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