Tj6 BACTERIA 



nitrogen of the air or nitrates and ammonium salts. The 

 majority, however, can only make use of the nitrogen of complex 

 organic compounds such as albuminoids and amides. 



Milk, beef-broth, beer-worts and decoctions of various fruits 

 are liquid media largely employed in the artificial nutrition of 

 bacteria. Solid jelly-like media are essential for the separation 

 and growth of certain species ; most of them consist of beef- 

 broth, solutions of peptone, sugar or other substances to which is 

 added a certain amount of gelatine or agar-agar. 



7. Just as in a natural pasture, wood, or meadow, it is usual to 

 find a number of distinct species of plants growing together, so 

 it is usual in any decomposing organic substance or wherever 

 bacteria are met with to find a mixture of species present. 

 Moreover, as it is possible to cultivate in a field or garden one 

 species of green plant by itself, so is it possible to grow in 

 nutrient solutions one species of bacterium free from admixture 

 with all others, in which latter case the growth is termed a fure 

 culture. 



Into the details of the pure cultivation, or the separation and 

 growth of individual species of bacteria we cannot here enter : 

 it may, however, be mentioned that it is only by pure cultivation 

 that a correct knowledge is obtained of the physiological powers 

 of these organisms. 



8. Sterilisation : Pasteurisation, — In the cases of those bacteria 

 which give rise to diseases or to objectionable fermentations 

 such as the souring of milk, the putrefaction of flesh, and other 

 chemical changes described in the following chapter, methods 

 must be adopted to check their activity or to destroy them 

 altogether. 



Milk, beer, flesh and all substances from which all living 

 organisms have been removed or destroyed are said to be sterile 

 and will keep indefinitely if, after the process of sterilisation the 

 further access of bacteria is prevented. 



A large number of methods of sterilisation are in daily use, the 



