NUTRITION OF HETEROTROPHIC BACTERIA 87 



freely on ammonium salts and simple carbon compounds, 

 whilst the parasitic M. tuberculosis and Johne's bacillus 

 have complex requirements when freshly isolated, but on 

 prolonged culture in the laboratory gradually become less 

 fastidious and able to grow on ordinary or even synthetic 

 media. 



Our knowledge of the general nutritional requirements 

 of bacteria has been summed up by Knight, who divides 

 the organisms into four groups with increasing complexity 

 of demands, corresponding to progressive loss of synthetic 

 power, as follows : — 



1 . Carbon derived from carbon dioxide ; nitrogen 



from inorganic sources (elementary nitrogen, 

 nitrites, nitrates or ammonia) ; energy from 

 light in the case of photosynthetic autotrophs 

 and from simple inorganic oxidations in the case 

 of chemosynthetic autotrophs. 



2. Carbon and energy from organic carbon com- 



pounds (carbon dioxide is not the main source 

 of carbon) ; nitrogen from inorganic compounds. 



3. Carbon and energy from organic carbon com- 



pounds ; nitrogen from amino -acids (some, 

 tryptophane, for example, are in many cases 

 essential) ; ammonium salts are not assimilated. 



4. Carbon and energy from organic compounds ; 



nitrogen from amino-acids, of which a con- 

 siderable number is usually required. One or 

 more growth factors are also required. 

 A fifth group, the viruses, may be added to this 

 Hst :— 



5. Live and reproduce only in living tissues ; that 



is, exhibit an almost complete lack of synthetic 



powers. 

 It may be stated that in the last year or two the 

 discovery of saprophytic filter-passing organisms, which 

 can thrive independently of living tissue, has been 

 reported. 



