68 



bacteriological chemistry 



Table 4 



{After Knight) 



The autotrophs live and grow in purely mineral media 

 which must contain the oxidisable substance characteristic 

 of the particular organism depending for its existence 

 on the oxidation of that compound. No organic nutrients 

 are required ; in fact the obligate autotrophs will not 

 grow on ordinary media containing organic carbon. 

 There are comparatively few obligate autotrophs ; the 

 most important among them are the nitrifying bacteria, 

 the purple sulphur bacteria and some of the iron bacteria. 

 The facultative autotrophs, of which there is a greater 

 variety, may derive their energy and growth require- 

 ments from the oxidation of inorganic substances, with 

 a corresponding reduction of carbon dioxide to give 

 the starting materials for synthesis, or they may grow 

 on already formed organic substances as a source of 

 carbon. Some of the sulphur and iron bacteria, together 

 with the hydrogen, carbon monoxide and methane 



