CHAEACTEEISTICS OF NITEITE-BACTEEIA 191 



§4. PHYSIOLOGICAL CHAKACTERISTICS OF THE 

 NITRITE-BACTERIA. 



These bacteria have a remarkable power of assimilating the 

 atmospheric carbon dioxide and utilizing it as a source of food. This 

 power is extremely interesting, because its discovery led to the 

 overthrow of the doctrine that had long been held by botanists, 

 namely that only plants possessing the green colouring matter chlwo- 

 phyll, and these only in the daylight, were able to assimilate carbon 

 dioxide from the atmosphere. Here, however, organisms were found 

 that could do this without the aid either of chlorophyll or of light. 

 Scarcely less interesting is their relation to organic matter. If any 

 organic matter be present in their nutrient medium, even in very 

 small quantities, growth cannot take place; that is, the addition of 

 organic substances has the same effect as the addition of antiseptics. 

 Indeed, it is remarkable that those organic substances that are most 

 readily assimilable by other bacteria are the most injurious in their 

 effects on the nitrite-bacteria. This applies particularly to glucose 

 and peptone, which are introduced into almost every medium that is 

 prepared for the nourishment of bacteria. The presence even of 0'025 

 per cent, of either of these two substances has an injurious effect, and 

 0-2 per cent, is sufficient to arrest growth altogether. These bacteria 

 must therefore derive their carbon and their nitrogen from inorganic 

 materials. Their carbon is obtained, as mentioned above, from the 

 carbon dioxide of the atmosphere, whilst their nitrogen is got from 

 the ammonium compounds which are always present in the soils, etc., 

 in which these bacteria are found. 



To secure the multiplication of nitrite-bacteria it is absolutely 

 necessary to supply them with some ammonium compound. Among 

 the various salts of that base, ammonium sulphate gives the best 

 results. 



As a general rule in bacterial cultures, the presence of substances 

 produced by the activities of any particular microbe — i.e. waste pro- 

 ducts — is injurious to its further growth. That rule applies in the 

 case of the nitrite-bacteria. Hence the more nitrites that are present 

 the less active is the growth of the nitrite-bacteria. It is curious 

 to note that the introduction of a small quantity of a nitrite into a 

 culture fluid before inoculation with nitrite-bacteria has a far greater 

 effect in preventing their growth than if the same quantity is intro- 

 duced, after growth has well started. 



