Lecture VIII. 63 



provide them with these elements. Thus for example, Bacillus 

 coli thrives when supplied with ammonium chloride and glycerine, 

 and Bacillus subtilis can utilise ammonium tartrate and glycer- 

 ine, while Bacillus pyocyaneus easily draws its nitrogen from 

 potassium nitrate and its carbon from glucose or glycerine. It 

 has also been found that by varying the source of carbon, the 

 power of utilising various sources of nitrogen may be extended or 

 restricted. 



A very remarkable case is presented by the nitrifying organisms 

 (to be described later) which even in absence of light are able to 

 supply themselves with carbon from carbon dioxide by means 

 of the energy supplied in the oxidation of ammonia or nitrous 

 acid. 



Again, bacteria are known which are able to assimilate the free 

 nitrogen of the atmosphere. 



The elements other than carbon and nitrogen required in the 

 formation of bacterial protoplasm are usually supplied from the 

 water and the soluble salts resulting from the disintegration of 

 proteins or from accidental mineral supplies. 



In respiration also bacteria present a large number of variations. 

 Thus some respire free oxygen like the higher plants and animals. 

 These are described as aerobic forms. Others respire combined 

 oxygen and these are the so-called anaerobic bacteria. Some- 

 times indifferent forms are found which can respire equally aer- 

 obically or anaerobically. When a stab infection is introduced 

 into a gelatine medium as in the tube of our cultures, aerobic 

 bacteria develop only at the surface of the medium, anaerobic 

 only in the depths, while the indifferent develop all along the line 

 of the stab. So from the inspection of the growth of a stab 

 infection we can judge whether the bacterium is aerobic, anaerobic 

 or indifferent. 



Then again there is much diversity in the substances which are 

 oxidised during respiration. Proteids, carbohydrates and alcohols 

 are commonly utilised. But a great variety of materials seem 

 to be pressed into service and some very remarkable cases are 

 known. 



We shall first consider the nitrifying organisms alluded to 

 above. 



As has been pointed out, proteins are finally reduced, by the 

 action of various saprophytic bacteria, to ammonium salts, free 

 nitrogen, free hydrogen, carbon dioxide and sulphuretted hydrogen. 

 The ammonia is further oxidised by the respiration of nitroso- 

 bacteria to nitrous acid, and the energy set Tree by this oxidation 



