196 OUTLINES OF BACTEEIOLOGY 



out of 32 species of bacteria examined by him 16 or 17 possessed this 

 reducing power, and of the 25 species examined by Warington no less 

 than 18 were found to have this power. Other observers have fully 

 confirmed these results. The list of these bacteria includes a number 

 of well known saprophytes and parasites, e.g. Bacillus coli communis, 

 Bac. iluorescens non-liquefaciens, and the microbes of cholera and 

 typhus. It is evident from the diversity of these organisms that 

 the power of reduction, under special circumstances, is possessed by 

 a very large number of bacteria, this power being in some way 

 advantageous to the organism exercising it. In fact it seems certain 

 that this reduction is undertaken in order to secure the oxygen for 

 purposes of bacterial respiration. That is to say, oxygen is made 

 use of to break up organic matter so that the bacteria may utilise the 

 energy that is thus liberated. 



Another kind of reduction which is effected by some microorganisms 

 is the change of nitrates and nitrites into nitrous and nitric oxides. 

 This was observed in connection with the tobacco- industry as early 

 as 1868. If saltpetre and sugar are introduced into a culture fluid 

 which has been inoculated with a portion of soil it is very common 

 to find that the saltpetre has partially disappeared, its place being 

 taken by nitrous and nitric oxides. That this is accomplished by the 

 microorganisms of the soil is evident from the fact that if sterilised 

 soil be used no such change takes place. One of the organisms of 

 denitrification, viz. Bacterium denitrificans a, does not produce free 

 nitrogen if asparagin be supplied to the nutrient medium (saltpetre 

 bouillon), but instead it produces these two oxides of nitrogen. In 

 spite of the fact that it has been known for a long time that some 

 bacteria can effect this kind of reduction, very few positive results 

 have been obtained, and no bacteria have hitherto been isolated 

 which normally effect the reduction of nitrates and nitrites into 

 nitrous and nitric oxides. 



The third kind of reduction is that known as denitrification, as a 

 result of which nitrates and nitrites are reduced to free nitrogen, 

 which in consequence escapes into the atmosphere. Denitrification 

 is caused by bacteria which doubtless utilise the oxygen which they 

 thereby gain for purposes of respiration. The activity of a denitri- 

 fying microbe in bouillon and similar culture fluids is readily 

 recognised by the foam on the surface of the fluid, which is caused 

 by the escaping nitrogen. In a saltpetre-bouillon culture fluid as 

 much as 79 per cent, of the contained nitrogen may disappear in 

 this way. It is consequently necessarj;^ to prevent manure remaining 



