34 A MANUAL OF BACTERIOLOGY 



resistant bacteria are then free to develop. Greig-Smith, 1 

 however, denies that phagocytic protozoa possess any 

 power of limiting the number of bacteria in the soil, and 

 ascribes the effect of soil sterilisation to an action on the 

 bacterio-toxins and nutrients of the soil. 



Besides nitrifying bacteria many de-nitrifying organisms 

 occur in the soil. They may (1) reduce nitrates to nitrites ; 

 (2) remove oxygen from nitrates and nitrites and form 

 ammonia ; (3) form nitrous and nitric oxides or nitrogen 

 from nitrates and nitrites. 



Fermentation. Another important group of changes 

 produced by micro-organisms is that comprised under 

 the comprehensive title of " fermentation," of which it is 

 difficult to give an accurate definition, for the distinction 

 between it and other chemical changes due to the activity 

 of micro-organisms is conventional rather than scientific. 

 The original conception of the term involved the occur- 

 rence of frothing of the fermenting liquid, owing to the 

 escape of gaseous products. Fermentation is brought 

 about by the action of ferments, two classes of which are 

 recognised, viz. the living or organised ferments, which, 

 in other words, are micro-organisms ; and the unorganised 

 or chemical ferments, bodies such as pepsin, which in 

 minute amount produce changes in a considerable quantity 

 of the substance acted upon, without themselves under- 

 going alteration. 



It is better to reserve the term " fermentation " for 

 the changes brought about by the organised ferments or 

 living organisms, and to call the unorganised ferments 

 enzymes, and the changes which they produce zymolysis. 

 As fermentations are investigated more critically, the 

 tendency is to find that they are brought about by enzymes, 

 extra -cellular or intra-cellular, so that in course of time 

 this distinction may no longer hold good. 



1 Proc. Linn. Soc. N.S.W., xxxvii, 1913, p. 655. 



