102 SEWAGE AND ITS PURIFICATION 



is effective only in symbiosis with B. coli : the latter supplies it 

 with its necessary ammoniacal food. B. coli^ reduces nitrate 

 to nitrite, which in turn is denitrified by the organism I. 



Enzymes. 



A great number of changes, most of them hydrolytic, are 

 accomplished by the large class of organic substances termed 

 *' enzymes," which, though not living, are products of animal 

 and vegetable life. They have been defined by Lehmann and 

 Neumann as " chemical bodies, which in minimum amounts 

 and without being used up are able to separate large amounts 

 of complicated organic molecules into simpler, smaller, more 

 soluble and diffusible molecules." The definition is not quite 

 accurate, as the milk ferment, for instance, actually coagulates 

 casein, or renders it insoluble, but it gives an idea of the 

 immense power that these enzymes possess, and the economy 

 of their use as distinguished from ordinary chemical or 

 mechanical means. Many of them are products of bacteria or 

 other fungi, and are powerful agents in resolving action, as a 

 bacillus is not only able to act in its immediate neighbourhood, 

 but also at a considerable distance, through the soluble ferments 

 it forms and disengages. 



The enzymes are soluble nitrogenous bodies, which can be 

 precipitated and rendered inert by strong alcohol, mercuric 

 chloride, and by boiling. They can be separated from bacteria 

 by filtration, when the enzymes pass through, while the 

 bacteria are retained. Other distinctions from the organisms 

 which produce them are : 



1. Enzymes can often work at a greater range of temperature ; 

 that is, are less susceptible to heat and cold than the living 

 bacteria. Therefore it is possible to find temperatures which 

 will inhibit, if not kill, bacteria without affecting enzymes.- 



2. Antiseptics, like chloroform, thymol, etc., which kill or 

 inhibit bacteria, do not prevent enzymes from acting. Thus 



^ Weissenberg, Archiv.f. Hygiene, 1897, xxx., 3. 



2 Brunton and Macfadyen {Proceedings of the Royal Society, 1890, xlvi., 542), 

 working on the liquefaction of gelatine by a variety of micro-organisms, found 

 that a temperature of 100° C. destroyed both the bacteria and the power of 

 liquefaction, while a temperature of 50° destroyed neither. Between 60° and 75° 

 the organisms were destroyed, but not the liquefying action, showing that it was 

 due to an enzyme secreted by the bacteria. They succeeded in isolating this 

 enzyme, and demonstrated its peptonizing effect apart from the bacteria which 

 produced it. It was hindered by acidity, and favoured by alkalinity. A diastatic 

 enzyme was also identified. These bacteria evinced adaptiveness to the media 

 in which they grew, and could digest animal fibre and carbohydrates, but not fats. 



