ACTION OF BACTERIAL FERMENTS. 29 



hydrogen, carbon dioxide, methane, etc. For an exact 

 knowledge of the destructive capacities of any particular 

 bacterium there must be an accurate chemical examination 

 of its effects when it has been grown in artificial media the 

 nature of which is known. The precise substances it is 

 capable of forming can thus be found out. Many sub- 

 stances, however, are produced by bacteria, of the exact 

 nature of which we are still ignorant, for example, the toxic 

 bodies which play such an important part in the action of 

 many pathogenic species. 



Many of the actions of bacteria depend on the produc- 

 tion by them of ferments of a very varied nature and com- 

 plicated action. Thus the digestive action on albumins 

 depends on the production of a peptic ferment analogous 

 to that produced in the animal stomach. Ferments which 

 invert sugar, which split sugars up into alcohols or acids, 

 which coagulate casein, which split up urea into ammonium 

 carbonate, have all been isolated from different bacteria. 



Such ferments may be diffused into the surrounding 

 fluid, or be retained in the cells where they are formed. 

 Sometimes the breaking down of the organic matter appears 

 to take place within, or in the immediate proximity of, the 

 bacteria, sometimes wherever the soluble ferments reach 

 the organic substances. And in certain cases the ferments 

 diffused out into the surrounding medium probably break 

 down the latter to some extent, and prepare it for a further, 

 probably intracellular, disintegration. Thus in certain 

 putrefactions of fibrin, if the process be allowed to go on 

 naturally, the fibrin dissolves and ultimately great gaseous 

 evolution of carbon dioxide and ammonia takes place, but 

 if the bacteria, shortly after the process has begun, are killed 

 or paralysed by chloroform, then only a peptonisation of the 

 fibrin occurs, without the further splitting up and gaseous 

 production being observed. That a purely intracellular 

 digestion may take place is illustrated by what has been 

 shown to occur in the case of the micrococcus ureae, which 

 from urea forms ammonium carbonate by adding water to 

 the urea molecule. Here, if after the action has com- 



