MECHANISM OF METABOLISM. 143 



bacteria. Among these exceptions is the Bact. lactis acidi. The ab- 

 sence of katalase in this species has been recommended as a diagnos- 

 tic test. It is possible that this enzyme is necessary for intracellular 

 oxidations. 



A number of other reductases are known. Nearly all of the reductions 

 mentioned in the paragraph on the products of mineral decomposition 

 are proved to be of enzymic nature; these processes will take place after 

 the cell is killed by a disinfectant or is ground to pieces. This can be 

 readily demonstrated by lacerating the cells with quartz sand. They 

 will then reduce nitrates to nitrites, sulphur to hydrogen sulphide. The 

 decolorization of litmus, methylene blue, indigo, and other organic dyes 

 is due in microbial cultures to enzymes which are almost exclusively 

 endo-enzymes. 



ADDITIONAL REMARKS ON THE RELATION or CELLS AND ENZYMES. 



Enzymes are produced only by living cells. After they are once 

 formed, they act like chemical compounds, independent of the cell which 

 produced them. Even the endo-enzymes follow only the law of enzyme 

 action and are not influenced by the cell which contains them. The 

 enzymes are mostly influenced by their own products, and when a certain 

 yeast ceases to ferment sugar at the concentration of 8.5 per cent of 

 alchohol, this means that the alcoholase of this yeast cannot tolerate 

 more than 8.5 per cent of alcohol. The inability of the cell to regulate 

 enzymic action may account for the fact that often a culture produces an 

 amount of fermentation products sufficient to kill all cells. This is ob- 

 served in the lactic, acetic and alcoholic fermentations, and perhaps 

 occurs in many others. 



Most cells produce more than one enzyme. Microorganisms feeding 

 upon various foods must form various enzymes. Frequently several 

 enzymes are necessary for the decomposition of one compound. Mucor 

 rouxii uses three enzymes in order to form alcohol from starch, first the 

 diastase to change starch to maltose, then maltase to change maltose to 

 dextrose and finally alcoholase to change dextrose to alcohol and carbon 

 dioxide. The number of enzymes formed by certain microorganisms is 

 surprising. Aspergillus niger has the reputation of forming almost all 

 enzymes which have ever been found in microorganisms. Penicillium 

 camemberti produces (after Dox) erepsin nuclease, amidase, lipase, 



