6o Biology of Micro-organisms 



Metabolism.^ — According to their activities, micro-organisms are 

 classed as — ■ 



Zymogens, when they cause fermentation. 



Saprogens, when they cause putrefaction. 



Chromogens, when they produce colors. 



Photogens, when they phosphoresce. 



Aerogens, when they evolve gas. 



Pathogens, when they cause disease. 



The metaboUc activities of micro-organisms occasion many well- 

 known changes in nature. Thus, it is through their energies that by 

 fermentative and putrefactive changes organic matter is gradually- 

 transformed from complex to simple compounds. It is by the 

 energy of bacteria that foul waters are gradually purified, and while 

 it is true that the presence of large numbers of bacteria in water 

 detracts from its ' potability, the very bacteria that cause its con- 

 demnation ultimately effect its purification by exhausting the 

 organic matter it contains in their own nutrition. In the treat- 

 ment of sewage by the "septic tank" method, the organic matter 

 contained in the water is consumed through the agency of anaerobic 

 and aerobic bacteria, until the water once more becomes clear and 

 pure, the bacteria dying out as the nutrition becomes exhausted. 



The promptness with which bacteria attack organic matter is 

 seen in the changes brought about in foods, some of which are 

 ruined in flavor or quality, though others are-thought to be improved. 

 Thus, the flavor of butter, sausage, and cheese, the aroma of wines, 

 and many other important gustatory characteristics of our foods 

 depend solely upon the activity of bacteria or other micro-organisms. 



Many of these activities are harmless, and, indeed, advantageous, 

 though the fact that they are not infrequently accompanied by 

 chemic changes, some of which are poisonous, make it necessary to 

 watch and time their operations lest acridity, acidity, insipidity, or 

 toxicity of the food replace the desired effect. 



Briefly considered, the best known phenomena resulting from 

 micro-organismal energy are as follows: 



Fermentation. — Fermentation is catalysis of carbon compounds 

 caused by catalysts or ferments resulting from micro-organismal 

 metabolism. The alcoholic fermentation, which is a familiar 

 phenomenon to the layman as well as to the brewer and chemist, 

 depends upon the activity of an yeast-plant, one of the saccharo- 

 myces fungi by which the sugar is broken up into alcohol and carbon 

 , dioxid, with some glycerin and other by-products. The following 

 equation shows the chief changes produced: 



CsHijOe = 2C2H6OH + 2CO2 

 Sugar Alcohol Carbon dioxid 



There are also several bacteria which produce the acetic fermenta- 

 tion, though it is generally attributed to Bacillus aceticus. There 

 are two equations to express this fermentation: 



