66 Biology of Micro-organisms 



of the salt, until from 1-15,000, which inhibit ordinary cul- 

 tures, it could accommodate itself to 1-2000. 



The various chemic agents act in different ways upon the 

 micro-organisms. Thus, they may combine with the pr'oto- 

 plasm to make a new and no longer vital cornpound ; or, they 

 may coagulate or dissolve or dehydrate or oxidize the proto- 

 plasm to a destructive extent. 



The addition of chemic agents to solutions containing 

 micro-organisms also changes the osmotic pressure. When 

 an active organism is living in its normal environment, it 

 contains within its plasm a greater concentration of solutes 

 than are to be found in the surrounding fluid. Under these 

 circumstances the pressure on the inside of the ectosarc 

 or other cell membrane is greater than that on the outer 

 side, and the cell is in a state of turgor. If now salts are added 

 so that the solutes on the outside exceed those on the inside, 

 water is drawn out and the protoplasm is made to shrink or 

 condense. According to the degree of this change the 

 organism will be embarrassed, made impotent, or destroyed. 

 On the other hand, when micro-organisms have enjoyed 

 a concentrated medium like blood-serum and are suddenly 

 transferred to distilled water, so much water may be sud- 

 denly drawn into their protoplasm that they swell up and 

 may burst and go to pieces. This is particularly true of the 

 delicate protozoa like the trypanosoma. 



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 metabolic 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 condemnation 

 ultimately effect its purification by exhausting the organic 

 matter it contains in their own nutrition. In the modern 



