40 THE FUNDAMENTAL PRINCIPLES. 



the most prominent of which are those that bear the respec- 

 tive names. This class of bacteria are called the zymogenic. 



2. Putrefaction is caused by a variety of bacteria called 

 saprogenic. 



3. Pigments and colors are produced by a number of bacteria 

 called chromogenic. Sometimes the pigment is secreted by the 

 cells and diffused in the surrounding media, the bacteria 

 remaining uncolored. At other times the pigment is limited 

 to the cell-protoplasm and membrane, the surrounding medium 

 remaining uncolored. As a rule, these pigments are produced 

 only in an atmosphere of oxygen. 



4. Some bacteria, called photogenic, phosphorescent, or 

 fluorescent, have the property of emitting and producing light. 



5. Bacteria secrete poisonous substances which are some- 

 times highly deadly, and which from their character are 

 classed as ptomaines and toxalbumins. 



The ptomaines are crystallizable basic substances, closely 

 allied to the vegetable alkaloids. 



The toxalbumins are non-crystallizable substances, similar 

 to albumin or protein. 



It is by the poisonous effects of these toxins that most 

 bacteria affect the human body. 



6. Some bacteria liquefy gelatin, and this fact is made use 

 of in differentiating the different species. This liquefaction 

 has been demonstrated to be caused by a soluble peptonizing 

 ferment secreted by the bacteria cells, and after filtration of 

 a bouillon culture of the liquefying bacterium the filtrate free 

 from bacteria possesses the same power of liquefying gelatin. 



7. Some bacteria produce acids and other alkalies as may 

 be demonstrated by their action in certain fluid media. 



8. Various gases, such as carbon dioxide, marsh gas, hydro- 

 gen sulphide, etc., are the products in some instances of 

 bacterial growth. 



9. A number of bacteria produce odors without the apparent 

 production of gases. 



10. A few produce aromatics, and are used extensively in 

 the arts. 



11. Again, a certain class peptonize milk by means of 



