IMMUNITY AND ANTITOXINS 241 



Tke products of bacteria are chiefly six : 



1. Pigment. We have already seen how many organisms 

 exhibit their energy in the formation of many coloured pig- 

 ments. They are, as a rule, " innocent " microbes. Oxygen 

 is required for some, darkness for others, and they all vary 

 according to the medium upon which they are growing. 

 Red milk, yellow milk, and green pus afford examples of 

 pigment produced by bacteria. 



2. Gas. Quite a number of the common bacteria, like 

 Bacillus coli, produce gas in their growth ; hydrogen (H), 

 carbonic acid (CO 2 ), methane (CH 4 ), and even nitrogen 

 (N) being formed by different bacteria. Many gases pro- 

 duced during fermentative processes are the result, not 

 directly of the growth of the bacillus causing the ferment- 

 ation, but indirectly owing to the splitting up of the fer- 

 menting fluids. 



3. Acids. Lactic, acetic, butyric, etc., are common types 

 of acids resulting from the growth of bacteria. 



4. Liquefying Ferment. As we have seen, bacteria may 

 be classified with regard to their behaviour in gelatine 

 medium, whether or not they produce a peptonising ferment 

 which liquefies the gelatine. 



5. Phosphorescence. Some species of bacteria in sea-water 

 possess the power of producing light. 



6. Organic Chemical Products. When a pathogenic bacil- 

 lus grows either in the body or in a test-tube, it produces as 

 a result of its metabolism certain poisonous substances called 

 toxins. These may occur in the blood as a direct result of 

 the life of the bacillus, or they may occur as the result of a 

 ferment produced by the bacillus. They are of various 

 kinds according to the various diseases, and by their effect 

 upon the blood and body tissues they cause the symptoms 

 of the disease in question. We know, for instance, that a 

 characteristic symptom common to many diseases is fever. 

 Now, fever is produced by the action of the albumoses 



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