406 THE QUESTION OF IMMUNITY AND ANTITOXINS 



1. Pigment. — We have already seen that many organisms exhibit 

 their energy in the formation of various pigments. These are, 

 as a rule, " innocent " bacteria. Oxygen is required for the pro- 

 duction of pigment by some of these species, absence of light by 

 others, and they all vary according to the medium upon which 

 they are growing. Eed milk, blue milk, and green pus are illus- 

 trations of materials owing their colour to pigment produced by 

 bacteria. Chromoporous bacteria are those in which the pigment is 

 diffused out into the surrounding medium; chromophorous bacteria 

 are those in which the pigment is stored in the cell protoplasm of 

 the organism. 



2. Gas. — A large number of the common bacteria, like B. coli, 

 produce gas in their growth; hydrogen (H), carbonic acid (COg), 

 methane (CHJ, sulphuretted hydrogen (HgS), and even nitrogen 

 (N) being formed by different bacteria. 



3. Acids. — Lactic, acetic, butyric, etc., are common types of acids 

 resulting from the growth of bacteria. 



4. Liq-mfying Ferment. — As we have seen, bacteria may also be 

 classified with regard to their behaviour in gelatine medium, as to 

 whether or not they produce a peptonising ferment which liquefies 

 the gelatine. 



5. Phosphorescence. — Some species of bacteria, for example, certain 

 species in sea-water, possess the power of producing light (photogenic 

 bacteria). 



6. Many organisms are capable of producing indol (a substance 

 formed by bacterial action from proteids by alimentary decomposi- 

 tion), or other metabolic substances as end-products. 



7. Organic Chemical Products.—When a pathogenic bacillus grows 

 in the body, it produces as a result of its metabolism certain poisonous 

 substances termed 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. Toxips are of various kinds, 

 and by their effect upon the blood and body tissues they cause 

 the symptoms of the various diseases. We know, for instance, 

 that a characteristic symptom common to many diseases is fever, 

 which is produced by the action of the albumoses (bodies allied 

 to the albumins) upon the heat-regulating centres in the brain. 

 Whenever we have a bacillus growing in the body which has 

 the power of producing a toxin albumose, we obtain fever as a 

 result of that product acting upon the brain. Albumoses, as a 

 matter of fact, cause a number of symptoms and poisonous effects, 

 but the mention of one as an illustration will suffice. Toxins 

 act, broadly speaking, in two ways. They have a local effect 



