SYMBIOSIS, ANTIBIOSIS AND COMMENSALISM 53 



pigment formation, which is a striking cultural characteristic of many 

 bacteria", is wholly unknown. The pigment they produce does not 

 protect them against strong light, and achromogenic strains may be 

 cultivated from the chromogenic varieties without apparent loss in the 

 cultural or chemical characters of the organisms. It is very probable 

 that these pigments are chiefly waste products of metabolic origin. 



Pigments are produced in darkness and sunlight rapidly destroys 

 many of them. Oxygen is not necessary for their production, for the 

 non-colored leukobase is the form in which the pigment is excreted 

 by bacteria, but oxygen is necessary for the development of color from 

 this leukobase. 



Pigment-producing bacteria may be grouped into four classes : 



1. Bacteria producing photodynamic pigment. Certain sulphur 

 bacteria which produce bacteriopurpurin. 



2. Phosphorogenic bacteria which produce a luminous substance 

 somewhat analogous to that of glow-worms. These organisms are 

 chiefly marine forms, as B. phosphorescens. 



3. Fluorogenic bacteria which produce a pigment soluble in water 

 and culture media; this usually exhibits complementary colors as it 

 is viewed by reflected and transverse light respectively. 



4. Chromogenic bacteria. The pigment produced is usually insol- 

 uble in water and soluble in organic solvents. The color varies accord- 

 ing to the organism producing it. The more common colors are red, 

 orange, yellow, green, blue, violet, brown, and black pigment. These 

 colored pigments are usually referred to as lipochromes because of 

 their solubility in organic solvents and their general relationship to 

 fats. Many of them give well-defined and constant absorption when 

 they are viewed spectroscopically in solutions. 1 



M. SYMBIOSIS, ANTIBIOSIS AND COMMENSALISM. 



The biological relations of bacteria are of the greatest importance 

 in the economy of nature and in the production of disease. Bacteria 

 do not grow in pure culture in nature, although they may do so in the 

 tissues of man or animals, as disease-producing bacteria (pathogenic 

 bacteria). In nature, where the reduction of dead complex organic 

 material to mineralized salts is the striking function of bacteria, the 

 successive steps in the degradation of organic matter are carried on 

 by different kinds of microbes. The various steps appear to vary 



1 Sullivan, Jour. Med. Research, 1905, xiv, 109. 



