SYMBIOSIS, ANTIBIOSIS AND COM M ENS A LI SM 51 
L. PIGMENTS. 
With the exception of bacteriopurpurin, which occurs in the sulphur 
bacteria and is supposed to be photodynamic and, therefore, some- 
what analogous to the chlorophyll of the higher plants, the significance 
of 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 culti\ated 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 bands when 
they are viewed spectroscopically in solutions.^ 
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 difterent kinds of microbes. The various steps appear to vary 
somewhat, but the process is on the whole an orderly and definite one. 
1 Sullivan: Jour. Med. Research, 1905, 14, 109. 
