THE BIOLOGICAL ACTIVITIES OF BACTERIA 59 



bacteria in their metabolic processes liberate energy. It has been 

 shown by several observers that slight quantities of heat are given 

 off from actively growing cultures. The functions, furthermore, of 

 reproduction, motility, and enzjone formation may be looked upon as 

 forms of energy liberation. In addition to this, certain bacteria have 

 been observed which may liberate energy in the form of light. 



Light Production by Bacteria. — ^The production of light by bacteria 

 is a power possessed chiefly by certain species inhabiting salt water. 

 Thus, much of the phosphorescence observed at sea, though more fre- 

 quently due to Medusa and other invertebrate animals, is caused by 

 these bacteria. Numerous species which produce this phenomenon 

 have been isolated, too many, and too unimportant, to be individually 

 described. All of them are aerobes and require highly complex food 

 stuffs. They are closely allied to the putrefactive bacteria, and in 

 the sea are usually found upon rotting animal matter.^ The production 

 of light seems directly dependent upon the free access of oxygen, since 

 no light appears under anaerobic conditions. Their luminous quality, 

 moreover, is not a true phosphorescence, in that it does not depend 

 upon previous illumination and develops a^ well in cultures kept in the 

 dark as in those which have been exposed to light. ^ 



The Formation of Pigment by Batcteria (Chromobacteria) . — A large 

 number of bacteria, when cultivated upon suitable media, give rise to 

 characteristic colors which are valuable as marks of differentiation. 

 For each species, the color is usually constant, depending, to a certain 

 extent, upon the conditions of cultivation. In only a few of the 

 pigmented bacteria is the pigment contained within the cell body, and 

 in only one variety, the sulphur bacteria, does the pigment appear to 

 hold any distinct relationship to nutrition. In most cases, the coloring 

 matter is found to be deposited in small intercellular grammes or globules. 

 The absence of any relationship of the pigment to sunlight, as is the case 

 with the chlorophyll of the green plants, is indicated by the fact that 

 most of the chromobacteria thrive and produce pigment equally well in 

 the dark as they do in the presence of light. Among the most common 

 of the pigment bacteria met with in bacteriological work are Staphy- 

 lococcus pyogenes aureus. Bacillus pyocyaneus. Bacillus prodigiosus, 

 and some of the green fluorescent bacteria frequently found in feces. 



The chemical nature of these pigments has been investigated quite 

 thoroughly and it has been shown that they vary in composition. 



1 Pfliiger's Arch. f. Phys., xi, 1875. ' Fischer, Cent. f. Bakt., iii, 1888. 



