THE BIOLOGICAL ACTIVITIES OF BACTERIA 67 



the green plants, is indicated by the fact that most of the chrorno- 

 bacteria 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 Staphylo- 

 coccus 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 com- 

 position. Some of the pigments, like that of Staphylococcus aureus, 

 are probably non-protein and of a fatty nature. 32 They are insoluble 

 in water but soluble in alcohol, ether, and chloroform. Because of 

 their probable composition, they have been spoken of as "lipo- 

 chromes. " Other pigments, like the pyocyanin, which lends the 

 green color to cultures of Bacillus pyocyaneus, are water soluble 

 and are probably of protein composition. Pyocyanin may be 

 crystallized out of aqueous solution in the form of fine needles. The 

 crystals may be redissolved in chloroform. Aqueous solutions retain 

 their color. Solutions in choloroform, however, are changed grad- 

 ually to yellow. 



The power of pigment production of various bacteria depends in 

 each case upon cultural conditions. In most cases, this simply sig- 

 nifies that pigment is produced only when the microorganism, finding 

 the most favorable environmental conditions, is enabled to develop 

 all its functions to their fullest extent. Thus, a too high acidity or 

 alkalinity of the culture medium may inhibit pigment formation. 

 Oxygen is necessary for the production of color in some bacteria, 

 since the bacteria themselves often produce the pigment only as a 

 leuko-body which is then oxidized into the pigment proper. A 

 notable example of this is the pigment of B. pyocyaneus. In other 

 cases, temperature plays an important role in influencing color pro- 

 duction. Thus, Bacillus prodigiosus does not produce its pigment 

 when growing in the incubator. By persistent cultivation in an 

 unfavorable environment, colored cultures may lose their power of 

 pigment production. 



Sulphur Bacteria. Wherever the decomposition of organic mat- 

 ter gives rise to the formation of H 2 S, in cess-pools, in ditches, at 

 the bottom of the sea, and in stagnant ponds, there is found a 

 curiously interesting group of microorganisms, the so-called sulphur 



82 Schroetcr, Cent, f . Bakt., xviii, 1895. 



