THE CHEMISTRY OF BACTEEIA. 115 



lieve that, in most of these cases, the pigment is not a prim- 

 ary, but rather a secondary product. That is to say, the 

 bacterial cell elaborates a colorless, or leuco-product which, 

 on contact with the oxygen of the air, becomes converted 

 into the pigment. Almost invariably, pigment production 

 will be observed on the surface of the culture media, where 

 there is free access of oxygen. 



In a few instances, as in the case of the Spirillum ru- 

 brum, the pigment is formed in the absence of air. This 

 would indicate that in the particular case, the pigment was 

 a primary product, made directly by the cell. 



The pigment production in a given species is subject to 

 considerable variation. At one time the growth may pos- 

 sess a very bright color, and again, at another time it may 

 be very pale or colorless. The chromogenic property is, 

 therefore, easily influenced by even trivial conditions. Un- 

 der no condition, however, does a given organism give rise 

 to different pigments, although, in a few instances, several 

 pigments may be formed at the same time. The temperature, 

 reaction and composition of the medium, and the oxygen 

 supply are factors especially deserving attention. 



As a rule, pigment production is diminished, or sup- 

 pressed, when the cultures are grown at about the body tem- 

 perature, 37-39°. The most brilliant pigments are obtained 

 when the cultures are grown at about 15°. Prolonged cul- 

 tivation, at a high temperature, may result in the produc- 

 tion of permanent, colorless varieties. 



Exposure to sun-light has a similar effect as a high 

 temperature. In many instances, an insolation of a few 

 hours may give rise to colorless varieties. 



The reaction of the medium has a considerable influence 

 upon pigment production. This will especially be pointed 

 out in connection with the bacillus of blue milk. As to the 

 composition of the medium, it should be said that a starchy 

 surface, such as that of a potato, seems to be most favor- 

 able to pigment production. 



