CHROMOGENIC BACTERIA 193 



chromes and others to the ptomains. The reaction and 

 chemical constitution of the media on which bacteria are grow- 

 ing markedly influences chromogenesis in some cases. Acids 

 and alkalies usually produce some change in the pigment when 

 they are applied to it. Occasionally the color may be de- 

 stroyed, changed to a different color, or to a color which 

 will return to the original when the requisite reaction is re- 

 stored. E. F. Smith and others give the following examples : 

 The yellow pigment of Bacterium campestre remains un- 

 changed and Bacillus prodigiosus changes from red to carmine 

 in certain acid solutions and yellowish brown in certain alkalies. 

 The blue color of Pseudomonas syncyanea is produced only in 

 acid milk. The blue-green fluorescent color often seen in some 

 chromogenic bacteria is produced only in alkaline media. 

 Some chromogenic bacteria produce alkalies (NH 3 , etc.). 

 This depends on the supply of nitrogen and carbon. In 

 gelatin Pseudomonas pyocyanea does not form alkali, but in 

 ammonium succinate it does. 



In this last-mentioned group of blue-green fluorescent organ- 

 isms, of which Pseudomonas fluorescens is a typical example, 

 it is probable that there are two distinct pigments produced. 

 Jordan has studied these pigments carefully and states that 

 there is present a blue pigment, called pyocyanin, which is 

 visible by gaslight and soluble in chloroform, and a green 

 fluorescent pigment, called fluorescin, which is yellowish 

 in gaslight and insoluble in alcohol and chloroform. Bac- 

 teria readily lose their power to produce pyocyanin. 



