90 PRINCIPLES >F BACTERIOLOGY 



Rennin-like ferments (substances having the power of coagulating milk 

 with neutral reaction, independent of acids) are found not infrequently 

 among bacteria. The B. prodigiosus, for instance, in from one to two 

 days coagulates to a solid mass milk which has been sterilized at 55 

 to 60 C. These ferments have not been thoroughly investigated ; they 

 are probably present, however, in all species of bacteria which coagu- 

 late milk, even though the organisms also produce acid. 



Fermentation yields products that are poisonous to the ferment; 

 hence fermentation ceases when the nutriment is exhausted or the fer- 

 mentation is in excess. Different kinds of fermentation obtain specific 

 names, according to the products. Thus acetic, yielding acetic acid; alco- 

 holic or vinous, yielding alcohol; ammoniacal, yielding ammonia; amylic, 

 yielding amylic alcohol; benzole, yielding benzoic acid; butyric, yielding 

 butyric acid; lactic, yielding lactic acid; and viscous, yielding a gummy 

 mass. 



Pigment Production. Pigments have no known importance in con- 

 nection -with disease, but are of interest and have value in identifying 

 bacteria. Their chemical composition is not generally known. 



RED AND YELLOW PIGMENTS. Of the twenty-seven red and yellow 

 bacteria studied by Schneider, almost all produce pigments soluble in 

 alcohol and insoluble in water. The larger majority of these possess 

 in common the property of being colored blue-green by sulphuric acid 

 and red or orange by a solution of potash. Though varying consider- 

 ably in their chemical composition and in their spectra, they may be 

 classified, for the most part, among that large group of pigments com- 

 mon to both the animal and vegetable kingdoms known as lipochromes, 

 and to which belong the pigments of fat, yolk of egg, the carotin of 

 carrots, turnips, etc. 



VIOLET PIGMENTS. Certain bacteria produce violet pigments, also 

 insoluble in water and soluble in alcohol, but insoluble in ether, benzol, 

 and chloroform. These are colored yellow when treated in a dry state 

 with sulphuric acid and emerald-green with potash solution. 



BLUE PIGMENTS, such as the blue pyocyanin, are also produced by 

 the so-called fluorescent bacteria, along with a pigment named bac- 

 teriofluorescin. In cultures the fluorescence is at first blue; later, as 

 the cultures become alkaline it is green. 



Numerous investigations have been made to determine the cause of 

 the variation in the chromogenic function of bacteria. All conditions 

 which are unfavorable to the growth of the bacteria decrease the pro- 

 duction of pigment, as cultivation in unsuitable media or at too low 

 or too high a temperature, etc. The B. prodigiosus produces no 

 pigndent at 37 C., and when transplanted at this temperature, even 

 into favorable media, the power of pigment production is gradually lost. 

 B. pyocyaneus does not produce pigment under anaerobic conditions. 



Ordinarily colorless species of bacteria sometimes produce pigments. 

 Thus yellow colonies of the pneumococcus have been observed, and 

 colored varieties of the streptococcus pyogenes. Occasionally colored 

 and uncolored colonies of the same species of bacteria may be seen to 



