348 APPLIED BACTERIOLOGY 



to the agency of bacteria of free and fixed nitrogen bj 

 plants, it will be seen that these discoveries have revolu- 

 tionised some of the older ideas respecting vegetable 

 physiology, and certainly may be said to have marked an 

 epoch in the history of scientific agriculture. 



Chromogenic Bacteria and Colouring Matters. — As before 

 stated, many of the saprophytic bacteria, as the result of 

 their vital action, give rise to many beautiful colouring 

 matters, such as red, yellow, green, violet, etc. In some 

 cases these colouring matters are contained in the substance 

 of the bacteria themselves, or within the sheath ; in others 

 the organisms in themselves are quite colourless, and pro- 

 duce colouring matter as the result of the decomposition of 

 the nitrogenous albuminoid matter contained in the nutrient 

 media. The bacillus of the disease of milk known as 

 ' blue milk ' (B. cyanogenus), when grown on gelatine, is 

 of a pale-blue colour ; but the organism soon colours the 

 whole of the medium of a dirty -green colour, which is soon 

 replaced by a muddy-brownish tint. Many bacteria, to 

 produce their characteristic colouring matter, require to be 

 grown under certain conditions ; many require light for the 

 formation of the pigment; others, again, require a lo«r 

 temperature — for instancCj the colouring matter of the 

 B. prodigiosus will develop at blood-heat. Again, many 

 bacteria lose their power of pigmentation after continued' 

 subculture on one kind of nutrient medium ; this may be 

 generally restored, as, in the case of the B. prodigiosus, the 

 chromogenic power is restored by growth on potato. 



The brilliant blood-like colouring matter of the B. pro- 

 digiosus was the cause of the phenomenon known as the 

 ' bleeding host ' or ' bloody sweat.' The moist consecrated 

 wafers, after being left on the altar in the church over- 

 night, would be found the next day to be covered with little 

 blood-like drops, which rapidly grew larger. What else, it 



