282 APPLIED BACTERIOLOGY 



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 low 

 temperature — for instance, 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 phenomena 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 

 was asked, could it be but blood, which could but mean 

 some terrible portent of great calamity ? It is needless to 

 say that great capital was made out of this ' miracle ' by 

 the Church in the Middle Ages. It was a miracle which 

 priest and layman could believe in with perfect honesty — 

 oiie of which, owing to the want of apparent cause, the 

 supernatural may have seemed the natural explanation. 



Many of the bacterial colouring matters strongly re- 

 semble the aniline dyes in their behaviour to acids 

 and alkalies, and in their appearances on media. Some 

 cultures, after keeping, take on the peculiar metallic lustre 

 so characteristic of the aniline dyes. Many of these 

 bacterial pigments are soluble in water, while others are 

 insoluble in water, but soluble in alcohol, ether, chloro- 

 form, etc. 



The colouring matter of the B. prodigiosus can be ex- 

 tracted with ether ; that of the B. pyocyaneus is soluble 



