92 MICROBES AND TOXINS 



is therefore not like that of the salts of strontium and barium, 

 a case of re-emission of light formerly absorbed. It is the 

 discharge in the form of light of energy absorbed in another 

 form. 



Production of Pigments. Numerous bacteria exist, 

 whose cultures possess colour, green, violet, red, blue, black, 

 and fluorescent ; these colours have nothing in common with 

 the green colour of chlorophyll plants, for they are diffused 

 throughout the cells, whereas chlorophyll is agglomerated in 

 distinct masses. The bacteria which produce coloured cultures 

 without their cells themselves containing the pigment are the 

 more numerous ; the pigment is therefore an excretory product 

 which diffuses into the medium or collects in little masses which 

 can be seen under the microscope at the side of the bacteria. All 

 the coloured bacteria might be put in this category if one 

 admits that the bacteria which contain a diffuse pigment ought 

 to be classed as algae. 



Staphylococcus aureus and various sarcinae produce colonies 

 of a golden yellow : the pigment is a fatty substance (lipochrome) 

 insoluble in water but soluble in alcohol, benzine, chloroform, 

 ether and carbon bi-sulphide and capable of saponification ; it 

 turns to blue or bluish-green on the addition of sulphuric acid 

 and to orange or red on the addition of alkalies. 



Everyone has heard of the miracle of the " bleeding host" 

 the sacred bread which becomes covered, more by accident 

 than miracle, with red spots having a reddish-brown, somewhat 

 metallic lustre. It only means that it has become invaded 

 by one of the commonest bacteria, one which is present abund- 

 antly in air, milk and dust, especially at the end of summer and 

 in autumn, the B. prodigiosus. The pigment is insoluble in water, 

 soluble in alcohol ; sulphuric acid turns it into reddish-brown, 

 alkalies into yellow. Reducing agents decolorise it as does 

 light, though only after some time. 



There appears occasionally on the surface of milk a bluish 

 colour, sometimes as a uniform film, sometimes in rings or 

 marbling ; this is due to the bacillus cyanogenes. The colour 

 is soluble in water, insoluble in alcohol, ether, and chloroform. 



