PHYSIOLOGY OF THE MICROBES 95 



Grown in pure culture in sterile milk, the colour is merely 

 grey ; to get the typical blue the collaboration of an acid- 

 producing bacterium is necessary. In nature this is provided 

 for by the lactic bacilli. The B. cyanogenes produces at the 

 same time a green fluorescent pigment. 



The Bacillus pyocyaneus (Gessard) is the colour-producing 

 bacterium which has been most studied. It used to be thought 

 that it was the cause of blue pus ; but it confined itself really 

 in the days before antiseptics to diffusing its blue pigment 

 through the linen of the dressings. This blue colour, " pyo- 

 cyanin" is soluble in water and chloroform, insoluble in 

 alcohol ; it becomes pink in acid solution, yellowish in alkaline, 

 and is a base closely approaching the ptomaines. 



The B. pyocyaneus produces in addition a fluorescent pigment 

 and a green pigment not fluorescent ; and, finally, old cultures 

 take on a smoky brown tint. By heating, by inoculating on 

 special media, and by animal passages it is possible to dis- 

 associate or to associate these different colours in the same 

 microbe and to create different strains or even a non-pigmented 

 variety ; the green fluorescent pigment is particularly associated 

 with phosphatic food, but the strains thus obtained depend on 

 the medium and on the technique employed; they are not 

 fixed, and are rather transitory varieties than true strains. 

 The chromogenic function lending itself thus to modification 

 it is obvious that it is not one of the essential properties of the 

 bacterium. 



With the microbe, as with higher creatures, habits are more 

 easily changed than nature. 



The majority of the chromogenic bacteria produce their 

 pigment at moderate temperatures, 20 to 25 C. ; at 37 C, the 

 B. prodigiosus and the sarcina grow excellently, but produce 

 no pigment. They prefer a slightly acid medium, but 

 fluorescence requires the medium to be alkaline. The 

 starches are excellent food materials, which explains why the 

 B. prodigiosus grows so well on the Sacred Host. The essential 

 nutritive material is oxygen, and with certain exceptions none 

 of them produce pigment when shut off from air. 



