126 MICRO-ORGANISMS AND DISEASE [chap. 



glycose or grape-sugar, alcohol, lactic sugar, urea or gum, &c., 

 as the case may be, are by the growth of particular microbes 

 changed in the manner of fermentation into other sub- 

 stances. 



6 Many bacteria have the power to produce pigments : 

 these appear either on all media on which their growth 

 occurs, or only on particular media. , In the first case the 



Fig 24. — Surface (Streak) Culture on Gelatine of the Common Bacterium 



LACTIS. 



pigment formation is real, in the second only apparently 

 so. Thus a variety of bacilli, e.g. bacillus subtilis, bacillus 

 mesentericus, bacillus coli, bacillus of glanders, when grow- 

 ing on potato, form a brownish or yellowish-brown smeary 

 layer, but do not produce any pigment on other media ; the 

 bacillus anthracis turns agar brownish after the growth has 

 reached a certain long duration, &c. 



