CHAPTER V 



THE CHEMICAL COMPOSITION OF BACTERIA, 

 YEASTS AND THE LOWER FUNGI 



THE problem of the composition of micro-organisms 

 can be approached in two ways : either by the 

 purely qualitative method of microscopical examina- 

 tion after appropriate selective staining or by chemical 

 methods of isolation, which may be made quantitative 

 as well as qualitative. 



As examples of the microscopical method may be 

 quoted the use of osmic acid which stains fats ; the 

 blue colour given by starch aiid the red-brown colour 

 by glycogen with iodine, and the blue colour given by 

 cellulose in presence of zinc chloriodide. Certain dyes, too, 

 are selective in their action ; Sudan III, for instance, 

 dyes fat globules red but leaves unstained other portions 

 of the cell ; the nucleoprotein of metachromatic or 

 volutin granules is stained selectively by such nuclear 

 stains as polychrome methylene blue. These methods 

 are of value in showing the distribution of the constituents 

 in the cell, particularly in the case of the larger cells, 

 such as those of yeasts, but their use is obviously attended 

 with great difficulty when they are applied to such minute 

 cells as those of bacteria in which, generally speaking, 

 details of internal structure are not easily visible. The 

 microscopical methods also suffer from the drawback 

 that they only identify groups of substances, and usually 

 do not distinguish between the members of such groups. 

 Thus Sudan III stains all fats alike, and gives no clue 

 as to the particular sort of fat in a given organism. 



The chemical methods afford a means of separating 

 the various constituents from one another and allow 



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