CHEMICAL COMPOSITION OF BACTERIA, ETC. 61 



nucleic acids as such and not nucleoproteins, since they 

 give nucleic acid staining reactions and are not digested 

 hy trj^psin and pepsin. 



The proteins and nucleoproteins will be considered in 

 more detail in Chapter XVIII. 



Carbohydrates. — Estimates of the carbohydi\ate con- 

 tent of micro-organisms are not very accurate and vary 

 from 12 per cent, in some water bacilli to about 28 per 

 cent, in the diphtheria and tubercle bacilli. The yeasts 

 may contain from 27 to 63 per cent, and the moulds 7-8 

 to 40 per cent. 



The cell membranes of bacteria have been claimed by 

 certain workers to contain cellulose, but this has never 

 been satisfactorily proved except in the case of Aceto- 

 bacter xyliniim, which synthesises quite large yields of 

 cellulose from a variety of sugars. Cellulose appears not 

 to occur in yeasts or moulds. 



Less complex polysaccharides, in the sense of smaller 

 molecules though not necessarily from a chemical point 

 of view, are very common constituents of nearly all 

 micro-organisms. The obvious examples are the " soluble 

 specific substances " so characteristic of many species of 

 bacteria, the capsules and gums of other species, the 

 glycogen of yeast and the polysaccharides of many moulds. 

 These polysaccharides may be built up from glucose, 

 galactose, mannose, fructose, pentoses, glycuronic acids 

 or mixtures of these units, as will be seen when their 

 study is resumed in Chapter XIX. 



The presence of chitin, a polysaccharide built up of 

 glucosamine units, in bacteria and yeasts is stiU very 

 doubtful, although its presence in the cell wall of many 

 moulds seems to be established quite definitely. 



Polysaccharides are found combined with protein in 

 the mucoproteins which constitute the capsules of many 

 bacterial species . 



" Reserve carbohydrates " are found in many bacteria 

 and yeasts and have been given various names and 



