CHAPTER VI 



BACTERIAL POLYSACCHARIDES 



The polysaccharides synthesised by bacteria have received 

 detailed attention from carbohydrate chemists, as not only 

 do they present a wide range of new structures, but the poly- 

 saccharide found in the capsules of some organisms determines 

 their immunological specificity. Polysaccharides are formed 

 by bacteria in capsules, extracellular gums and slimes, and 

 probably as stores of energy. 



Gums and slimes 



One of the earliest studies of Pasteur was concerned with 

 the " viscous fermentation " occurring in sugar solutions, and 

 he showed that the slime formation was due to infection of 

 the material with certain organisms. We know now of several 

 species of bacteria which are able to synthesise hydrophilic 

 polysaccharide gels which are composed mainly of either 

 glucosan (" dextran ") or fructosan (" levan "). Organisms 

 such as B. mesentericus and B. subtilis, when growing in the 

 presence of sucrose, give rise to a fructosan in which the 

 fructofuranose residues are linked as follows: — 



H 0H>| IVh OH^ 



H 1 r CH^OH H ^ f CH^OH 



OH H OH H 



The organisms attack sucrose to form fructosan, but cannot 

 produce the gum from glucose or fructose alone, a mixture of 

 glucose and fructose, or from invert sugar. This suggests 

 that energy is required to link the fructofuranose residues, and 

 this energy is derived from the hydrolysis of the sucrose mole- 

 cule. A cell-free enzyme has been obtained from B. subtilis 

 which will carry out the synthesis of fructosan from sucrose : 



sucrose > fructosan -f glucose. 



U9 



