120 



BACTERIAL POLYSACCHAEIDES 



In other cases the extracellular gum or slime is a glucosan. 

 Organisms such as Leuconostoc dextranicus, Betacoccus arabi- 

 nosus, etc., will synthesise glucosan when grown in the presence 

 of sucrose. The glucosan has a long chain structure in which 

 the repeating unit is 



Bacterial cellulose 



Acetobacter xylinum (see p. 149) produces a slimy envelope 

 when growth takes place in the presence of sucrose or glycerol. 

 The envelope is composed of a polysaccharide which has the 

 same structure as that of vegetable cellulose, i.e. a chain 

 structure consisting of cellobiose as the repeating unit: 



CH,OH 



Capsule polysaccharides 



Many organisms produce capsules and these are often, 

 though not invariably, composed largely of polysaccharides. 

 These are of considerable interest since they appear to confer 

 specificity upon the immunological response. For example, 

 the genus Pneumococcus can be divided into 32 types by 

 serological methods. If a serum to Type I is prepared by 

 injection of the intact organism into an animal, then that 

 serum reacts with Type I pneumococcus only. However, if 

 a serum is prepared against the proteins of the Type I pneu- 

 mococcus — in the absence of the polysaccharide — then the 



