378 METABOLISM OF MICROORGANISMS 



on hydrolysis such as gkicuronic acid, D-arabinose, and inositol. Some 

 of the most notable polysaccharides, their important characteristics, and 

 the bacteria producing them are as follows : 



(a) Cellulose. This is true cellulose, identical in chemical and physical 

 properties with that found in higher plants. It is produced by Acetobacter 

 xylinum and other members of this genus. 



(b) Polysaccharides with marked physiological and chemical proper- 

 ties are produced by many pneumococci. The polysaccharide produced 

 by Type III consists of alternate glucose and glucuronic acid units bound 

 together through oxygen by a /i^-linkage from carbon 4 of the glucose 

 to carbon 1 of the glucuronic acid and a second ^S-linkage between carbon 

 3 of the glucuronic acid and carbon 1 of the glucose. There appear to 

 be over 600 units each of glucose and glucuronic acid in the polysaccharide 

 chain. The polysaccharide has marked antigenic properties; when in- 

 jected into a rabbit, it evokes production of antibodies and immunity 

 against infection with Type III pneumococcus. 



(c) Dextrans. Many bacteria produce dextrans, but L. mesenteroides 

 is the best known dextran-producer. One reason for the current interest 

 in this bacterium is that it produces a dextran that is now being manu- 

 factured as a substitute for blood plasma. L. mesenteroides is found as 

 a contaminant in sugar factories, and the dextran produced interferes 

 seriously with manufacturing operations. A 10 per cent sucrose solu- 

 tion is fermented in about 24 hours and gives a yield of 25-35 per cent 

 dextran, based on sucrose used. The dextran comes from the glucose 

 part of the sucrose molecule, but glucose itself does not give any dextran, 

 although the microorganism grows well on this sugar. On a glucose 

 medium the microorganism behaves as a heterolactic. The dextran 

 appears to be formed from sucrose according to the following equation: 



n(CioHooOn) -> (CgHioOs)?! + /^(CeHisOs) 

 Sucrose Dextran Fructose 



Some of the glucose and most of the fructose are fermented to lactic acid, 

 acetic acid, ethyl alcohol, carbon dioxide, and mannitol. Potent enzyme 

 preparations which bring about the rapid formation of dextran and 

 fructose from sucrose have been obtained from the culture solution. 



The dextran has a branching structure with apparently a-l,6-linkages in 

 the main chain and a-l,4-linkages at the branching points (pp. 50 and 60) . 

 The molecular weights of dextrans from different strains of L. mesenter- 

 oides are enormous, e.g., 25 to 80 millions. These dextrans are too large 

 for use directly as blood plasma substitutes. They are partly degraded 

 by controlled acid hydrolysis and fractionated to give products of suitable 

 size, e.g., molecular weights of about 75,000. Only about 10 per cent 

 of the original dextran is obtained as material suitable for clinical use. 

 An extensive search is now in progress for microorganisms that will pro- 



