CARBON SOURCES 



131 



Lactose. This sugar is probably present in the milk of all animals. 

 Hydrolysis of lactose by acids or lactase yields a molecule each of glucose 

 and galactose. This sugar is hydrolyzed by emulsin and is therefore a 

 /3-glycoside. 



CH2OH 



Lactose 



Sucrose. This sugar is of common occurrence in plants. On hydroly- 

 sis one molecule of glucose and one of fructose are formed; a mixed a- and 

 j8-glycoside linkage unites the sugar moieties. Sucrose apparently is 

 utilized by fewer fungi than maltose, but more extensively than lactose 

 (see Table 22). 



CH2OH 



CH2OH 



Sucrose 



In addition to the three common disaccharides (maltose, lactose, 

 and sucrose), many other oligosaccharides are known. Owing to cost 

 and relative unavailability, these sugars have not been studied inten- 

 sively. Some of these "rare" sugars are used in differential media in 

 bacteriology. Brief mention will be made here of some of these sugars. 

 The nonreducing disaccharide trehalose (mushroom sugar) is syn- 

 thesized by various fungi and is fermented by many yeasts. Trehalose 

 on hydrolysis yields glucose ; it differs from maltose in the position of the 



