NITROGEN METABOLISM 210 



67. ietcDii does not l)ring about the '' Sticldaml leactioii '' 

 but breaks down glutamic acid, aspartic acid or serine 

 to give carbon dioxide, ammonia, acetic acid and butyric 

 acid, together with some lactic acid and ethyl alcohol 

 from aspartic acid. The same products are formed in 

 the dissimilation of pyruvic and fumaric acids, with 

 malic acid in addition from the latter. Glucose is not 

 attacked by CI. tetani. CI. tetanomorphum and CI. 

 cochleariiim can derive energy for growth from the 

 breakdown of glutamic acid alone. 



Ring Compounds. — The purines, pyrimidines and 

 similar compounds can be used by many bacteria but 

 not by all. Thus Esch. coll will not utilise uric acid or 

 hypoxanthine, whereas Aerobacter aerogenes can do so. 

 Hippuric acid can support the growth of the hsemolytic 

 streptococci. CI. acidi-urici and CI. cylindrosporum can 

 utilise uric acid and some other purines. 



Proteins, Proteoses and Higher Polypeptides. — Com- 

 pounds of these types are not utilised directl}' by bacteria 

 or yeasts, even by the most actively proteolytic organisms. 

 Purified proteins as a sole source of nitrogen will not 

 permit the growth of bacteria, most probably because the 

 cell membranes are not permeable to them. Before 

 they can be utilised they must be broken down into 

 simpler substances which can penetrate into the cell, 

 where they can be acted upon by the true metabolic 

 enzymes of the cells, the endo-enzymes. This 

 preliminary breakdow^n of complex proteins to the 

 simpler nitrogen compounds which can be used is the 

 function of exo-cellular, hydrolytic enzymes elaborated 

 by the bacteria. Until enough of such proteoljrtic enzymes 

 are produced by an inoculum of bacteria proteins cannot 

 be made available. Usually, of course, there is sufficient 

 nitrogen available in a simple form in a medium to enable 

 the culture to start growing and produce enough proteo- 

 lytic enzymes to allow the organism to utilise any proteins 

 present. Such organisms as Esch. coll, which do not 



