220 BACTERTOLOGICAL CHEMISTRY 



nor even on such sinij^le compounds as the dipeptides, 

 which are formed by the union of two amino-acids. Even 

 the active proteolytic bacteria, like Proteus, cannot utilise 

 pure protein as the sole source of carbon and nitrogen, 

 since not enough exo-enzjnne is carried over with the 

 inoculum to break the protein down to diffusible frag- 

 ments on which the cells must depend for their growth, 

 and in the absence of growth, of course, no enzymes can 

 be formed. The addition of small quantities of some 

 simple nitrogen source is sufficient to allow growth to 

 start and proteolytic exo -enzymes to be formed, and then 

 the proteins can be hydrolysed. 



Protein Sparing Action. — It is often claimed that the 

 presence of carbohydrate in media reduces the utilisation 

 of proteins and the production of proteolytic enzymes. 

 Kendall states that addition of glucose to gelatin media 

 delays the formation of the proteolytic enzymes until 

 all the sugar has been fermented. He explains this by 

 saying, " When the sugar is exhausted the organism is 

 forced to derive its energy from the protein constituents, 

 and the enzyme is then formed to bring about the necessary 

 changes in the protein to make it assimilable." That this 

 is not the true explanation is suggested by Berman and 

 Rettger, who state that the inhibition is due to the acid 

 produced by the fermentation of the sugar. In the 

 case of B. suhtilis, which ferments glucose only slowly, 

 or of Aerobacter cloacce, which yields products which are 

 not strongly acid, the presence of sugar has little or no 

 effect on the breakdown of peptone or protein. With 

 Esch. coli or Proteus, which give much acid, the growth and 

 chemical activities are quickly brought to a standstill 

 unless the medium is so heavily buffered that the pH 

 value never falls low enough to inhibit the breakdown of 

 protein. When such buffering is employed the breakdown 

 of protein proceeds as vigorously as in the absence of glucose. 



It has also been shown that in the case of Proteus 

 there is an optimum ^^H at 8-0 for the production of pro- 



