672 ARTHUR ISAAC KEXDALL 



.will act quite as readily upon protein media containing glucose as upon 

 protein media from which glucose is absent. 



The foregoing illustrations typify a very general property of bacteria, 

 and of other living things for that matter, with respect to metabolism. 

 It has long been a physiological dictum that i 'carbohydrate spares body 

 protein" (Howell(a) ), meaning by that that an animal requires a definite, 

 if minimal, amount of dietary protein to maintain the nitrogen equilibrium 

 of the adult organism. This minimal amount of nitrogen is indispensable 

 for the repair of structural wear and tear, and for the replacement of 

 nitrogenous losses in secretions, enzymes and other nitrogen-containing sub- 

 stances, which are of necessity constantly lost to the body. The fuel or 

 energy requirement of the organism, on the contrary, amounting to many 

 times the minimal nitrogen requirement, can be met by the feeding of 

 non-nitrogenous food, as carbohydrate and, to a lesser degree, organic 

 acids or fat. 



Bacterial nutrition presents the same fundamental phenomena of 

 structural and energy requirements. The former absolutely requires 

 nitrogen as one element in its make-up, whereas the latter may be satisfied 

 by non-nitrogenous organic substances. Of these, the carbohydrates as a 

 class are of paramount importance, although of varying degrees according 

 to specific characteristics of the organisms under investigation. Precisely 

 as saprophytic bacteria were found to be more energetic cleavers of protein 

 than parasitic and pathogenic bacteria, so the saprophytic types are some- 

 what more energetic cleavers, both in kind and amount, of carbohydrate 

 than the pathogenic types. Hence, a majority of the progressively patho- 

 genic bacteria, as typhoid, dysentery, diphtheria and many others, utilize 

 the hexoses [especially glucose], but fail to utilize the bioses, as lactose and 

 saccharose. The pathogenic bacteria produce less deep seated changes 

 even in the hexoses than do the saprophytic types. In general, the 

 changes induced by the former result in the formation of lactic and acetic 

 acids, whereas the latter frequently oxidize a not inconsiderable portion of 

 the hexose to carbon dioxid and hydrogen. 



Returning to the conditions prevailing in cultures of diphtheria, colon 

 and cholera organisms referred to above, it will be found that plain or 

 sugar-free media offer to bacteria protein and protein derivatives [pep- 

 tone, polypeptids and amino acids] , as the sole source of structure and of 

 energy. The glucose media offer precisely the same protein and protein 

 derivatives for structure non-nitrogenous substances are not suitable for 

 structure, generally speaking and, in addition, a choice between this 

 protein or protein derivative and carbohydrate for energy. To sum- 

 marize : 



The marked difference discernible between the significant products 

 formed by bacteria in non-saccharine media, where both structure and 

 energy requirements are of necessity obtained from the nitrogenous protein 



