80 BACTERIAL METABOLISM 
in the glucose broth appears to be largely the nitrogenous waste 
incidental to the utilization of protein for structural purposes: the 
relatively large amount of ammonia observed in the corresponding 
sugar-free broths is the combined "structural waste" and the "deam- 
ination" incidental to the utilization of protein for their energy require- 
ment. In some instances, for example in certain strains of the gas 
bacillus (B. welchii) there is a slight, secondary change in the protein 
constitutents of the medium when carbohydrate is vigorously decom- 
posed. This results in the formation of a histamine-like substance 
which, very small in amounts has relatively great physiological activity.^ 
The progressively pathogenic bacteria, as the diphtheria, typhoid and 
dysentery bacilli, produce much less ammonia in sugar-free media 
than do the less pathogenic organisms under parallel cultural conditions. 
Vm. SIGNIFICANCE OF BACTERIAL METABOLISM, WITH SPECIAL 
REFERENCE TO THE SPARING ACTION OF UTILIZABLE 
CARBOHYDRATE FOR PROTEIN. 
Considerable emphasis has been placed upon the sparing action 
of utilizable carbohydrate for protein in the preceding pages. It now 
remains to summarize the salient features of this aspect of bacteri- 
ology and to indicate briefly by means of a few illustrations precisely 
how a comprehension of the principles underlying bacterial metab- 
olism may be made use of in controlling, or at least influencing the 
action of these microorganisms upon their environment. The examples 
selected are chosen rather with a view of indicating the extreme range 
of the subject than for completeness along any limited line of investi- 
gation. 
1. The Composition of Bacteria.— Experiments quoted previously 
(page 58) show very clearly that the percentage composition of the 
bacterial cell varies according to the medium in which it is grown. 
Particularly striking is the difference in nitrogen content when the 
same bacterium is grown in media of the same nitrogenous composition 
and reaction with and without the addition of utilizable carbohydrate. 
2. The Recognition of Bacteria.— The recognition of many kinds of 
bacteria, as for example members of the intestinal group, depends upon 
the reactions these organisms induce in various sugars. Thus B. 
alcaligenes ferments no sugars; B. dysenteriae ferments glucose with 
the production of acid; B. proteus ferments glucose and saccharose 
with the evolution of gas and the production of acid; B. coli ferments 
glucose and lactose with the evolution of gas and the production of 
acid; B. coli coagulates milk, while B. proteus characteristically pep- 
tonizes it. All of these reactions are explained perfectly upon the 
theory that utilizal)le carbohydrate protects protein from bacterial 
1 Kendall and Schmitt: Jour. Infec. Dis., ibid., 1926, 39, 250. 
