668 ARTHUR ISAAC KEXDALL 



the organism must escape from the tissues in significant numbers to chan- 

 nels in communication with the outside world, and eventually reach other, 

 suitable hosts. Such organisms incite specific epidemics. They are pro- 

 gressively pathogenic from host to host. 



It is a striking fact that the evolution of bacteria from saprophytic 

 types through parasitic to pathogenic types has been attended by a marked 

 decrease in the chemical activities of the microbes. For example, the con- 

 trast in chemical activity between the powerfully proteolytic members 

 of the saprophytic hay bacillus group, which are without virulence, 

 through the ordinary skin Staphylococcus to the exquisitely fastidious 

 Meningococcus is only equaled by the increased pathogenic! ty of these 

 latter organisms. Generally speaking, intense chemical activity appears 

 to be incompatible with pathogenicity (Kendall). 



The facts adduced thus far relate to general properties of bacteria j 

 they furnish little or no information relative to the specificity of bacteria 

 and of bacterial action. Bacteria, in the last analysis, are "living chem- 

 ical reagents," as Professor Folin once characterized them, and the 

 specificity of bacterial action is largely, if not almost wholly, a problem 

 of the chemistry of their interchange with their environment. 



The ultimate chemistry of bacterial action, particularly that relating 

 to the pathogenic organisms, is as yet unsolved. The formula for diph- 

 theria and tetanus toxins, the nature of the poisons of the typhoid and 

 dysentery bacilli, are problems for the bacteriological chemists of the 

 future to solve. Nevertheless, all bacteria of interest or of importance to 

 man exhibit certain rather general relationships with respect to their 

 energy requirements, which are of interest and of increasing importance 

 in the solution of certain problems of medicine, A discussion of these re- 

 lationships will necessitate a survey of the general phenomena of bacterial 

 nutrition. 



3. Chemical Requirements for Bacterial Development: 



a For Structure. b For Energy. 



The cytoplasm of bacteria contains nitrogen, carbon, hydrogen and 

 oxygen, together with other elements in lesser amounts, in about the same 

 proportions as those found in other living cells. The phosphoric acid 

 content is higher than that found in the cells of a majority of higher 

 plants or animals, however. 7 It is obvious that the growth of bacteria in 

 the abstract depends upon the availability of these elements, together with 

 those of lesser occurrence, in proper amounts and in proper combinations. 

 For purposes of discussion, attention will be directed specifically toward 



* Thus, the ash of Bacillus xerosis contains 34 per cent of phosphorus calculated 

 as phosphoric acid, the tubercle bacillus 55 per cent, the cholera vibrio about 45 per cent. 



