1922] 



ROSEN — A BACTERIAL DISEASE OF FOXTAIL 351 



of change did not occur in different batches of tubes. New 

 tubes in particular, even after hardening in several dippings of 

 sulphuric acid-potassium dichromate cleanser and after several 

 sterilizations in the autoclave, persisted in giving off considerable 

 alkali. Hence in any series of 10 tubes, some made by one con- 

 cern and some by another, or some old and others new, it was 

 possible to obtain various acidities and alkalinities using the 

 same medium. This in all probability is the reason for obtain- 

 ing a series of poured plates with the same organism, some of the 

 plates showing colonies with characteristic precipitate and others 

 lacking in this feature. For the effect of autoclaving on the re- 

 action of the medium see articles by Clark (II, '15, p. 130), An- 

 thony and Ekroth (II, 16), Norton (II, 19) and Davis (II, '20). 

 Besides glassware, other factors may change the reaction of a me- 

 dium. Grace and Highberger (II, '20) noted changes in acidity, 

 particularly in uninoculated glucose broth, and offered no ex- 

 planation for the change. Foster and Randall (II, '21) found an 

 increase in acidity of broth which they concluded was due not to 

 a taking up of carbon dioxide, but to an opening up of the COHN 



groups as a result of the hydrolysis of the protein constituents. 



Here it may be pointed out that not only has titration as meas- 

 ured by the hydrogen-ion concentration given more accurate re- 

 sults but the use of this method has stimulated careful analyses 

 of physiological processes as related to active acidity or alkalinity 

 with the discovery of numerous interesting and often valuable 

 relationships, particularly with reference to human pathogens. A 

 few instances will be cited. Clark and Lubs (II, 15) differenti- 

 ated between organisms of the colon-aerogenes group by the use 

 of indicators whose color changes were noted in terms of hydro- 

 gen-ion concentration. Ayers (II, 16) found that streptococci 

 reached a more or less definite hydrogen-ion concentration in 

 culture media, a fact which, he decided, helped in the classifica- 

 tion of these bacteria. Later, in 1918, Ayers, Johnson, and Davis 

 found that a non-pathogenic strain of Streptococcus reached a 

 higher hydrogen-ion concentration than the pathogenic strains. 

 They concluded that the difference in hydrogen-ion concentration 

 may be used as one means of distinguishing non-pathogenic from 

 pathogenic streptococci. Finally, Avery and Cullen (II, 19) 



found that they were able to distinguish human from bovine 



strains of Streptococcus because of the fact that human strains 



in nutrient broth develop a lower hydrogen-ion concentration. 



