July i6. 1917 A Substitute for Litmus for Use in Milk Cultures 109 



.color of different batches will be reproducible. This implies that the 

 initial Ph will be the same in all cases. So far as this affects the growth 

 and metabolism of a culture, it is important that it should be repro- 

 ducible, which it certainly is not when the procedures that have been 

 used in the preparation of litmus milks are modified at will, with no very 

 clear conception of the important aims. 



The possible usefulness of diluted milk has not been fully appreciated. 

 While our own experiments have been very few, the writers think that 

 one or two of the more general aspects of the subject are worthy of 

 notice. 



Dilution of milk tends to raise the Pg. In certain instances this may 

 be a distinct advantage and a better way of adjustment than the addition 

 of alkali. In one preliminary experiment it was found that the change 

 in Pjj during the sterilization of a milk diluted five times was less than 

 that in the undiluted sample. The most suggestive aspect of the subject 

 is the relative buffer effects in diluted and undiluted milk. The buffer 

 effect of milk is considerably higher than that of most culture media. ^ 

 Consequently a culture must elaborate an unusual quantity of acid or 

 alkali to induce a given change in Pg and a consequent change in the color 

 of an indicator. By diluting the milk the relative buffer effect is lowered, 

 and a proportionally smaller degree of acid or alkali fermentation is 

 required to induce a given change in the indicator color. Dilution also 

 permits a better view of the color. 



Obviously such facts are not the only ones to be considered, and in lieu 

 of sufficient data to permit a systematic treatment of the use of diluted 

 milk, the writers will confine themselves to the presentation of a series 

 of experiments with cultures in undiluted skim milk in which the rela- 

 tive value of litmus and bromcresol purple was tested. 



The organisms used were several acid-forming cultures of Bacillus 

 coli, B. aerogenes, B. hidgaricus, and streptococci, three alkali producers 

 of Mr. S. H. Ayers, of the Dairy Division, some cultures of B. proteus, 

 and the following pathogenic bacteria kindly sent to the writers by Prof. 

 C. E. A. Winslow from the collection of the American Museum of Natural 

 History : 



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1 Clark, W. M. "the reactign" of bacteriologic culture media. In Jour. Infect. Diseases, v. 17, 

 no. I, p. 109-136, 7 charts. 1915. 



