582 S. HENRY AYERS AND COTJRTLAND S. MUDGE 



The washed-agar solution is prepared as described. No adjust- 

 ment of reaction is necessary, as the final hydrogen-ion con- 

 centration of the medium will be about pH 6.7. 



BACTERIAL COUNTS ON MILK-POWDER YEAST MEDIUM 



The counts obtained on the milk-powder yeast agar, as well 

 as on standard extract and standard infusion agar, are shown 

 in table 3. The figures show that the counts on the milk-powder 

 yeast were much higher than on standard extract agar and that 

 they agree closely with the counts on standard infusion agar. 



Yeast extract was used extensively in other combinations 

 with good results, and while we have only a few results on the 

 medium just described it is believed it can be used to advantage 

 with skhnmed-milk powder as a substitute for peptone and 

 meat extract. Further work is necessary before this point can 

 be definite^ settled. 



The milk-powder yeast agar does not appear to give quite such 

 high counts with all samples as the powder, peptone, meat- 

 extract agars A and B, but it has the advantage of being more 

 nearly uniform in composition. This is obvious for the reason 

 that the yeast extract can be made in the laboratory in a definite 

 way from a dry preparation consisting of a definite species of 

 pure yeast. 



The milk-powder yeast agar is a clear medium on which colonies 

 of the strong acid-forming bacteria can be seen, although not so 

 distinctly as on the other media described in this paper. Fep- 

 tonizers can be readily observed by flowing the plate with 5 

 per cent acetic acid. 



THE USE OF SKIMMED-MILK POWDER 



Dissolved skimmed-milk powder is used as a foundation for 

 the media described in this paper and not as merely something 

 to be added to some other medium. The use of skimmed-milk 

 powder must be considered in this light in order that its value 

 may be fully appreciated. 



