4 6 



Scientific Proceedings (95). 



it has been the custom to have the same concentration of the 

 different ingredients in carbohydrate media. The usefulness of 

 media could be greatly increased if the composition of each medium 

 were more closely adapted to the purpose in hand. Thus, for 

 isolating bacteria on a plate by the process of excluding all the 

 lactose-fermenting bacteria, one should have the initial reaction 

 as nearly neutral as possible; one should incorporate as high an 

 amount of lactose as consistant with the concentration of peptone 

 in the medium; one should buffer the medium only enough to 

 prevent excessive penetration of the acid into the surrounding 

 agar, but not so much as to delay the appearance of free acid 

 within and immediately around the colony. On the other hand, 

 when preparing a liquid medium in which one desires to follow 

 gas formation in addition to that of acid, one should be careful 

 to have a fair excess of buffer. Even a small amount of free acid 

 produced can be easily distinguished in a comparatively thick 

 layer of a tube of liquid culture; thus the excess of buffer will not 

 interfere materially in diagnosing the acid production, but it will 

 greatly improve the growth by keeping down the hydrogen ion 

 concentration and will facilitate the production of gas. Again, 

 if one desires to follow the growth of bacteria through a longer 

 period of time in order to determine the character of later stages 

 of metabolism (late alkalinity for instance) one should correlate 

 the respective amounts of carbohydrate and peptone. If one 

 desires to determine the power of reduction, one must use a dif- 

 ferent indicator from that which ought to be used if the free 

 acidity alone is to be determined, for in that case one should select 

 an indicator which is not easily affected by the reduction. 1 



1 This work is a part of the investigation of food poisoning, conducted under the 

 direction of Dr. M. J. Rosenau, professor of preventive medicine and hygiene, Har- 

 vard Medical School. The investigations are done under the auspices of the Ad- 

 visory Committee on the Toxicity of Preserved Foods of the National Research 

 Council, and under a grant of the National Canners Association. 



