APPENDIX 143 
are more often acid than alkaline, it is proposed that acid media be 
designated by the plus sign and alkaline media by the minus sign, 
and that the degree of acidity or alkalinity be noted in parts per 
hundred; thus a medium marked + 1.5 would indicate that the 
medium was acid and that 1.5 % of n/1 sodium hydroxid is required 
to make it neutral to phenolphthalein, while —1.5 would indicate 
that the medium was alkaline and that 1.5 % of n/1 acid must be 
added to make it neutral to the indicator. 
Limits of accuracy of the proposed method for the control of the 
reaction of media: 
The available data are as yet insufficient to warrant any conclu- 
sions upon this point. The limits of accuracy seem to vary with 
the ingredients employed in preparing nutrient media, different 
samples' of meat infusion, pepton, and gelatin appearing to react 
differently with the acids and alkalis and in a way which is not 
understood. 
This method, nevertheless, when carefully carried out, and when 
the media before titration are thoroughly mixed and are of the 
prescribed volume, give fairly uniform results. 
Standard reaction of media (provisional) : 
Experience seems to vary somewhat as to the optimum degree 
of reaction which shall be uniformly adopted in the preparation of 
standard culture media. To what extent this is due to variation in 
natural conditions as compared with variations of laboratory pro- 
cedure, it seems impossible to state. Somewhat different degrees 
of reaction for optimum growth are required, not only in or upon 
the media of different composition and by bacteria of different 
species, but also by bacteria of the same species when in different 
stages of vitality. 
The bulk of available evidence from both Europe and America 
points to a reaction of +1.5 as the optimum degree of reaction for 
bacterial development in inoculated culture media; and while this 
experience is at variance with that in several of our own laboratories, 
it has been deemed wise to adopt +1.5 as the provisional standard 
reaction of media, but with the recommendation that the optimum 
growth reaction be always recorded in species descriptions.” 
Journal Am. Public Health Association, January, 1898. 
