CULTURAL CHARACTERS OF BACTERIA 27 



sodium hydrate per litre necessary to effect complete neutraliza- 

 tion. The question as to what is the best reaction of media for 

 general work is not an easy one to settle, and one on which 

 bacteriologists differ. Reactions are now commonly expressed 

 by plus or minus signs, the former representing an acid and the 

 latter an alkaline condition, the number following the sign 

 representing the percentage of normal acid or alkali present in 

 the medium. Thus + 1.5 would indicate that the medium con- 

 tained 1.5 parts per 100 or 1.5 per cent of free normal acid, 

 while — 1. 5 would indicate that the medium contained an 

 equivalent quantity of free alkali. The committee of the 

 American Public Health Association, in 1898, adopted a medium 

 whose titre was -|- 1.5 as the best for general work. This re- 

 action may be adopted if suitable, but it has been found by the 

 writer that many bacteria completely failed to grow in media of 

 this reaction. This is especially true of a large number of soil 

 bacteria, which almost invariably require a neutral or slightly 

 alkaline medium. I would therefore recommend a medium 

 whose reaction is 0.5 per cent acid to phenolphthalein as one of 

 more general applicability. It cannot be too strongly impressed 

 upon the reader that whatever the reaction, its measure should be 

 stated in all descriptions of cultural characters. To obtain uniform 

 results it is important that media should not only have identical 

 reactions, but that they should be prepared according to a fixed 

 and uniform rule. The method here presented is that of the 

 Laboratory Committee of the American Public Health Associa- 

 tion. In the following formula it is noticed that Liebig's meat 

 extract is recommended instead of fresh meat infusion, not only 

 because it adds simplicity to the preparation of media, but 

 because media of more uniform composition can be obtained. 

 Furthermore, I have found the meat extract quite free from 

 muscle sugar, so objectionable when fresh meat infusions are 

 used. 



