116 REPORT OF COMMITTEE 



power. These media are to be made up according to the 

 directions given by the committees of the American Pubhc 

 Health Association on water analysis and on milk analysis 

 (1916, 1917), except that white of egg maybe used for clari- 

 fication if desired. It is recommended, however, that instead 

 of using phenolphthalein in adjusting the reaction of these 

 media the simpler and more accurate procedure be adopted of 

 adjusting to the neutral point of brom thymol blue^. Bring 

 the media to such an acidity as to turn this indicator a dis- 

 tinct grass-green (neither yellow green nor blue green). This 

 color indicates approximately ''true neutrality," i.e., a hydrogen- 

 ion concentration between Pg = 6.6 and Ph = 7.4, a variation 

 which seems to have no appreciable effect on bacterial activi- 

 ties. Another equally satisfactory method of adjusting media 

 to this hydrogen-ion concentration is to bring them to such 

 an acidity as to cause the first faint trace of permanent pink 

 to appear with phenol red.^ 



Sugar broths. Just before sterilization 1 per cent of the 

 required carbohydrate is to be added to beef-extract broth. 

 Otherwise proceed as for sugar-free broth. Adjust reaction 

 with brom thymol blue or phenol red. 



Plain gelatin. Proceed as for beef-extract gelatin, but omit 

 beef-extract and peptone. Clarify with white of egg. 



Nitrate broth. For routine work this medium should contain: 



Peptone 1 gram 



KNO, 1 gram 



Distilled water 1000 cc. 



Filtering is unnecessary, and reaction requires no adjustment. 

 Modification of this formula is necessary for some organisms 

 (see p. 124). 



Starch agar. Add 0.2 per cent of water-soluble starch to the 

 ordinary beef-extract agar. 



Indicator media. Saccharine media with some indicator to 

 show acid production are frequently used. Litmus is the most 



^ 0.04 per cent di bromo thymol sulphonphthalein in 95 per cent alcohol. 

 ' 0.02 per cent phenol sulphonphthalein in 95 per cent alcohol. 



