130 CONN, HARDING, KLIGLER, FROST, PRUCHA AND ATKINS 



of china blue with phenol red. Both of these combinations are 

 colorless or nearly so at pH = 7.0, turning blue as the reaction 

 becomes acid and red as it becomes alkaline. Rosolic acid has 

 the advantage over phenol red of having a more alkaline range 

 (pH = 7.3 to pH = 9) than phenol red, hence giving the 

 Bronfenbrenner combination a sensitive range from pH = 5 to 

 pH = 9. Rosolic acid is insoluble in water, but it is possible to 

 keep it in concentrated alcoholic solution (as above recommended 

 for brom cresol purple and cresol red) so that only 1 cc. of 

 alcohol is added to a liter of medium. The concentration of 

 china blue in the medium should be 0.0025 per cent, that of 

 rosolic acid or its sodium salt 0.005 per cent, while in combination 

 with china blue a 0.001 per cent solution of phenol red is recom- 

 mended. Either of these combinations should have distinct ad- 

 vantage over the combination of brom cresol purple with cresol 

 red; but there has been as yet no opportunity to compare them. 



Variations of these media. For certain organisms the above 

 formulae are not the best — many pathogenic bacteria, for in- 

 stance require more peptone than is provided in the above formula 

 for broth, while some organisms are best studied in media of a 

 hydrogen-ion concentration different from that recommended 

 above. In such cases the individual investigator is free to modify 

 the media to suit his own purposes; but whenever other than these 

 routine formulae are used, the fact should be stated on the chart. 

 In employing a reaction other than that of neutrality it is 

 recommended that instead of using the titrimetric method, the 

 reaction be adjusted to some definite shade of brom cresol purple, 

 if a more acid reaction is desired, or of phenol red if it is to be 

 more alkaline. 



Optional media. In many laboratories other media than those 

 specifically mentioned on the chart are in general use, such as 

 potato, blood serum, agar stabs, and so forth. Blank spaces are 

 left on the chart for recording characteristics on any optional 

 media. 



