METHODS OF PURE CULTURE STUDY 117 



common indicator, enough of which should be added in saturated 

 aqueous solution to give the medium a distinct blue color (taking 

 care that the litmus solution used is not so alkaline as to alter, 

 appreciably, tfie reaction of the medium). Litmus, however, 

 does not give accurate results in terms of hydrogen ion concen- 

 tration; so except for certain special purposes (see p. 124) it is 

 recommended that brom cresol purple be used. Prepare this 

 by dissolving 0.4 g. of di bromo ortho cresol sulphonphthalein 

 in a minimum amount of alcohol, making up to 1 litre with 

 water. Add 40 cc. of this solution to a litre of the medium. 

 The blue or purple color given to media by this indicator begins 

 to fade with the slightest production of acid in the medium and 

 disappears completely at an acidity corresponding to Ph=5.0, 

 considerably below the curdling point of milk. 



Variations of these media. For certain organisms the above 

 formulae are not the best — many pathogenic bacteria, for instance, 

 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 them to suit his own purposes; but whenever other than 

 these standard formulae are used, the fact should be stated on 

 the chart. In employing a reaction other than that of neu- 

 trality 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. 



INVIGORATION OF CULTURES 



Provided a medium can be found upon which the organism 

 to be studied grows vigorously, it should always be invigorated 



