226 



BERTHA VAN H. ANTHONY AND C. V. EKKOTH 



immediate use in plating. If, for example, the whole is to be 

 neutral to phenolphthalein when entirely finished an over neu- 

 tralization is necessary to allow for the acid changes during the 

 re-heating, as in making Bordet Gengou medium. 



Since, in spite of the addition of soda for the correction of a 

 medium further hydrolysis occurs when heat is applied, especi- 

 ally in the autoclave, it is impossible to know the exact reaction 

 a medium will have when sterilization is complete or when the 

 medium is re-melted. In practical work, however, it has been 

 found that an over neutralization of 0.1 to 0.3 per cent has given 

 good results when the titration is performed at 30°C. 



The re-sterilization of media without suitable correction to allow 

 for the effects of heating is to be avoided if a very definite end 

 point is desired. 



PEPTONES 



The present necessity of finding substitutes for Witte's pep- 

 tone, so long the standard in bacteriological work, has led us 

 to test the reaction of various peptones on the market. A 1 

 per cent solution of each in distilled water was boiled one minute 

 and then filtered through cotton and filter paper. When cool, 

 each was titrated at room temperature and then again after 

 the same sample had been boiled one minute, that is, the same 

 procedure was followed as in titrating the meat infusions (page 

 214). 



The following table shows the reactions of eight peptones, 

 including Witte's. 



TABLE II 



Reaction of peptones {titrated with phenolphthalein) 



Armour 



Atkinson 



Difco 



Eimer & Amend. 

 Fairchild culture 



Leitz 



Squibb 



Witte 



ROOM TEMPER- 

 ATURE PIQURE 

 (AT 20 °C.) 



+ 0.6 

 + 0.4 



+0.6 

 + 1.0 

 +0.7 

 +0.4 

 +0.3 

 +0.3 



RISE AFTER 

 BEING BO LED 



ONE MINUTE 

 IN CASSEROLE 



+0.4 

 +0.2 

 +0.2 

 +0.4 

 +0.4 

 +0.3 

 +0.1 

 +0.1 



BOTIINQ 



TEMPERATUKB 



FiaURB 



+ 1.0 

 +0.6 

 +0.8 

 + 1.4 

 + 1.1 

 +0.7 

 +0.4 

 +0.4 



