THE USE OF AGAR SLANTS IN DETECTING 

 FERMENTATION 



H. J. CONN and G. J. HUCKER 



Agricultural Experiment Station, Geneva, New York 

 Received for publication March 25, 1920 



In a recent paper (Conn, 1919) one of the writers described 

 the use of agar slants instead of liquid media for detecting acid 

 production by two soil organisms, stating the method to be much 

 more satisfactory for these particular bacteria than the old 

 method. Subsequently this method was given as an alternate 

 procedure in the report of the Committee on the Descriptive 

 Chart (1919). We have recently used the method for a great 

 variety of soil and milk organisms with such great success that 

 it seems worthy of more specific mention. The technic is as 

 follows : 



One per cent of the desired sugar is added to an agar medium 

 favorable to the growth of the organisms to be tested. The 

 reaction is adjusted by the use of brom thymol blue to a pH- 

 value between 6.8 and 7.2. Then an indicator is added whose 

 range lies just to the acid side of pH = 7.0, such as china blue 

 or brom cresol purple; or better yet, a mixture of the former 

 with cresol red or sodium rosolate (as recommended by Bronfen- 

 brenner, Schlesinger and Soletsky, 1920), or of the latter with 

 cresol red, in order to detect decreases as well as increases in 

 hydrogen-ion concentration. The following concentrations of 

 indicators are recommended: Brom cresol purple 0.001 per cent; 

 Brom cresol purple purple and cresol red each 0.0005 per cent; 

 China blue 0.0025 per cent; China blue 0.0025 per cent plus 

 sodium salt of rosolic acid 0.005 per cent. 



It is convenient to keep the indicators in concentrated alcoholic 

 stock solutions of such a strength that a definite amount can 

 be measured out per litre of medium: i.e., a 1.6 per cent stock 



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