SOLID CULTURE MEDIA WITH A WIDE RANGE OF 



HYDROGEN OR HYDROXYL ION 



CONCENTRATION 



FREDERICK A. WOLF and I. V. SHUNK 



From the Botanical Laboratory, North Carolina Experiment Station, West Raleigh, 



North Carolina 



Received for publication October 11, 1920 



A considerable number of investigations, made during the 

 past few years, have extended our knowledge of the profound 

 influence exerted by acids and alkalis upon the growth of micro- 

 organisms. This is especially true in the case of such fungi and 

 bacteria as lend themselves readily to cultivation on artificial 

 media. Investigations have also clarified many problems related 

 to these media themselves, such as the influence of acids and 

 alkalis on colloidal hydration and jellification, the buffer action 

 of proteins and salts, the devising of improved colorimetric and 

 electrometric technic for the measurement of hydrogen and 

 hydroxyl ion concentration, etc. The point covered in the 

 present study, which grew out of an attempt to determine the 

 limit of tolerance of certain organisms to acid and alkali on 

 solid media, does not appear to have been brought out in any 

 foregoing investigation. In previous studies use has been made 

 of liquid media for very tolerant organisms, even for forms 

 which are known to thrive best on solid media, since it has been 

 impossible to make agar or gelatin with high pH values solid. 

 It is the present purpose, therefore, to show that acids and alkalis 

 need not materially modify the physical properties of agar and 

 gelatin media within, and even far beyond, the limits of tolerance 

 of any living organism. 



METHODS 



The media were prepared by adding either 1 or 2 per cent 

 commercial agar or 10 or 15 per cent bacto-gelatin to a bouillon 

 consisting of 0.3 per cent Liebig's beef extract, 1 per cent Armour's 



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