I. Solid Culture Media with a Wide Range of Hydrogen or 
Hydroxyl Ion Concentration* 
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 microorganisms. This is especially true in the case 
of such fungi and bacteria as lend themselves readily to cultivation on artifi¬ 
cial media. Investigations have also clarified many problems related to these 
media themselves, such as the influence of acids and alkalis on colloidal hydra¬ 
tion and jellification, the buffer action of proteins and salts, the devising of 
improved colorimetric and electrometric technic for the measurement of hydro¬ 
gen 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, there¬ 
fore 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 Lie¬ 
big’s beef extract, 1 per cent Armour’s peptone and 0.5 per cent sodium clilorid. 
They were then heated in an autoclave, flasked, and sterilized, for fifteen min¬ 
utes at 15 pounds pressure in the case of agar, and 10 pounds pressure in the 
case of gelatin. No attempt was made to adjust the reaction of the media to 
neutrality prior to sterilization. The acid used was hydrochloric, with a spe¬ 
cific gravity of 1.20 or it possessed an HC1 concentration of 39.11 per cent. 
The sodium hydroxid had a specific gravity of 1.226 or an NaOH concentra¬ 
tion of approximately 20 per cent. Strong acid and alkali were employed to 
eliminate the factor of dilution of the media. Upon removal from the auto¬ 
clave the agar was cooled to about 50°C. and the gelatin to about 40°C. before 
the addition of appropriate quantities of acid or alkali, and were maintained 
at these temperatures while 10 cc. portions were withdrawn with a pipette 
and put into test tubes. The acid or alkali was added to these 10 cc. portions 
with a 1 cc. pipette graduated in tenths. After the addition of the acid or 
alkali the tubes were well agitated and were further cooled with the results 
indicated in the tabulations which follow. 
EXPERIMENTAL 
Only those proportions of agar or gelatin which are commonly employed in 
making culture media were used in this study, but they indicate, as would be 
♦Reprinted from Jour. Bact. 6: No. 3, 325-330, 1921. 
