CULTURE MEDIA 



media are best sterilized at 15 pounds pressure for 15 to 30 

 minutes. Gelatin media are sterilized in flowing steam on three 

 consecutive days or in the autoclave at 10 pounds pressure for 

 30 minutes. Soil is extremely difficult to sterilize. Small 

 containers with about 5 to 10 grams of soil are heated in the 

 autoclave at 15 to 20 pounds pressure for 2 hours one day, or 1 

 hour on each of 2 consecutive days, or in flowing steam on at 

 least 7 consecutive days. 



The relation between pressure and temperature is shown in 

 the table below: 



The reaction of a medium has a decided influence upon the 

 decompositions brought about by sterilization. The lower the 

 pH of the medium, the greater will be the hydrolyzing effect of 

 the sterilization, not only of gelatin, but also of agar, so that at 

 pH 4.0, as much as 3 per cent agar has to be used to obtain a 

 soUd medium. When the medium is alkaline, the iron of the 

 solution will be precipitated out. This may necessitate the 

 addition of a small amount of sterile iron salt, or the sterilization 

 of the medium by filtration through a Berkefeld filter. 



It is recommended that culture media be inoculated as 

 soon as possible after sterilization. Because of the adsorbed 

 air, old culture medium is unfit for the growth of strict anaerobic 

 bacteria. 



Silica gel media need not be sterilized. The washing with 

 sterik \ vter, followed by the use of highly selective media is 

 sufficient to assure sterility, as far as air contaminations are 

 concerned. Their use is adapted only to the very selective 

 organisms. 



