48 MEDICAL MYCOLOGY 



organism is present. The period of sterilization must of course be measured 

 from the time the desired temperature is attained and it may require 10-15 

 minutes, even with a strong heating unit, to reach this temperature. 



These temperatures, especially if they are prolonged beyond the minimum 

 necessary to secure complete sterilization, may injure or transform the more 

 labile organic compounds, especially if the acidity or alkalinity of the medium 

 is such as to favor hydrolysis of the higher carbohydrates to hexoses, etc. 



The autoclave may be heated by electricity or gas or connected with a 

 steam supply pipe if there is sufficient pressure maintained. Care should be 

 taken to see that sufficient water is present and that the lid is wiped free from 

 dust and dirt before each sterilization. After the autoclave is full of steam 

 and the thermometer registers 100° C, the vent is closed. It is not advisable 

 to leave the autoclave without observation, although if the safety valve is 

 properly set, steam will escape after the desired pressure is reached. As soon 

 as this occurs one may safely cut down on the heat supply, since a rapid 

 escape of steam soon exhausts the small supply of water, and often dislocates 

 the cotton plugs or causes the medium to boil up and wet the plugs. When 

 the necessary time for sterilization has elapsed, the gas is turned off, and the 

 autoclave is allowed to cool until the temperature reaches 100° C. before open- 

 ing. An experienced operator may open the cock and allow the steam to 

 escape slowly before the pressure is wholly down, but this procedure is not 

 advised for a beginner. 



Under field conditions, I have found one of the various aluminium pres- 

 sure cookers now on the market very useful, although only a comparatively 

 small amount of media may be sterilized at one time. 



Filtration. — The passage of the medium through a filter with pores smaller 

 than bacteria is a possible though tedious process which may have to be used 

 in cases where biologic products are so altered by heat that it is impossible 

 otherwise to retain them in condition to use as media. 



Asepsis. — The careful excision of bits of plant tissue, under condition of 

 asepsis approaching that obtaining in the operating room of a hospital, with 

 thoroughly sterilized instruments, usually in a special room (culture chamber) 

 where the air is kept relatively free from dust or microorganisms, is sometimes 

 practiced. Such tissues should be incubated for a sufficient time to insure 

 that they have not been accidentally contaminated during their preparation. 

 This method in its simpler form is one of the oldest ways of securing culture 

 media, but is not much used today. 



CULTURE MEDIA 



Media may be roughly classified as liquid and solid. The former were 

 much used by the earlier workers, but at present are little used except in 

 certain physiologic studies where solids interfere with chemical procedures 

 and in a few other special cases. They are still employed in studies of spore 

 srermination. 



