74 Microbes and You 



steam to enter the chamber and to escape through the drain before 

 closing all outlets. As soon as the temperature approaches 121° C, 

 the pressure gauge, if it is functioning properly, should register 

 close to 15 pounds. At the conclusion of the sterilization period, 

 do not release the pressure quicklij or the liquids in the autoclave 

 will "blow their tops" and spill the carefully prepared media. For 

 best results, merely shut off the steam inlet valve and allow the 

 pressure to decrease gradually. If the autoclave has no serious 

 leaks in the gaskets or valves, this cooling-off period should reduce 

 the pressure to zero in from 10 to 20 minutes after closing the 

 steam valve. 



Tubes or flasks of liquids being autoclaved are commonly 

 stoppered with plugs of non-absorbent cotton to allow ready access 

 to live steam. Under no circumstances should absorbent cotton be 

 substituted, since once the cotton becomes soaked with moisture, it 

 loses its capacity to retard the entrance of contaminating organisms 

 into the sterile fluids. The importance of the introduction by 

 Schroder and von Dusch of cotton plugs to microbiology cannot be 

 overemphasized. 



Dry Heat Sterilization. Indirect Heat. Most of the glass- 

 ware, such as pipettes and petri dishes, used in microbiological 

 laboratories must be dry after sterilization, and this precludes the 

 wet autoclave as a means of sterilizing much of this equipment. 

 However, a common baking oven, heated with gas, kerosene, or 

 electricity, serves as a useful instrument for dry heat sterilization. 

 In addition to glassware, all oils and greases should be subjected 

 to drv oven treatment to sterilize them. A number of hospitals 

 still mistakenly "sterilize" such things as mineral oil and vaseline 

 gauze in the autoclave, forgetting the old adage that oil and water 

 do not mix. Onlv the outer exposed portion of such greasy sub- 

 stances which come in intimate contact with the live steam are 

 being subjected to moist heat at 121° C. The rest of the material 

 is practically water-free, and 121° C. dry heat is not sufficient to 

 sterilize in the usual 15-minute contact period. The chart in Table 

 5 indicates that protein in the absence of moisture does not coagu- 

 late until the temperature approaches 160° C. Therefore, when 



