170 Sterilization and Disinfection 



must be sterilized by exposure to streaming steam or steam under 

 pressure, in the steam sterilizer or autoclave, before they can be 

 pronounced sterile. 



It must always be borne in mind that after sterilization has been 

 accomplished it is necessary to protect the sterilized objects and 

 media from future contamination. 



To Schroder and Van Dusch belongs the credit of having first 

 shown that when the mouths of flasks and tubes are closed with 

 plugs of sterile cotton no germs can filter through. This discovery 

 has been of inestimable value, and has been one of the chief means 

 permitting the advance of bacteriology. If, before sterilizing, 

 flasks and tubes are carefully plugged with ordinary (non-absorbent) 

 cotton-wool, they and their contents will remain free from the 

 access of germs until opened. Instruments may be sterilized wrapped 

 in cotton, to be opened only when ready for use; or instruments 

 and rubber goods sterilized by steam can subsequently be wrapped 

 in sterile cotton and kept for use. It is of the utmost importance to 

 carefully protect every sterilized object, in order that the object of 

 the sterilization be not defeated. As the spores of molds falling 

 upon cotton sometimes grow and allow their mycelia to work their 

 way through and drop into the culture-medium, Roux has em- 

 ployed paper caps, with which the cotton stoppers can be pro- 

 tected from the dust. These are easily made by curling a small 

 square of paper into a "cornucopia," and fastening by turning up 

 the edge or putting in a pin. The paper is placed over the stopper 

 before the sterilization, after which no contamination of the cotton 

 can occur. 



II. Sterilization and Protection of Culture-media. — As almost 

 all of the culture-media contain about 80 per cent, of water, which 

 would evaporate in the hot-air closet, and so destroy the material, 

 hot-air sterilization is inappropriate for them, sterilization by stream- 

 ing steam being the only satisfactory method. The prepared 

 media are placed in previously sterilized flasks or tubes, carefully 

 plugged with cotton-wool, and then sterilized in an Arnold's steam 

 sterilizer. 



The temperature of boiling water, ioo°C., does not kill the spores, 

 so that one exposure of the culture-media to streaming steam is of 

 little use. The sterilization must be applied in a systematic 

 manner — intermittent sterilization — based upon a knowledge of 

 sporulation. 



In carrying out intermittent sterilization the culture-medium 

 is exposed for fifteen minutes to the passage of streaming steam or 

 to some temperature judged to be sufficiently high, so that the adult 

 micro-organisms contained in it are killed. As the spores remain 

 uninjured, the medium is stood aside in a cool place for twenty-four 

 hours, and the spores allowed slowly to develop into adult organisms. 



When the twenty-four hours have passed, the medium is again 



