STERILIZATION 



37 



steamed the second day. The third exposure killing bacteria, developing from 

 spores that might have escaped the first two exposures. For spores to develop 

 into bacteria, conditions favorable to growth must prevail during the periods 

 between steamings. The material upon, or in which the spores are lodged, must 

 contain available bacterial food and the temperature must be about 2oC. to 

 35C. It will be seen that intermittent or fractional sterilization is not adapt- 

 able to sterilization of instruments, etc. 



Quite a number of culture media and many pharmaceuticals undergo 

 chemical changes and are injured 

 by autoclaving, boiling in brine or 

 steaming for an hour. These are 

 sterilized according to the degree of 

 heat they can withstand. Some are 

 placed in the steam sterilizer for J^ 

 hour for 3 consecutive days, others 

 are given 20 minutes or 10 minutes 

 each day for 3 consecutive days. 

 Preparations that cannot be exposed 

 to a temperature of iooC. for any 

 time without injury, are sterilized 

 by heating them in a water bath at 

 90, 80, 70 or 6oC. from 15 

 minutes to i hour each day for 5 

 or 6 consecutive days, according to 

 their degree of stability. 



Since some spores and a few 

 bacteria can resist temperatures 

 below the boiling point for many 

 hours, to insure the sterility of preparations that cannot be heated 

 above 80 or 9oC., ingredients that may be autoclaved or steamed, are 

 first sterilized that way. Sterile utensils are used throughout preparation and 

 care taken to avoid contamination by air, etc. As an example, let us consider 

 the preparation of litmus lactose gelatin. A saturated aqueous solution of 

 litmus is made, also plain bouillon; these are separately sterilized in autoclave. 

 Gelatin which has been kept sealed in dust-proof packages to prevent possible 

 contamination is dissolved in sterile bouillon, sterile litmus added, and then i 

 per cent, of lactose, the lactose having been kept in an air-tight container and 

 weighed out on a sterile watch glass. By means of a sterile test-tube filler, the 

 litmus lactose gelatin is run into cotton-stoppered test-tubes or flakes previously 

 sterilized in a hot-air sterilizer. This litmus lactose gelatin, which cannot 

 withstand steaming for prolonged periods, which has been guarded against con- 

 tamination and made of sterile ingredients, so far as possible, usually contains 

 few bacteria and fewer spores. It is placed in the steam sterilizer for 15 minutes 

 each day on 3 consecutive days, being kept at room temperature between 

 steamings. In nearly all cases complete sterility is produced. 



FIG. 6. ARNOLD STEAM STERILIZER. 



