STERILIZATION 73 



not necessary to make plate or tube cultures to determine the 

 fitness of the gelatin for bacteriological work. Incidentally it 

 may be remarked that a gelatin which is unsuitable for bacterio- 

 logical work is also unfit for use as human food. 



The agar should be the highest grade obtainable, and if the 

 shredded form is used it should always be washed in sterilized 

 distilled water before making into culture media. 



With regard to the other ingredients required in making cul- 

 ture media, such as dextrose, lactose, maltose, saccharose, glycerin, 

 salt litmus, etc., etc., special efforts should be made to get these 

 as pure as possible. The degree of purity should be determined 

 by actual tests. 



2. Sterilization. — Thorough sterilization of all culture media 

 is absolutely necessary. It is, however, known that heating pro- 

 duces some marked changes in the molecular composition of the 

 media, even inducing actual chemical decomposition. It is 

 therefore desirable to make the time of heat exposure as brief as 

 possible. Ordinarily it is therefore preferable to use the auto- 

 clave, bringing the temperature up to 120° C. (15 lb. pressure) 

 for a period of 15 min. This temperature will sterilize all 

 media. A shorter period does not insure complete sterilization 

 and a longer exposure is apt to produce inversion of the sugars 

 used and also permanently lower the melting point of the gelatin. 

 Solid media as gelatin and agar should be liquefied before placing 

 in the autoclave. 



The following rules should be strictly observed in using the 

 autoclave : 



a. The sterilizer should be hot when the media are introduced. 

 About 100° C. Let all air escape from chamber. 



b. At the end of the period of sterilization (15 min.), remove 

 the media and cool them as rapidly as possible. 



Compliance with these rules will reduce to a minimum the 

 tendency toward liquefaction of the gelatin and a tendency to 

 decompose the various chemicals used, due to prolonged heating. 



