AGAR MEDIUM 175 



Or, the gelatin may be added to meat juice before it is 

 boiled, then this is heated to about body temperature (not 

 too hot, or the proteins will be coagulated too soon) until 

 the gelatin is dissolved. Then the material is standardized 

 and thoroughly boiled and filtered. The proteins of the 

 meat juice coagulate and thus clear the medium without 

 the addition of egg white. Commercial gelatin is markedly 

 acid from the method of manufacture, hence the medium 

 requires careful titration, even when made from a standard- 

 ized broth. 



Gelatin should be sterilized by discontinuous heating at 

 100° on three successive days, because long boiling or heat- 

 ing above 100° tends to hydrolyze the gelatin into gelatin 

 proteose and peptone and it will not gelatinize on cooling. 

 It may be heated in the autoclave for ten to fifteen minutes 

 at 10 pounds' pressure and sometimes not be hydrolyzed, 

 but the procedure is uncertain and very resistant spores may 

 not be killed. The medium should be put into the culture 

 tubes in which it is to be used as soon as filtered, and steril- 

 ized in these, since, if put into flasks these must be steril- 

 ized, and then when transferred to tubes for use, it must be 

 again sterilized unless great care is taken to have the tubes 

 plugged and sterilized first, and in transferring aseptically 

 to these tubes. These repeated heatings are very apt to 

 decompose the gelatin, so it will not "set" on cooling. The 

 prepared and sterilized tubes of gelatin should be kept in 

 an ice-box or cool room, as they will melt in overheated 

 laboratories in summer or winter. 



Agar Medium.— Agar agar, usually called agar, is a com- 

 plex carbohydrate substance of unknown composition 

 obtained from certain seaweeds along the coast of Japan, 

 Southeastern Asia and our Pacific Coast States. It occurs in 

 commerce as thin translucent strips or as a powder. It resem- 

 bles gelatin only in the property its solutions have of gelatin- 

 izing when cooled. Gelatin is an albuminoid closely related to 

 the proteins, agar a carbohydrate. Agar is much less soluble 

 in water, 1 or 1.5 per cent of agar giving a jelly as dense as 10 

 to 15 per cent of gelatin. It dissolves only in water heated 

 to near the boiling-point (98° to 99° C), and only after much 



