AGAR MEDIUM 163 



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 sterilized in 

 these, since, if put into flasks these must be sterilized, 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 ob- 

 tained from certain seaweeds along the coast of Japan 

 and Southeastern Asia. It occurs in commerce as thin 

 translucent strips or as a powder. It resembles gelatin only 

 in the property its solutions have of gelatinizing 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°) and only after much longer 

 heating. This hot solution "jells," "sets" or gelatinizes at 

 about 38° and remains solid until again heated to near boil- 

 ing. Hence bacteria may be grown on agar at the body 

 temperature (37°) and above, and the agar will remain solid, 

 while gelatin media are fluid above about 25°. No patho- 

 genic bacteria and none of the saprophytes liable to be met 

 with in the laboratory are able to "liquefy" agar. 



An agar medium is conveniently prepared from broth by 

 adding 1 or 1.5 per cent, of the finely divided agar to the 

 broth and boiling until dissolved, standardizing, clearing, 

 filtering, and sterilizing. The agar must be thoroughly 

 boiled, usually for ten to fifteen minutes, and the water loss 

 made up by the addition of distilled water before titration. 

 Agar is practically neutral so that there is little difference 

 between the titration of the dissolved agar and the original 

 broth. The agar solution should be kept hot from the begin- 



