176 CULTURE MEDIA 



longer heating. This hot solution ''jells/' "sets" or gela- 

 tinizes at about 38° and remains solid until again heated to 

 near boiling. 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 

 pathogenic bacteria and none of the saprophytes liable to 

 be met with in the laboratory are able to ''liquefy" agar. 



Ajv 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, fil- 

 tering and sterilizing. The agar must be thoroughly boiled, 

 usually from 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- 

 ning to the end, except the cooling down to below 60°, 

 when the egg white for clearing is added. Though filtration 

 through paper is possible as with gelatin, if the agar solution 

 is thoroughly boiled and filtered boiling hot, it is more satis- 

 factory for beginners to use absorbent cotton wet with boil- 

 ing water and to pour the hot agar through the same filter 

 if not clear the first time. The solidified agar medium is 

 never perfectly clear, but always more or less opalescent. 

 The agar medium may be sterilized in the autoclave for 

 fifteen minutes at 15 pounds' pressure, as the high tempera- 

 ture does not injure the agar. 



Potato Media.— Potatoes furnish a natural culture 

 medium which is very useful for the study of many bacteria. 

 The simplest, and for most purposes the best, way to use 

 potatoes is in culture tubes as "potato tube cultures" (No. 

 8, Fig. 116). These are prepared as follows: Large tubes 

 are used. Large healthy potatoes are selected. Each end 

 of the potato is sliced off so as to have parallel surfaces. 

 With a cork-borer of a size to fit the tubes used, cylinders 

 about one and one-half inches long are made. Each cylin- 

 der is cut diagonally from base to base. This furnishes two 

 pieces each with a circular base and an oval, sloping surface. 

 The pieces are then washed clean and dropped for a minute 



