92 BACTERIOLOGY 



the medium is cooled to this temperature after solution has been 

 accomplished. It is titrated and the proper amount of alkali 

 added. An egg is beaten up in water and then stirred into the 

 medium. It is then boiled on the watpr bath for an hour, filtered, 

 tested, tubed and sterilized. 



Nutrient Agar. — To a liter of nutrient broth, prepared as 

 above described (page 84) add 15 grams of finely cut agar shreds. 

 Weigh the pan with its contents. Boil the material over the 

 direct flame for one to two hours, with constant stirring to avoid 

 burning, adding hot distilled water from timeto time to compensate 

 for the loss by evaporation. Instead of boiUng it is convenient 

 to cook the medium in the autoclave at 110° C. for 45 minutes 

 to an hour. In either case, the agar should be very completely 

 dissolved. The medium is then cooled to 60° C. and an egg pre- 

 viously beaten up in water is added and thoroughly mixed with the 

 agar. It is then boiled again for 10 minutes over the free flame, 

 with constant stirring at the bottom, or for 45 minutes on the 

 water bath, or for 15 minutes in the autoclave at 110° C. Distilled 

 water is added to restore the original weight, and the medium is 

 then filtered, usually through a layer, of cotton wet with hot 

 water, although filter paper may be used. Filtration is favored 

 by keeping the funnel hot, either with the not-watei funnel 

 heater or in a steam baith, and it may be hastened by the use of 

 suction. The filtrate need not be perfectly clear, and it usually 

 clouds on cooling unless it is acid in reaction. The reaction 

 should be alkaline to litmus. After fiilling into tubes or flasks, 

 agar should be sterilized in the autoclave at 110° C. for 30 to 

 35 minutes. 



Time may be saved by using the more expensive powdered 

 agar in place of the agar shreds and when a very clear medium 

 is desired the hquid agar may be allowed to sediment in a water 

 bath at 60° C. for some hours before filtration. 



Modifications of the Common Media. — Broth is made nearly 

 free from sugar by fermenting the meat infusion over night at 

 37° C. after inoculating it with B. coli, and then proceeding with 



