6o6 LABORATORY EXERCISES 



Put in autoclave, run up to two atmospheres of pressure, put out flame, and allow 

 to cool until below ioo°C. before opening (Fig. 213). Let the solution of agar cool 

 still further to about 7S°C., and then mix A and B, add (i per cent.) 10 grams dried 

 peptone and (0.5 per cent.) 5 grams common salt, bring to a boil for about three 

 minutes, neutralize and filter. The product is an absolutely clear jelly, which 

 never forms any precipitate. The whole process, with the exception of the time 

 the meat is steeping, requires only about one hour and a half. In both the above 

 methods of making agar, the filtration is very quick — from ten to twelve minutes 

 for the liter. It is not necessary to use a hot-water funnel, but wet the filter paper 

 with boiling water immediately before pouring in the agar. In the process with 

 fresh meat the clarification is effected by the coagulation of the albumen in the 

 meat water, hence solution B must not be added to A until cool enough to avoid 

 coagulation. In general the fresh meat is to be recommended, and the process is 

 easier than with the meat extract, though the latter has the advantage of cheap- 

 ness and convenience, since the meat extract can always be kept on hand, and the 

 time lost in soaking the fresh meat is saved. 



Methods of Inoculations of Agar-agar. — Agar is stored in test-tubes in one of two 

 waj's, viz.: as a straight, or cylindric medium; or, as an oblique, or slanted medium. 



1. Oblique or slanted medium. Here the medium has been allowed to solidify 

 with the tube in an inclined position, thus forming a flat surface extending nearly 

 to the mouth of the tube. Such slanted agar is used for "streak" (Strich cultur), 

 or "smear" cultivations. 



2. Straight, or cylindric, medium. Here the medium forms a cylindric mass in 

 the lower part of the test-tube and the upper surface is at right angles to the long 

 axis of the tube. Such a cylindric medium is suitable for stab culture when the 

 platinum needle is thrust deeply into the substance of the medium with the needle 

 held vertically. 



LESSON 13 



Various Nutrient A gars. — In addition to beef bouillon, or in place of it, various 

 substances organic and inorganic may be added to the agar with advantage. 



Litmus lactose agar is made out of ordinary nutrient agar by adding milk sugar 

 and enough pure litmus to give the tests. To 1000 c.c. of ordinary agar, preferably 

 that made from bouillon free from muscle sugar, add 10 grams of c.p. lactose and 

 20 c.c. of a saturated (water) solution of c.p. (lime-free) blue litmus. 



Glycerin agar, maltose agar may be made with any amount of the substance 

 desired, generally i or 2 per cent. (1000 c.c. agar plus 50 c.c. Schering's c.p. glyc-erin). 



Beerivort agar is conveniently made in i or 2 per cent, combinations of beerwort 

 and ordinary agar. Take a measured quantity of agar by volume and after it is 

 liquefied in the steam sterilizer add enough beerwort by volume to make a i or 2 per 

 cent, quantity of that liquid. 



Glucose agar is a useful culture medium. Take i or 2 per cent, of glucose by 

 weight (i gram = i c.c. by volume) and add to a measured volume of agar in the 

 liquid form. 



