192 Cultivation of Micro-organisms 



extract are added. The solution is boiled until the constituents 

 are dissolved, titrated, and filtered when cold. If it be filtered while 

 hot, there is always a subsequent precipitation of meat-salts, which 

 clouds it. 



Bouillon and other liquid culture media are best dispensed and 

 kept in small receptacles — test-tubes or flasks — ^in order that a single 

 contaminating organism, should it enter, may not spoil the entire 

 quantity. A convenient, simple apparatus for filling tubes with 

 liquid media consists of a funnel to which a short glass pipet is at- 

 tached by a bit of rubber tubing. A pinch-cock controls the outflow 

 of the liquid. The apparatus need not be sterilized before using, as 

 the culture medium must subsequently be sterilized either by the 

 intermittent method or in the autoclave after the tubes are filled. 

 The test-tubes and flasks into which the culture medium is filled 

 must, however, be previously sterilized by dry heat, unless the sub- 

 sequent sterilization is to be performed in the autoclave, when it 

 may be unnecessary. 



Sugar bouillon is bouillon containing in solution known percent- 

 ages of such sugars as glucose, lactose, saccharose, etc. As Smith* 

 has pointed out, if the quantity of sugar in the bouillon is to be ac- 

 curately known, it is necessary to. first destroy the muscle sugars in 

 the meat-infusion. This can be done by adding a culture of the 

 colon bacillus to the meat-infusion and permitting fermentation to 

 continue overnight, then finishing the bouillon and adding the 

 known quantity of whatever sugar is desired. About i per cent, 

 of dextrose, lactose, saccharose or galactose is all that is required. 

 More may be injurious. If the bouillon be made from meat ex- 

 tract, fermentation may not be necessary. 



The sugar bouillons should not be sterilized in the autoclave, as 

 the high temperatures chemically alter the sugars. 



GELATIN 



The culture-medium known as gelatin is bouillon to which 10 

 per cent, of gelatin is added. It has the decided advantage over 

 bouillon that it is not only an excellent food for bacteria, and, like 

 the bouillon, transparent, but also is solid at the room temperature. 

 Nor is this all: it is a transparent solid that can be made liquid or 

 solid at will. Leffmann and La Wall have examined commercial 

 gelatins and found that many of them contain sulphur dioxid in 

 quantities as great as 835 parts per million. As the varying quan- 

 tity of this impurity may modify the growth of the culture, pure 

 gelatin should be demanded, and all gelatin should be washed for 

 some hours in cold running water after being weighed and before 

 being added to the bouillon. It is prepared as follows: 



To 1000 cc. of meat-infusion or to 1000 cc. of water containing 

 * "Jour, of Exp. Med.," n, No. 5, p. 546. 



