78 LABORATORY EXERCISES IN BACTERIOLOGY. 



ant paste may be spread in dishes or introduced into tubes if desired. In this method 

 of preparation one may take advantage of the opportunity afforded for adjusting the 

 reaction of the medium, by boiling the paste in a sodium hydrate solution to correct the 

 reaction uniformly through the mass. 



Glycerine potato tubes differ from the above ordinary tubes merely in that a five per 

 cent, solution of glycerine is introduced into the tube before sterilization, up to the level 

 of the lower surface of the potato ; from which a small amount of the glycerine diffuses 

 gradually through the potato and over its surface. Tubertle bacilli may be grown upon 

 such a preparation. 



(d) Eisner's Medium. This is a modification of Holz's potato gelatine (used for the 

 same purpose as Eisner's medium), designed as an advantageous medium for the isola- 

 tion of the Bacillus typhosus and Bacillus coli communis from the variety of organisms 

 with which they are apt to be associated in the dejecta, and from each other. Upon 

 it the common saprophytic germs are almost completely inhibited, and the typhoid 

 fever organism develops at a later period than Bacillus coli and with such differences of 

 appearance in the colonies as to make possible the distinction of one from the other, 

 thus facilitating their separation. The medium may be made as follows: Five hundred 

 grams of pared potato are grated finely and the pulp placed in the refrigerator, in a porce- 

 lain dish, over night. The following morning the juice is expressed from the pulp and 

 filtered several times through a layer of absorbent cotton or through animal charcoal 

 (preferably the latter). The filtrate should now be titered with decinormal sodium 

 hydroxide solution to determine its reaction and the amount of water which will be 

 required to be added to reduce its acidity to the standard. In this quantity of water is 

 now boiled the amount of gelatine required to make a ten per cent, proportion of the 

 gelatine when the potato juice shall have been added, the reaction of the gelatine being 

 corrected to neutral point after it has been dissolved (and before adding the potato 

 juice) by means of normal sodium hydroxide solution. This done, and the bulk of the 

 fluid corrected for evaporation to that necessary to properly dilute the potato juice, the 

 latter is slowly added to the gelatine and mixed, and the whole boiled for five to ten 

 minutes, filtered, and distributed to tubes or flasks, and sterilized in the usual inter- 

 rupted manner in steam. Thus far the medium constitutes Holz's potato gelatine. 

 Eisner's medium is made from this by adding freshly when required for use one cubic 

 centimeter of a sterilized solution (ten per cent, strength) of potassium iodide to each 

 ten cubic centimeters of the medium. It will be found advantageous for incubation at 

 body heat that one-half the required amount of gelatine be substituted by a correspond- 

 ing amount of agar (one per cent, of agar). 



2. A number of other carbohydrate media are employed for culture purposes, as car- 

 rots, turnips, apples, bread-paste, etc. They are used for the most part for the cultiva- 

 tion of the moulds and of chromogenic forms of bacteria, but are scarcely of sufficient, 

 importance to be further detailed here. The various sugars as culture media will be 

 noted in connection with the media in which they are usually used. 



II. PROTEID MEDIA. 



By experience it has been found that nutrient media containing proteid substance 

 are upon the whole the most favorable for the culture of the greatest number of known 

 bacteria, particularly the pathogenic forms ; and it is probable that with further study 

 the selection of such material will be far more exact than at present apprehended. The 

 usual source at present is adult beef ; yet it is probable, from the comparatively limited 



