CULTURE MEDIA. 41 



aqueous extract is then obtained by filtration through muslin by 

 pressure. This extract is cooked, filtered, and carefully neutralized 

 by the addition of a solution of carbonate of sodium, which is added 

 drop by drop. Usually we add to this one-half per cent of chloride 

 of sodium. The addition of ten grammes of peptone to a litre of 

 this meat infusion constitutes the flesh-peptone solution which is 

 largely used in the preparation of solid culture media, to be described 

 hereafter. 



The addition of five per cent of glycerin to the above infusion 

 makes a useful liquid medium for the cultivation of the tubercle ba- 

 cillus (Roux and Xocard). The liquid should be again neutralized 

 after adding the glycerin, which commonly has an acid reaction. 



Dunham's Peptone Solution. This is used principally for de- 

 termining whether bacteria under investigation are capable of pro- 

 ducing indol. One part of pure dried peptone is added to 100 parts 

 of distilled water, and to this is added one-half per cent of sodium 

 chloride. The addition of rosalic acid to this solution affords a 

 means of determining whether bacteria cultivated in it produce an 

 acid or an alkaline reaction of the medium. The pale rose color im- 

 parted to the peptone solution by the addition of rosalic acid becomes 

 more intense when the solution becomes alkaline, and it fades out en- 

 tirely when it becomes acid. To obtain this reaction add 2 parts 

 of the following solution to 100 parts of Dunham's peptone solution: 

 rosalic acid (corralline), 2 parts; alcohol (eighty per cent), 100 parts. 



Bouillon is made by cooking the chopped meat one pound in a 

 litre of water for about half an hour in a large glass flask or an 

 enamelled iron kettle. The filtered bouillon is then carefully neu- 

 tralized with sodium carbonate, and again boiled for an hour to pre- 

 cipitate all coagulable albuminoids. It is again filtered and dis- 

 tributed in test tubes or small flasks, in which it is subsequently 

 sterilized. For certain pathogenic bacteria a bouillon made from the 

 flesh of a fowl or of a rabbit is preferable to beef bouillon. 



Flesh infusion may also be made from one of the standard beef 

 extracts, such as Liebig's (five grammes to a litre of water). 



Various vegetable infusions may also be used as culture media, 

 such as yeast water, potato water, infusion of hay, of barley, or of 

 wheat, of dried fruits, beer wort, etc. 



SOLID CULTURE MEDIA. The introduction of solid culture 

 media, and especially the use of gelatin and agar-agar, as first 

 recommended by Koch (1881), for the isolation and differentiation of 

 species, was a most important advance in bacteriological technology. 

 We are concerned here only with the composition and preparation 

 of these media. 



Flesh-Pept one-Gelatin. This is made by adding ten per cent 



