CULTURE MEDIA. 43 



The advantages of the gelatin medium are that it is perfectly 

 transparent, that it is easily melted for making ''plates," and that 

 many bacteria exhibit in it special characters of growth by which they 

 may be differentiated from others which resemble them in form. 

 The principal disadvantage is the low melting point, which prevents 

 us from making use of this medium for cultivating bacteria in an in- 

 cubating oven at a higher temperature than about 22 C. for ten-per- 

 cent gelatin. 



This disadvantage is overcome by using agar-agar instead of 

 gelatin. This is prepared in Japan and other Eastern countries 

 from certain species of gelatinous algae. It comes to us in the form 

 of bundles of dried strips, which form a stiff jelly when dissolved in 

 water in the proportion of one to two per cent. This jelly remains 

 solid at a temperature of 40 C. and above. It was first employed 

 by Hesse, one of Koch's collaborators in the office of the imperial 

 board of health of Berlin. Koch, who was in search of a trans- 

 parent jelly which would stand the temperature required for the cul- 

 tivation of certain pathogenic bacteria (37 to 38 C.), quickly recog- 

 nized its value and introduced it into general use. 



The agar-agar jelly is more difficult to filter than the gelatin 

 medium, and some skill is required in order to obtain a transparent 

 solution. It will bear long boiling without losing its quality of 

 forming a stiff jelly. From ten to twenty grammes are added to a 

 litre of flesh infusion, or we may make a peptonized agar in accor- 

 dance with the following formula which is given by Salomonson : 

 Add to one litre of distilled water five grammes Liebig's extract, 

 thirty grammes peptone, five grammes cane sugar, fifteen grammes 

 agar. Cook for an hour, render slightly alkaline, and cool to below 

 60 C. Clarify and cook again for an hour or more. 



Glycerin-agar is made by adding five per cent of glycerin to 

 the peptonized agar made by the above formula or by the use of the 

 flesh-peptone infusion. This is a very favorable medium for the cul- 

 tivation of the tubercle bacillus first used by Roux and Nocard. 



Agar-gelatin, a medium which has recently come into favor and 

 is said to be very useful, as it resembles gelatin in transparency and 

 has a considerably higher melting point than ten-per-cent gelatin, is 

 made by adding fifty grammes of gelatin and 7. 5 grammes of agar 

 to a litre of flesh-peptone solution. Care should be taken not to cook 

 this longer than is necessary. 



In making all of these agar culture media the main difficulties 

 encountered result from the difficulty of dissolving the agar and the 

 slowness with which the solution passes through filtering paper. 

 These difficulties are best met as follows : Break up the sticks of agar 

 into small fragments and allow them to soak in cold water for twenty- 



