PREPARATION OF NUTRIENT CULTURE-MEDIA 69 



in flasks three-quarters of an hour for three successive 

 days. 



Bread Mash. Bread devoid of crust, dried in an oven, and 

 then pulverized and mixed with water until thick, and steril- 

 ized as above. 



Solid transparent media are prepared from materials 

 which are transparent and which can readily be converted 

 into liquids. Such are the gelatin and agar culture-media. 



Gelatin. Gelatin is obtained from bones and tendons, 

 and consists chiefly of chondrin and gluten. 



Agar-agar. This agent, which is of vegetable origin, 

 derived from sea-plants gathered on the coasts of India and 

 Japan, has many of the properties of gelatin, retaining its 

 solidity at a much higher temperature; it becomes liquid at 

 90 C. and congeals again at 45 C. (gelatin will liquefy at 

 35 C.), whereas 38 C. is the temperature at which most 

 pathogenic germs grow best. Agar cultures can be kept in 

 incubator for days and weeks without liquefying. 



Agar is not affected very much by the peptonizing action 

 of the bacteria. 



The crude agar should first be rinsed in water, and then in 

 5 per cent, acetic acid and clear water again, to rid it of im- 

 purities. If agar is boiled thoroughly over a hot flame or in 

 an autoclave, it can be filtered much more readily. The 

 main point is to see that all the agar is dissolved. 



Glycerin-agar. The addition of 4 to 6 per cent, of gly- 

 cerin to nutrient agar greatly enhances its value as a culture- 

 medium. 



Gelatin-agar. A mixture of 5 per cent, gelatin and 0.75 

 per cent, agar combines in it some of the virtues of both 

 agents. 



Blood-serum. Blood-serum, being rich in albumin, co- 

 agulates very easily at 70 C., and if this temperature is not 

 exceeded, a transparent solid substance is obtained upon 

 which the majority of bacteria develop, and some with 

 preference. 



