1896.1 MICROSCOPICAL JOURNAL. 159 



carbon and nitrogen are obtained. Most saprophytic 

 bacteria, as the ordinary germs of pntrefaction, grow best 

 at 25° to 30° C, while tlie optiniam temperature for the 

 parasitic varieties is that of the animal body in whicli 

 they are found. Extreme coll does not destroy bacteria,. 

 but all are destroyed by a tem[)erature of 100 C. main- 

 trained for some time. Soaie bacteria wil Idevelop read- 

 ily in a slightly acid culture medium, while other forms 

 will not grow if the least ti'ace of acid be present. . 



Germs causing disease in animals are c;illed patho- 

 genic, and almost invariably require neutral or slightly 

 alkaline materials for food. In order to obtain satisfac- 

 tory knowledge of the biological characteristics of bac- 

 teria, they must be grown in various ways. A great 

 variety of substances have been used as food for bacteria, 

 some are natural, others artiticial. Of the varieties of 

 pabulum the most important is blood-serum, obtained 

 under aseptic conditions from the blood of slaughtered 

 animals. This serum may be coagulated by heat, when 

 it is known as Koch's blood-serum, or, if a small amount 

 of beef bouillon is added, and then coagulated, it is called 

 Loeffler's blood-serum, whicn is used very extensively by 

 health boards in many of our larger cities for growing 

 diphtheria germs. Potatoes are frequently used, and are 

 very useful for bringing out the biological characteristic!^ 

 of "surface growths," of some forms of bacteria. Other 

 tuberous roots, milk, cooked fish, etc., may be used. Usu- 

 ally, however, artificial materials are employed in the 

 laboratory : beef bouillon, containing 1 tu 2 per cent pep- 

 tone and i per cent sodium chloride, is generally the basis. 

 In the manufacturing laboratory, broth of this kind is 

 used almost entirely for growing the various toxins used 

 for immunizing the animals which produce the anitoxins. 

 To the beer bouillon may be added from 10 to 20 per cent 

 gelatin, which forms the plain or nutrient gelatin, used 

 very extensively for making Stich or puncture cultures. 



