1896.1 MICROSCOPICAL JOURNAL. 159 



carl30ii and nit-rogt^n ar*^ obtained. Most saprojihytic 

 bacteria, a.s the ordinary geinis of ]Mitrefa('tion, grow best 

 at 25° to 30° C, while tlie o[)tiniiini temi)erature for the 

 parasitic varieties is that of the animal body in which 

 they are fonnd. EKtre.ue col I does not de.stroy bacteria, 

 l)nt all are ilestroyed by a leniiieratnre of 100 C. niaiu- 

 trained for some time. So.ne bacteria wii Idevelop read- 

 ily in a slightly acid cnliure medium, while other forms 

 will not grow if the least tr;tce of acid be present. 



Germs causing disease in animals are called patho- 

 genic, ami almost invtirialdy require neutral or slightly 

 alkaline materials for fooii. In order to obtain satisfac- 

 tory knowledge of tlie biological characteristics of bac- 

 teria, they must be gi-own 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-sernm, 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 

 Loetiler's blood-serum, whicii 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 characteristics 

 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 to 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. 



