92 TEXT-BOOK OF BACTERIOLOGY. 



cannot be employed, since the chances of their being polluted from 

 the beginning are far greater. 



Serum once consolidated does not lose its solidity again. It is 

 therefore particularly well adapted for the culture of bacteria at 

 high temperatures. 



But in consequence of this peculiarity it has not the advantage 

 possessed by other solid and transparent media that of being able 

 to return to the liquid state under certain conditions, and thereby 

 becoming easier to manipulate. Under the influence of certain 

 bacteria, however, it does soften in the same way as gelatin, and 

 then it is usually peptonized to a very considerable extent. 



On this account the employment of blood-serum is restricted to 

 somewhat narrow limits, and we can in many cases greatly facili- 

 tate our labors by employing glycerin agar instead. For certain 

 purposes serum of human blood and of substances similar to it as, 

 for instance, the fluid from dropsy (ascites), from hydroceles, and 

 from the ovary has been prepared as a culture medium. Human 

 serum is chiefly obtained from the placenta, which generally yields 

 a considerable quantity of blood. 



The whole series of solid transparent food-media which are 

 commonly employed have now been considered, and also the special 

 advantages offered by particular kinds : the ease with which gela- 

 tin can be prepared and employed; the power of agar to resist not 

 only high temperatures, but also the decomposing action of bacteria; 

 and the importance of glycerin agar and of blood -serum for the 

 breeding of strictly parasitical micro-organisms. But it would be 

 a great mistake if it should be considered that these constituted a 

 wealth of artificial culture media with which could be obtained the 

 desired object at all times and under all circumstances. 



By the addition of gelatinizing substances to the bouillon, for 

 instance, the latter is not in itself rendered more suitable to serve 

 as food for the micro-organisms. It is true that bouillon was 

 chosen with a view of offering the requisite conditions for develop- 

 ment to as many kinds of bacteria as possible, and up to a certain 

 point its success was undoubted, for great numbers of bacteria are 

 known which thrive in the ordinary culture media. 



Yet its nourishing powers must not be overrated. Numbers of 

 bacteria, perhaps the majority of all existing species, do not find in 

 it the conditions necessary for their existence, and therefore resist 

 all efforts to breed them artificially. A simple example may suf- 

 fice to prove this. 



If saliva be examined under the microscope in a cover-glass 

 preparation or as a hanging drop, generally an abundance of 



