l6 BACTERIOLOGICAL DIAGNOSIS. 



the former amount is used in the winter, the latter in 

 the summer. For general purposes 12^ per cent, may 

 be used in all cases. 



The special advantages of gelatin as a culture 

 medium are two-fold. In the first place a great many 

 organisms grow in or on it in a characteristic way, so 

 that a bacteriologist may be able to identify the organ- 

 ism by inspection of the culture. This arises partly 

 from the fact that some bacteria produce a ferment 

 which digests gelatin just as pepsin does ; these bac- 

 teria " liquefy " the gelatin, and the distinction between 

 the bacteria which have and those which have not this 

 property is very important for purposes of diagnosis. 

 Further, some bacteria liquefy rapidly and others 

 slowly, and this is another important point in the 

 identification of a germ. 



In the second place, the gelatin medium may be 

 melted at a temperature (about 25 C.) at which 

 bacteria are not killed. This fact is made use of in the 

 isolation of bacteria from a fluid which contains several 

 species by the process known as "plating" Suppose, 

 for instance, that we find by microscopic examination 

 that a specimen of pus contains two different species of 

 bacteria (perhaps a bacillus and a coccus) and we wish 

 to obtain the two organisms in pure culture so that we 

 can ascertain their nature and properties. We take a 

 tube of gelatin and melt it by placing it in warm water, 

 and then inoculate the medium with a minute quantity 

 of the pus.* We then shake it so as to distribute the 

 organisms throughout the melted fluid, and then pour 

 the latter into a flat dish (Petri's plate) so that the 

 gelatin flows out into a thin film and then sets. If 



* Another and preferable plan is to inoculate the medium first and 

 to melt it afterwards. 



