12 CLINICAL BACTERIOLOGY AND HEMATOLOGY 



the interval between the first and second sterilization most of the 

 spores which may be present will develop into mature bacteria, 

 and these will be killed by the second steaming. The third 

 sterilization is to kill off any bacteria which may not have 

 developed from spores in the first interval. A very similar 

 process is adopted by the gardener in freeing soil from weeds ; 

 the application of chemical weed-destroyers or a thorough hoeing 

 will destroy developed plants, but will not injure seeds which may 

 be contained in the soil, and these processes are repeated, intervals 

 being allowed to permit the development of the plants, until they 

 reach the stage in which they are vulnerable. 



To recapitulate : Mix the ingredients, and heat until they are 

 dissolved, render slightly alkaline, boil half an hour, filter. Then 

 place in sterilized flask or into test-tubes, and sterilize in the steam 

 sterilizer for half an hour on three successive days. 



Nutrient Gelatin is broth which has been solidified by the 

 addition of from lo to 15 per cent, of gelatin ; the former amount 

 is used in the winter, the latter in the summer. For general 

 purposes i2|^ 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 organism 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 bacteria " 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 



