12 CLINICAL BACTERIOLOGY AND HiEMATOLOGY 



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 

 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 our dilution has been properly made, 

 we shall have separated each organism frota its neighbours, and 

 each separate germ will grow up into a "colony," which will soon 

 be visible to the naked eye. In all probability we shall be able 

 to see that these colonies are of two kinds : one may liquefy 

 and the other not, one may be coloured and the other colour- 

 less, one may be round and the other angular, etc. Samples 

 of each sort of colony are then transplanted to fresh culture- 

 tubes, and again incubated. An example of this process is given 

 on p. 58. 



A slight modification of this process enables us to make an 

 estimate of the number of living bacteria which is present in a 

 given fluid. To do this we have to follow out the above process, 

 adding a definite measured quantity of the fluid to the culture-tube 

 of liquefied gelatin. The number of colonies which develop is 

 counted, and this gives us the number of bacteria in the sample of 

 fluid. For example, if j?g- c.c. diffused throughout a tube of 



