IO CLINICAL BACTERIOLOGY AND H^MATOLOGY 



more vigorous growths than can be obtained in the same time on 

 the old media, and quite repays the extra trouble spent in its prepara- 

 tion. It is prepared as follows : Take a fresh bullock's heart, and 

 remove the fat, .vessels, etc. (if this cannot be obtained fresh meat 

 may be used instead), and mince the muscle fine. To one heart add 

 4 litres of water, render faintly alkaline to litmus, and heat to 

 7o-8o C. Now cool down to 45 C. (approximately) add i per cent. 

 of trypsin (Douglas uses Allen and Hanbury's liquor trypsinae co.), 

 shake, place in the incubator, and allow digestion to go on for two 

 to three hours. Now render slightly acid and boil, to coagulate the 

 unaltered proteins. Strain through muslin. Render the filtrate 

 alkaline to litmus, add about -5 gramme of calcium chloride to the 

 4 litres and -25 salt (10 grammes to the 4 litres). Autoclave for one hour 

 at 115 C., to precipitate the phosphates. Filter, tube, and sterilize, 

 as before. 



If you are preparing broth to use for making agar, add the latter 

 before the last stage, so that the autoclaving which serves for the 

 removal of phosphates will dissolve the agar. 



NUTRIENT GELATIN is broth which has been solidified by the 

 addition of from 10 to 15 per cent, of gelatin; the former 

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

 general purposes \2\ per cent may be used in all cases. 



The special advantages of gelatin as a culture medium are 

 twofold. 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 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 



