PREPARATION OF CULTURE MEDIA 13 



melted gelatin and poured out into a thin film produced twenty 

 colonies, it follows that i c.c. of the fluid contained 200 bacteria. 

 This is a brief description of the essentials of the method adopted 

 in the quantitative examination of water and other fluids. 

 Requisites fov the Manufacture of Gelatin. — i. Broth. 



2. Gelatin. (Coignet's gold label gelatin is best, but any good 

 brand will do.) 



3. Dilute solution of sodium carbonate. 



4. Litmus-papers. 



5. Flasks, stirring-rod, funnel, and plugged test-tubes as for 

 broth. 



Method. — Measure the broth and add to it 12 J grammes of 

 gelatin for each 100 c.c. ; allow to soak for an hour or more, and 

 then heat until the gelatin is dissolved. Continue the heat, and 

 render the medium faintly alkaline, just as was done in the 

 preparation of broth. Now filter through a moistened filter-paper. 

 To avoid the setting of the gelatin during the filtration, it is best 

 to use a double-jacketed funnel containing hot water, but if this is 

 not at hand the whole apparatus (flask and funnel) may be placed 

 in the steam sterilizer (the lid being kept off" to avoid the drops 

 of condensed water which might otherwise fall into the funnel) 

 or in a warm (but not hot) oven, and left at a temperature of about 

 40° C, until the process is complete. 



The gelatin which is made by the above process is sufficiently 

 clear for most purposes. A more sightly medium may be made 

 by clarification of the above by white of egg. To the medium 

 (after neutralization, but before filtration) add the white of one 

 egg for each 250 or 300 c.c. of fluid, and shake thoroughly. Now 

 boil in the steamer for half an hour, and filter as before. 



Test-tubes are filled with gelatin just in the same way as with 

 broth, and the process must be carried out quickly to avoid 

 solidification of the medium. Some of the test-tubes are allowed 

 to cool in the vertical position, others lying in a sloping position, 

 so that the upper surface of the gelatin forms an ellipse some 

 3 inches long. The former tubes are inoculated by driving a 

 straight platinum needle charged with the material containing 

 the bacteria into the gelatin in the axis of the tube ; cultures 

 made in this way are called "stab cultures." The gelatin 

 " slopes " are inoculated by drawing the charged needle along the 

 surface of the medium, care being taken not to plough it up ; 

 cultures made in this way are called " stroke cultures." 



