40 



METHODS OF CULTIVATION OF BACTERIA. 



Fig. 9. — Hot-water funnel. 



This consists of an outer tin funnel, the neck of which is fitted 



with a perforated cork, through which is placed the stem of an 

 inner glass funnel. The interspace between 

 the two funnels is filled with water, which 

 is kept hot by a Bunsen under a side arm 

 let into the outer funnel. Whichever in- 

 strument be used, before filtering shake up 

 the melted medium, as it is apt while melt- 

 ing to have settled into layers of dif- 

 ferent density. The medium when tubed 

 is to be sterihsed in the autoclave seven 

 minutes. 



2 {b). Glycerin Agar. — To 2 {a) after 

 filtration add 6 to 8 per cent of glycerin 



and sterilise as above. This is used especially for growing the 



tubercle bacillus. 



2 {c). Glucose Agar. — Prepare as in 2 {a), but add i to 2 

 per cent of grape sugar along with agar. This medium is used 

 for the culture of anaerobic organisms at temperatures above 

 the melting-point of gelatin. It is also a superior culture 

 medium for some aerobes, e.g. the B. diphtheriae. 



3. Gelatin Media. — These are simply the foregoing broths, 

 with gelatin added as a solidifying body. 



3 {a). Peptone Gelatin. — To iioo c.c. of water brought to 

 the boiling-point in a saucepan of known weight, 100 to 150 

 grammes of "gold label" gelatin (preferably that of Coignet 

 et Cie., Paris) are added, the solution being rapidly made by 

 grasping the bunch of leaves by one end in the hand, and 

 stirring it around in the water. As soon as solution is accom- 

 phshed the fluid is no longer permitted to boil, as prolonged 

 boiling tends to destroy the gelatinising power of the medium. 

 The further technique is quite the same as that described for 

 making agar. The medium when tubed may be sterihsed for 

 five minutes in the autoclave at 120° C, without endangering 

 its subsequent gelatinisation, if upon removal from the autoclave 

 the tubes are placed in ice-water until solidified — -a long experi- 

 ence justifies the use of the autoclave under such conditions. 

 Too much boiling, or boiling at too high a temperature, as has 

 been said, causes a gelatin medium to lose its property of solidi- 

 fication. This transparent solid gelatin medium is that chiefly 



