30 LIQUID MEDIA 



Red rubber caps are the best ; they should be put into a wide- mouthed flask 

 or bottle, which is then plugged with wool and autoclaved and afterwards put in 

 the incubator to dry the plug. A cap can then be taken out with a pair of sterile 

 forceps whenever one is wanted. 



In the case of stock cultures, which are to be put away for some time, 

 evaporation maybe prevented by pouring a little melted paraffin [or sealing- 

 wax] over the top of the plug. 



[This latter method of sealing tubes or bottles is also of great use in the 

 cultivation of slow-growing organisms such as the tubercle bacillus. After 

 sowing the medium, the top of the plug is carefully sealed with melted 

 paraffin (or sealing-wax), and the culture can then be incubated as long as 

 is necessary without fear of the medium drying up, if the sealing has been 

 efficiently done.] 



Method recommended. [When paraffin is used, gently warm the upper 

 J cm. of the tube by turning it round in the flame, and then with a pipette 

 or ladle pour a few drops of melted paraffin, kept liquid in a water bath, on 

 to the warm wool and let it soak in to a depth of | cm. or so. To unseal 

 the plug, gently warm the upper part of the tube again, stick a needle or 

 pair of forceps into the plug and turn it round at the same time raising it. 



[Another simple method of preventing evaporation during cultivation in test- 

 tubes is to place them in a large wide-mouthed ground-glass stoppered jar, which 

 has been previously thoroughly washed out with a saturated solution of perchloride 

 of mercury. Place two or three folds of filter paper moistened with perchloride 

 solution at the bottom of the bottle, and after arranging the tubes put a trace of 

 vaseline on the stopper and close the bottle, turning the stopper round to obliterate 

 any air channels. Volatile antiseptics (e.g. formalin) are obviously unsuitable for 

 this purpose.] 



SECTION I. LIQUID MEDIA. 1 

 1. Media made from animal tissues and fluids. 



There is a great variety of these media. A description of those in most 



general use only will be given here ; and the most frequently used of all, 



peptone beef broth, will be taken as a type and the technique of its 

 preparation given in fullest detail. 



Peptone beef broth. 



This medium is in everyday use. It will be referred to in future simply 

 as broth. 



Preparation. 1. Take 500 grams of lean beef. Cut away all fat, tendon 

 and aponeurosis. Mince it, and leave it to macerate in a litre of cold water 

 for 6 to 12 hours. 



2. Heat gently to boiling in an enamelled saucepan, stirring constantly, 

 and keep the mixture boiling for 10 minutes. 



3. Pour on to a thick clean cloth, express as far as possible all the fluid 

 out of the meat, and while still warm filter the fluid through a thick filter 

 paper (Chardin or Prat-Dumas) moistened with water to keep back the fat. 



4. Pour the filtered broth into an enamelled saucepan, and add 



Dry peptone (Chapoteaut) - - 10 grams, or 1 per cent, of the 



volume of water used. 



Salt, - 5 grams, or 0*5 per cent. 



Sodium phosphate, - - about 1 gram. 



Boil again, stirring meanwhile to dissolve the peptone 



1 The present chapter will be limited to a description of the culture media of general 

 application. Media applicable only to particular organisms will be dealt with when the 

 latter are under consideration. 



