PREPARATION OF MEDIA 63 



to be sterilised, it is not safe to do this in the hot-air 

 steriliser unless the heating and cooling are carried out 

 very slowly, as they are very liable to crack. It is prefer- 

 able, after cleaning and plugging with sterile wool, to 

 steam in the steam steriliser or the autoclave, the heating 

 and cooling being conducted slowly. 



Culture Media 



Culture media are employed (a) for the isolation of 

 micro-organisms from the medium in which they are 

 naturally present, and for their subsequent cultivation in 

 the laboratory, and (6) for the identification of the 

 organisms so isolated and cultivated. 



The preparation of the culture media in common use 

 is described below, but that of certain special media will 

 be mentioned elsewhere as required. The requisite pro- 

 cedures comprise (a) preparation of sterilised vessels, 

 (b) preparation of the media, (c) neutralisation or stan- 

 dardisation of the media, (d) filling into the vessels, and 

 (e) sterilisation. The preparation of sterilised vessels has 

 been described above (p. 62). For ordinary laboratory 

 cultures test-tubes are generally used. Media which are 

 solid at ordinary temperatures, e.g. agar, gelatin, and 

 coagulated serum, are prepared either as deep, upright 

 tubes (Fig. 9 A), for which 8-15 c.c. of the medium are 

 required for a tube, or as sloping tubes (Fig. 9 c), for which 

 4-5 c.c. are required for a tube. Of fluid media 5-15 c.c. 

 are used for a tube. The prepared media are introduced 

 into the test-tubes, etc., through a funnel, care being taken 

 to avoid soiling the mouths or the wool plugs may stick. 

 Fluid media may be run in from a large burette or from a 

 funnel with a piece of rubber tubing, with a spring clip, 

 attached to the stem. The filled tubes are then sterilised 

 in the steam steriliser (p. 51) by steaming for twenty to 

 thirty minutes on two or three successive days, or in the 



