270 APPENDIX 



GELATIN 



No gelatin media should be employed having a melting 

 point below 25 C. The percentage of gelatin added may 

 be increased to bring the melting point up to the desired 

 figure. With most gelatin on the market n per cent 

 seems to be preferable to the standard of 10 per cent, 

 provided the gelatin is weighed out without correcting for 

 contained moisture, as appears to have been the custom. 

 Ten per cent, or even 20 per cent of moisture commonly 

 occurs in laboratory gelatin, and unless this is taken into 

 account in weighing, the stiffness of the media is materially 

 affected and the bacterial results obtained considerably 

 modified. All gelatin should be tested for moisture before 

 using by drying a sample for half an hour at 105 C. The 

 stock should be kept under uniform conditions in tight 

 containers, so that the percentage of water present may 

 then be properly accounted for and the weight on a dry 

 basis be used in making up the medium. 



AGAR 



For bacterial counts 10 gm. of agar per liter should be 

 used. The smaller amount seems to be sufficient to carry 

 the added water and the medium is less stiff. This appears 

 to give higher and more consistent counts. Fifteen grams 

 may be employed for keeping cultures. 



North medium is especially valuable for keeping cultures, 

 particularly cocci. This is composed as follows: 



500 c.c. Extract of 500 gms. of chopped beef. 

 500 c.c. Distilled Water. 

 10 gm. Agar. 

 20 gm. Gelatin. 

 20 gm. Peptone. 

 5 gm. Sodium Chloride. 

 Reaction neutral. 



It is well to determine the reaction of the media after 

 sterilization, as during this process the reaction often changes 



