GENERAL PRINCIPLES 155 



clear j and if necessary neutralize again. Add sterile 

 litmus until a violet hue is produced. Tube, and sterilize. 

 A good medium for observing change of reaction. 



STERILIZATION AND DISINFECTION. 



1. Dry heat : 



(a) Bright red heat of flame : for platinum needles. 



(b) Dull red heat of flame : for knives, glass rods, 



etc. 



(c) Hot air : 170 C. for i hour : for glass-ware and 



cotton-wool. 



2. Moist heat : 



(a) Boiling in water at 100 C. : for 5 minutes kills 



all non-sporing forms: for i hours kills 

 spores also. 



(b) Steam at 100 C. : Koch's steam sterilizer : i 



hours' full steaming, or 15 minutes' full 

 steaming on three successive days : used for 

 all media. 



(c) High-pressure steam : in autoclave: at 115 C., 



2 minutes for germs, 15 minutes for spores. 

 Never used for gelatin media, or will not re- 

 solidify. Never used for carbohydrate 

 media, as decomposed into other sugars. 



3. Chemicals: 5 per cent carbolic, o-i per cent per- 



chloride of mercury, etc. Allow to remain in con- 

 tact for half-an-hour. 



Discontinuous sterilization at 57-75 C. is used for 

 media, like blood serum, that are changed at higher tem- 

 peratures. The object is submitted to 60 minutes' heating, 

 and kept at 20 to 37 C. until next day, when the heating 

 is repeated, and the same procedure repeated for 3 to 8 

 days. This is to cause spores present to assume the 

 vegetative form and then to kill the same on re-heating. 



CULTURAL METHODS. 



1. Inoculation of liquid media, solid media, living media. 



2. Isolation of pure cultures by (a) serial inoculation ; 

 (6) plate cultivation (serial dilution) ; (c) differential 



