40 METHODS OF STUDYING MICROORGANISMS 



exposure at 70 C. or 158 F., varying directly with the 

 density of the medium in which it is. The spore-forming 

 organisms are characterized by a vastly greater 

 resistance. This is due to the peculiar property of 

 spores of resisting deleterious agencies. 



Low temperatures are much less destructive than 

 high ones. The typhoid and diphtheria organisms may 

 resist 200 below zero C. or 300 F., while some of the 

 more delicate organisms quickly die at zero. In sterili- 

 zation that method is chosen which will do the least 

 damage to any object to be conserved. Simple boiling 

 should be undertaken whenever practicable, and immer- 

 sion for five minutes in boiling water will destroy the 

 vegetative forms of all bacteria. For spores, however, 

 at least of the disease-producing kind, two hours is 

 necessary. It is advisable to add 1 per cent, of sodium 

 carbonate to the water. This assists in killing of spores 

 and metal objects are not so apt to rust. This simple 

 boiling for ten minutes is sufficient for dry cleaned 

 syringes and surgical instruments in the absence of 

 infective material known to contain spores. Steriliza- 

 tion in live steam is the most practical method of 

 killing bacteria, as it can be carried out in the kitchen. 

 In the laboratory it is done by the Arnold sterilizer 

 (Fig. 18). It is the custom to employ what is called 

 fractional sterilization. This method is the exposure 

 of the material to be disinfected to the temperature of 

 100 C. or 212 F., which is the temperature reached by 

 the steam in the inner chamber, for fifteen minutes on 

 three successive days. On the first occasion vegetative 

 forms are killed and the spores remaining are permitted 

 to pass into the vegetative state over night. On the 

 second occasion these will then be killed. A third ex- 



