72 Microbes and You 



Intermittent Heating (Arnold sterilization). This technic is 

 advocated in the sterihzation of theraio-labile substances ( broken 

 down by heat) which can withstand a temperature of 60 to 65° C, 

 the point where most vegetative protein coagulates. The underly- 

 ing principle of intermittent heating is to destroy the vegetative 

 ( non-spore-containing ) cells present in the material to be sterilized, 

 incubate the product at a temperature which will encourage 

 germination of spores, heat once more to destroy the new crop of 

 vegetative cells, re-incubate, etc., until the third heating has dis- 

 posed of the last germinated spores. This method does have 

 certain advantages, but few microbiological laboratories employ 

 the technic extensively. 



Autoclaving. One of the most efficient and reliable methods of 

 sterilizing liquids is the use of steam under pressure. The instru- 

 ments employed in laboratories for accomplishing this are called 

 AUTOCLAVES, and they resemble home pressure cookers. Water 

 boils at 100° C. at atmospheric pressures of 760 millimeters of 

 mercury, but if we build up the pressure in a sealed container, 

 water can be kept from boiling until the temperature goes well 

 above 100° C. Table 6 reviews the relationship of steam under 

 pressure at sea level to temperatures attained. 



Table 6 

 CORRELATION OF STEAM PRESSURE WITH TEMPERATURE 



TEMPERATURE 

 POUNDS OF STEAM PRESSURE DEGREES C. DEGREES F. 



The usual rule of thumb for operating an autoclave is to subject 

 the liquids being sterilized to 15 pounds of live steam pressure 

 for at least 15 minutes. But as the volume of material to be 

 sterilized increases, sufficient time must be added to allow the 



