10 GENERAL MICROBIOLOGY 



The physical nature of the medium, the extraordinary 

 resistance of the spores of certain species of bacteria or 

 both in combination, may require that this intermittent 

 heating be carried on over a longer period of time, i.e., 

 four, five, six, etc., days in succession for the same or a 

 longer period each time, or that the period between inter- 

 mittent heatings be lengthened from twenty-four hours 

 to forty-eight hours. 



Tyndall's method is valuable in that media of delicate 

 composition may be sterilized without producing undesirable 

 changes, such as are often produced by the high tempera- 

 ture of the autoclav. 



3. Sterilization in Flowing Steam at 100 C. Continuous 

 or Discontinuous, (a) Continuous Heating. Simple boiling 

 or exposure to steam at 100 C., even though the exposure 

 be prolonged, is not a reliable method of sterilization. When 

 microorganisms have been dried, their resistance to the 

 effects of heat is much enhanced, and especially is this the 

 case when they are mixed with substances of a colloidal 

 nature. Certain resistant forms of protoplasm known as 

 spores may not be destroyed by one heating to 100 C., 

 even when the temperature has been maintained for sev- 

 eral minutes. 



(6) Discontinuous Heating. General use for the ster- 

 ilization of media. 



This principle of sterilization advanced by Tyndall 

 finds its widest application in bacteriological work with 

 the use of flowing steam. High-pressure steam may be 

 utilized to good advantage if a central heating station is 

 available. The Arnold sterilizer makes use of steam for 

 the sterilization process and lends itself readily to both the 

 continuous and discontinuous method. 



4. Sterilization by Superheated Steam (under pressure 

 and therefore above 100 C.). Water, syringes, surgical 

 dressings, bedding, india-rubber apparatus, filters, old 

 cultivations, culture media, etc., not injured by high tern- 



