66 



BACTERIOLOGY 



The temperature employed varies according to the effect desired. 

 A temperature of 60° C, maintained throughout a water liquid 

 for twenty minutes will kill most vegetative bacteria, and practi- 

 cally all pathogenic bacteria which do not form spores. Such 

 partial steriUzation is called Pasteurization. Boiling water, 100" 

 C, kills vegetative bacteria in a very short time, less than two min- 

 utes for most bacteria, and the spores of many species are destroyed 



by boiling for 5 to 30 minutes. 

 Some spores, however, for example 

 those of some varieties of B. vul- 

 gatus, may survive a boiHng tem- 

 perature for several hours. Boil- 

 ing is one of the most useful 

 practical methods of disinfection. 

 Nearly all pathogenic bacteria 

 are quickly killed in boiling 

 water. Surgical instruments are 

 commonly boiled in water to which 

 sodium carbonate, i to 2 per cent, 

 has been added. Rusting and 

 corrosion may also be prevented 

 by adding 10 per cent of borax 

 to the water in which metal in- 

 struments are boiled. Steriliza- 

 tion of bacteriological media is 

 usually done by means of stream- 

 ing steam, rather than by immer- 

 sion iri boiling water. The Koch steam sterilizer is a compara-, 

 tively simple device for this kind of sterilization. It is a tall, 

 cylindrical, tin vessel covered with asbestos or felt. The lower 

 portion is fiUed with water; on the side is a water-gauge indicating 

 the height of the water, in order that one may observe when there 

 is danger of the sterilizer boiling dry. Over the top there is a 

 tight-fitting cover. The steam is generated by a Bunsen burner 

 standing underneath. A perforated shelf placed some distance 



Pig. 29. — Koch's steam sterilizer. 



