32 



METHODS OF CULTIVATION OF BACTERIA. 



adjusting the safety valve so as to blow off at the correspond- 

 ing pressure. One exposure of media (when in small bulk) to 

 the latter temperature for seven minutes is sufficient to kill all. 

 organisms or spores, but if the bulk is great, then it is advisable 

 to prolong the exposure to fifteen minutes. Here, again, care 

 must be taken when gelatin is to be steriHsed. It must not be 

 exposed to a temperature above 105° C, and must be sterilised 

 by the intermittent method.^ Certain precautions are necessary 

 in using the autoclave. In all cases it is necessary to allow 

 the apparatus to cool well below 100° C. before opening it or 

 allowing steam to blow off, otherwise there will be a sudden de- 

 velopment of steam when the pressure is removed, and fluid 

 media will be blown out of the flasks. Sometimes the instru- 

 ment is not fitted with a thermometer. In this case care must 

 be taken ,to expel all the air initially present, otherwise a mix- 

 ture of air and steam being present, the pressure read off the 

 gauge cannot be accepted as an indication of the temperature. 

 Further, care must be taken to ensure 

 the presence of a residuum of water 

 when steam is fully up, otherwise the 

 steam is superheated, and the pressure 

 on the gauge again does not indicate 

 the temperature correctly. 



B. (4) Sterilisation at Low Tempera- 

 tures. — Most organisms in a non-spored 

 form are killed by a prolonged exposure 

 to a temperature of 57° C. This fact has 

 been taken advantage of for the sterili- 

 sation of blood serum, which will coagu- 

 late if exposed to a temperature above 

 that point. Such a medium is sterilised 

 on Tyndall's principle by exposing it for 

 an hour at 57° C. for eight consecutive 

 days, it being allowed to cool in the in- 

 terval to the room temperature. The apparatus shown in Fig. 6 

 is a small hot-water jacket heated by a Bunsen placed beneath 

 it, the temperature being controlled by a gas regulator. To 



Fig. 6.- 



- Steriliser for blood 

 serum. 



1 This medium will retain its gelatinising power perfectly well when sterilised for 

 five minutes in the autoclave, if, on removal therefrom, it be at once placed in the ice- 

 chest to cool. 



