BIOCHEMICAL TESTS 99 



Thermal Death-point. The accurate determination of the exact- 

 temperature that is necessary to destroy various species of bacteria 

 is frequently of great economic importance. Efficient steriliza- 

 tion and pasteurization can be accomplished only when these facts 

 are known. Many methods have been suggested. In the labora- 

 tory the determination is frequently made by subjecting freshly 

 inoculated tubes of broth to different temperatures in a water-bath 

 for ten minutes each. For reliable results more accurate methods 

 are needed. One of the commonest and best is the use of the 

 Sternberg bulb. This is blown of thin glass. A definite amount of 

 culture is introduced and the neck sealed in the flame. The bulbs 

 are completely immersed in a water-bath and suspended by wires 

 or by some other method, so that they do not come in contact 

 with the walls of the bath, and heated. A number of bulbs are 

 prepared and one heated five minutes, another ten minutes, at 50. 

 The temperature is raised two degrees and two more bulbs are ex- 

 posed. For sporeless bacteria the test should be made to 70, 

 and still higher for those that produce spores. The bulbs are cooled 

 quickly after their exposure, and their contents mixed with agar in 

 a Petri dish, or added to a tube of other suitable medium. This 

 is then incubated for several days. The minimum temperature 

 required to destroy the bacteria can readily be determined by a 

 comparison of the tubes. 



Efficiency of Disinfectants. The efficiency of disinfectants 

 is determined by testing their action on pure cultures of bacteria. 

 Koch's method, which has been commonly used, consists in drying 

 the organism on silk threads, immersing them for varying lengths 

 of time in the disinfectant to be tested, washing in sterile water, 

 and placing them upon the surface of agar. 



Hill's method is a modification of that of Koch, and is rather 

 more accurate and relatively simple. Sterilized glass rods are 

 coated at the tip with the bacteria to be tested by dipping them 

 into a broth culture to a depth of an inch. These are placed in 

 test-tubes and carefully dried in a thermostat. They may then be 

 immersed to a somewhat greater depth in the disinfectant to be 

 tested for definite periods of time, rinsed carefully in sterile water, 

 and placed in tubes containing broth. 



