Determining the Thermal Death-Point 257 



A SIMPLE METHOD OF DETERMINING THE 

 THERMAL DEATH-POINT. 



James B, Kendrick and Max W. Gardner, 



Purdue University Agricultural Experiment Station. 



Methods of determining- the thermal death-point of bacteria have 

 been described by NovyS Sternberg', Smith^ and many others. The 

 apparatus ordinarily used is illustrated by Novy and by Smith. The 

 standard procedure consists of a ten-minute exposure of a suspension of 

 the bacteria in a capillary tube or test tube to the temperature desired 

 by immersing the tube in a hot water bath. The methods vary as to 

 the medium and the type of tube used for the suspension, the regulation 

 of the temperature, and the mode of testing the viability of the organ- 

 isms. 



In recent studies of the thermal death-points of certain bacteria 

 causing plant diseases, some modifications of the older methods have been 

 adopted. A heavy suspension of each organism was made in a flask 

 of sterilized distilled water, and equal amounts were then transferred 

 with a sterilized pipette to small sterilized test tubes, one cm. in diam- 

 eter and seven cm. in length, with walls of fairly uniform thickness. 

 With plant pathogenes there seems to be no especial need of using bouil- 

 lon or salt solution for these suspensions. 



For the water bath a five-gallon wooden candy bucket was placed 

 in a sink under a combination steam and cold water faucet from which 

 a piece of hose led down into the bucket (fig. 1). By turning on 

 either the cold water or the steam and stirring with a stick, the tem- 

 perature of the water was easily adjusted and controlled. The tem- 

 perature in such a container is more easily controlled than in the 

 smaller metallic types because of the large volume of water and the 

 relatively low thermal conductivity of the wood. If steam is not availa- 

 ble, hot water can be used to raise the temperature since plant patho- 

 genic bacteria are not spore-formers and have low thermal death-points. 

 A. G. -Johnson and others working at the University of Wisconsin have 

 made use of a somewhat similar container. 



The test tubes containing the organisms were inserted in holes 

 bored in a large, flat, cork float about 15 cm. in diameter so as to project 

 well down into the water. By means of a certified thermometer reading 

 to one-tenth of a degree inserted through a cork into a similar test tube 

 containing water and suspended at the center of the float, the exact 

 temperature of the contents of the tubes could be controlled. The tem- 

 perature of the water bath was adjusted before the cork float with the 

 test tubes was placed on the surface. After the contents of the tubes 

 had reached the temperature of the bath, a matter usually of about one 

 minute, the float was allowed to remain ten minutes and then was 



Novy, Frederick G. Laboratory work in bacteriology. 1-563. 1899. p. 513. 

 ■ Sternberg, George M. A text-book of bacteriology. 1-708. 1901. p. 154. 

 Smith, Erwin F. Bacteria in relation to plant diseases. J:l-285. 1905. p. 75. 



"Proc. 38th Meeting, 1922 (1923)." 

 17—25870 



