402 Journal of Agricultural Research voi.iu.no. s 



quired 30 minutes heating at 7o°-72° C. (158° to 161.6° F.), in order to 

 destroy them. Zelenski (9) also found strains of B. co/i which resisted high 

 temperatures. 



In view of this uncertainty regarding the thermal death point of B. 

 colt and its relation to pasteurizing temperatures, the following experi- 

 mental work concerning the ability of colon bacilli to survive pasteuriza- 

 tion was undertaken. The number of cultures used in the experiments 

 was 174. 



METHOD OF DETERMINING THE THERMAL DEATH POINT 



The colon cultures were grown first in plain, neutral extract broth for 

 18 hours and then inoculated by means of a small-bore pipette into litmus- 

 milk tubes. Four drops constituted an inoculation in each milk tube. 

 In making the inoculations care was taken not to have any of the culture 

 touch or any of the inoculated milk wash upon the sides of the tube, either 

 during the handling or during the subsequent heating. 



The inoculated milk tubes, with the exception of the control tubes, 

 were heated in a water bath in which the water was agitated, and the tem- 

 perature of the milk was recorded in a control tube by a thermometer 

 placed in the milk. The temperature in the tubes was not allowed to 

 vary more than half a degree in either direction. In all experiments the 

 heating period was 30 minutes at a given temperature. After the heat- 

 ing, the tubes of milk were quickly cooled to about 10° C. (50° F.), incu- 

 bated at 37° C. (98.6° F.), and the reactions recorded after 24, 48, 72, and 

 96 hours. Growth in the tube indicated that the organism was not 

 destroyed at the particular temperature to which the milk had been sub- 

 jected. In every case the tubes were run in duplicate, and in general 

 both tubes had to show growth before the test was considered positive. 

 The only exceptions to this were cases in which only one of the tubes 

 showed growth after the highest heating temperature; in such cases one 

 tube was considered a positive reaction and the organism was recorded 

 as surviving the process. 



This method of determining the thermal death point was used, in order 

 to render the conditions of heating similar to pasteurization. 



THE THERMAL DEATH POINT OF THE CULTURES AS A WHOLE 



Studies were made of the thermal death point of 174 cultures of colon 

 bacilli isolated from the following sources: 154 from cow feces, 16 from 

 milk and cream, 2 from flies, 1 from human feces, and i from cheese. (The 

 cultures were supplied through the courtesy of Mr. L. A. Rogers, in 

 charge of the research laboratory of the Dairy Division.) All of the 

 organisms would be classified as colon bacilli according to the usual 

 cultural tests for Bacillus coli. These cultures, with the exception of 

 two not studied and' three noted below, were typical colon bacilli of the 



