Feb. 15. 1915 Colon Bacilli and Pasteurization 405 



In the above experiments two litmus-milk tubes were inoculated in 

 the usual way with each of the 12 cultures and were heated for 30 minutes 

 at 62.8° C. (145° F.). The first experiment shows the record of the 12 

 cultures which survived in the determination of the thermal death point 

 of the cultures as a whole. When these 12 cultures were again heated, 

 only 4 survived, on the third trial 8, on the fourth trial 6, on the fifth 

 trial 9, and on the sixth trial none survived. 



These results are important since they show that at 62.8° C. (145° F.) 

 colon bacilli may or may not survive a process of pasteurization. It is 

 evident that this is a critical temperature and that occasionally colon 

 bacilli may survive, owing in all probability to the resistance of a few 

 cells in the culture. This explanation is supported by the figures in the 

 summary of Table II, in which is seen the number of times that both 

 the litmus-milk tubes showed growth; also when both tubes were nega- 

 tive and when one tube was positive and one negative. 



It may be seen, also, that the same culture on repeated heating does 

 not give the same results. It is evident that certain strains of colon 

 bacilli have a thermal death point which is close to 62.8° C. (145° F.), and 

 although they represent only a small percentage of the cultures we stud- 

 ied, the fact that such cultures exist complicates the colon test for effi- 

 ciency of pasteurization. 



The apparent scarcity of these resistant colon bacilli and the fact that 

 62.8° C. (145° F.) is near their thermal death point explains our failure 

 to find them in the samples pasteurized by us under laboratory conditions, 

 as stated previously in this paper. 



HEAT RESISTANCE OF COLON BACILLI 



From these results it seems that the colon bacilli as a rule have a low 

 majority thermal death point and the cultures survive the higher tem- 

 peratures only by reason of the resistance of a few cells. 



Gage and Stoughton (3) found this to be true of a few cultures which 

 they studied, and although they tried to breed a race with a high majority 

 thermal death point, their efforts were not successful. We have also 

 tried to breed a resistant type, but thus far without success. 



In our experiments the thermal death point determinations were made 

 in duplicate tubes of litmus milk, and the appearance of growth was 

 recorded after an incubation period of 24, 48, 72, and 96 hours. In every 

 case control tubes not heated showed a marked acid reaction in 24 hours, 

 but in the heated tubes with the same inoculation the growth was often 

 delayed, so that sometimes no reaction was noticed until after 96 hours, 

 incubation. When the reaction was delayed, it showed that a portion of 

 the bacteria in the milk were destroyed by the heating. When the heat- 

 ing has little or no effect, the heated tubes should show a positive reaction 



