12 BULLETIN 342, U. S. DEPARTMENT OF AGRICULTURE. 



found that certain strains of steptococci are able to survive pas- 

 teurizing temperatures. 



The thermal death points of 139 cultures of streptococci isolated 

 from cow feces, from the udder and mouth, and from milk and 

 cream, showed a wide variation when the milk was heated for 30 

 minutes under conditions similar to pasteurization. At 140° F., the 

 lowest pasteurizing temperature, 89 cultures, or 64.03 per cent, sur- 

 vived; at 145° F., the usual temperature for pasteurizing, 46, or 33.07 

 per cent, survived; and at 160° F., 2.58 per cent survived; all these 

 were destroyed at 165° F. The streptococci from the udder were, 

 on the whole, less resistant, and those from milk and cream more 

 resistant to heat than those from the mouth and feces of the cow. 



Two classes of streptococci seem to survive pasteurization: (1) 

 Streptococci which have a low majority thermal death point: (the 

 temperature at which a majority of the bacteria are killed), but 

 among which a few cells are able to survive the pasteurizing tempera- 

 ture. This ability of a few bacteria may be owing to certain resist- 

 ant characteristics peculiar to them or may be caused by some pro- 

 tective influence in the milk. (2) Streptococci which have a high 

 majority thermal death point, and which, when such is the case, sur- 

 vive because this point is above the temperature of pasteurization. 

 This ability to resist destruction by heating is a permanent charac- 

 teristic of certain strains of streptococci. 



It is evident that certain varieties of streptococci are able to survive 

 pasteurization, while others are probably always destroyed. Numer- 

 ous investigators have studied the thermal death point of streptococci 

 isolated from patients having septic sore throat and have found that 

 the organism was destroyed by pasteurization at 145° for 20 minutes. 

 These results, together with the protection which proper pasteuriza- 

 tion seems to afford against epidemics of that disease caused by milk 

 supplies, indicate that the varieties of streptococci associated with or 

 responsible for the disease are among the varieties which have a low 

 thermal death point. 



In a similar study (11) of the ability of colon bacilli to survive 

 pasteurization it was found that certain strains could survive pas- 

 teurization at 145° F. for 30 minutes. On examining 174 cultures of 

 colon bacilli it was found that at 140° F., the lowest pasteurizing tem- 

 perature, 95 cultures survived; at 145° F., the usual temperature for 

 pasteurization, 12 survived. In each case the heating period was 30 

 minutes. Considerable variation was observed in the thermal death 

 point of the colon bacilli which survived at 145° F. When the cul- 

 tures which withstood the first heating were again heated it was 

 found that many did not survive, and in each subsequent heating 

 different results were obtained. Colon bacilli have a low majority 



