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. a 
Tn 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 
