20 BULLETIN 342, U. S. DEPARTMENT OF AGRICULTURE 
SURVIVAL OF STREPTOCOCCI 
The general groups of bacteria that survive pasteurization have 
been discussed. A more specific group will now be considered. It 
has been the custom of some authorities to consider the presence of 
streptococci in pasteurized milk to be an indication of ineffectiveness 
in the process. As already pointed out, pathogenic streptococci are 
readily destroyed by proper pasteurization. In a study of the subject 
(5) it was found that certain strains of streptococci are able to survive 
pasteurizing 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., 89 cultures, 
or 64.03 per cent, survived; at 145°, 46, or 33.07 per cent, survived; 
and at 160°, 3 cultures, or 2.16 per cent, survived; and all these w T ere 
killed at 165°. The streptococci from the udder were, on the wmole, 
less resistant and those from milk and cream more resistant to heat 
than those from the mouths and feces of the cows. 
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 temper- 
ature. This ability of a few bacteria to survive may be due to certain 
resistance peculiar to them or it may be due to some protective influ- 
ence in the milk. (2) Streptococci which have a high majority thermal 
death point, and which, when such is the case, survive because this 
point is above the temperature of pasteurization. This ability to 
resist destruction by heating is a permanent characteristic of certain 
strains of streptococci. 
These streptococci which have a high thermal death point above 
the pasteurizing temperature undoubtedly play an important part in 
the occasional high counts found in pasteurized milk. Such counts are 
sometimes observed when the count of the raw milk runs the same as 
usual. As the proportion of these heat-resistant types varies in milk, 
their numbers may at times reach such figures that their survival 
through the pasteurizing process gives an abnormally high-count 
product. The presence and variation of their numbers in milk, 
therefore, are matters which must be considered in connection with 
bacterial standards for pasteurized milk. 
It is evident that certain varieties of streptococci are able to survive 
pasteurization, while others probably are always killed. It has been 
found that the streptococci causing septic sore throat are killed by 
pasteurization at 142° F. for 30 minutes. 
These results, together with the protection which proper pasteuri- 
zation affords against epidemics of that disease due to milk supplies, 
indicate that the varieties of streptococci associated with or responsi- 
ble for septic sore throat are among the varieties which have a low 
thermal death point. 
THE COLON TEST FOR EFFECTIVENESS OF PASTEURIZATION 
In a study (7) of the ability of colon bacilli to survive pasteurization 
it was found that certain strains could survive heating at 145° F. for 
30 minutes. On examining 174 cultures of colon bacilli it was found 
that at 140°, the lowest temperature used, 95 cultures survived; at 
145°, 12 survived. In each case the heating period was 30 minutes. 
