EFFECT OF TEMPERATURE ON BACTERIA. 53 
Milk kept at 20°.—1. Here the growth of bacteria was due 
largely to the lactic organisms, Groups I. and II. reaching in 36 
hours a number of 500,000,000 per cubic centimeter, compris- 
ing 59 per cent. of the whole. ‘This sample also showed a 
larger number of Group III. than usual, 124,000,000 per cubic 
centimeter, comprising 16.4 per cent. 
2. ‘The neutral bacterial group IV. continued to increase in 
numbers to the end, and in this respect this sample differed 
from the other two, in which this group disappeared in the 
later stages. 
3. It will be noticed that the development of the lactic 
organisms, as before, has retarded the development of the 
liquefiers, which rapidly diminished in percentage although 
slightly increasing in numbers. 
Milk kept at 37°.—1. Here the results were especially 
striking, inasmuch as at the time of curdling, 18 hours, the 
Group III., Bacterium aerogenes, comprised 94 per cent. of all 
bacteria; the only other species present at this time being a few 
of the liquefiers. In this sample of milk, therefore the exces- 
sive development of Group III. caused the disappearance of all 
others except a few liquefiers. 
2. Plate 2 shows in a very striking manner the surprising 
effect of temperature in delaying the growth of bacteria. 
While at 37° the rapid development began at about the 8—1oth 
hour, and at 20° at about the 2oth hour, at 10° this growth 
was held back until the 72nd hour, and even then it was only 
the neutral bacteria that developed, the lactic species being 
held back until the 120th hour. ‘The plate also shows that the 
neutral group and the liquefiers begin to grow first, and in- 
crease regularly until the lactic bacteria begin to be abundant, 
when their further increase is practically stopped. If this plate 
is compared with Plate 1, it will be seen that in Plate 1 
where the lactic bacteria did not develop (at 10°), the other 
species continued to increase in numbers even to the time of 
curdling. 
