EFFECT OF TEMPERATURE ON BACTERIA. 37 
4. ‘The development of the ordinary lactic species, Bact. 
lactis acidz, in practically all cases checks the growth of other 
species of bacteria, and finally kills them, since the bacteria 
regularly decrease in actual numbers after the lactic bacteria 
have become very abundant. The development of lactic bac- 
teria thus serves as a protection both to the milk and the per- 
son drinking it, since it prevents the growth of other bacteria. 
This effect is probably due to the production of lactic acid, and 
is not noticeable in milk only a few hours old. The presence 
of large numbers of lactic bacteria in milk is not an indication 
that the milk is unwholesome. 
5. In practically all samples of milk kept at 20°C. the mul- 
tiplication of the Bact. lactis acidi begins quickly, and pro- 
gresses with great rapidity. They grow so rapidly that they 
produce acid enough to curdle the milk in about 4o hours, the 
growth of other species being held in check. Milk, when 
curdled at this temperature, shows a smooth acid curd, with 
no gas bubbles, such as a butter maker or a cheese maker likes 
to see, and contains commonly over 90 per cent. of Bact. lactis 
acidi, ‘This is the temperature favorable to ordinary dairy 
processes. 
6. <A totally different result appears in milk kept at 37°C. 
The results are somewhat more variable than at 20°. Occa- 
sionally the Bact. lactis acidi grows vigorously at this tempera- 
ture, but the common effect is that a different type of lactic 
bacteria develops. The type favored by this temperature is 
the species B. lactis aerogenes. It isa gas producer, and pro- 
duces a different type of acid from that produced by B&B. aczdz 
lipo eiozaue Zeit... Hyg.; XXXVIIE oor, and: Utz. 
Cena ebact 11. 3x) p. 600, 1904). ~It forms a curd full of 
gas bubbles, and is an enemy of the butter and cheese maker. 
If B. coli communis is in the milk, this also grows luxuriantly 
at 37°. The souring and spoiling of milk occur very rapidly 
at this temperature. 
These comparisons are in harmony with those of Utz (loc. 
cit.), and also of Bockhout and de Vries (Cent. f. Bact. II. 
XII., p. 89, 1904), although obtained by different methods. 
