56 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
lactic acid than the coli, but the latter are more active in 
proteid decomposition. Carbon dioxide, hydrogen, car- 
buretted hydrogen, and nitrogen are the gases produced, 
while the acids formed are lactic, acetic, and succinic. In 
the early stages of this change, the milk has a sweetish- 
sour refreshing taste and an odor that is not unpleasant, 
especially when the aerogenes bacteria are operating, but 
later the taste is unclean, while the odor is stable-like, and 
finally the taste becomes nauseating and salty and the 
odor is like that of decomposing manure and urine. Milk 
undergoing this form of fermentation and decomposition 
may prove harmful to persons drinking it, especially in- 
fants and adults with weak digestion. 
The principal representatives of this group of bac- 
teria are the Bacillus coli and the Bacillus aerogenes, also 
called Bacillus lactis aerogenes and Bacterium acidi lac- 
tict (Hueppe). The Bacillus coli is a short, thick, oval 
organism, which is motile, and which forms on solid media 
colonies which are usually flat, leaf-shaped and partially 
translucent, sometimes moist and globular. Some vari- 
eties render the milk alkaline and do not curdle it nor 
produce any other visible change; others peptonize the 
casein. Several varieties of coli are pathogenic, e.g., the 
bacilli of calf cholera, the Bacillus enteritidis (Gartner) 
and the Bacillus phlegmasia iiberis, which is one of the 
causes of parenchymatous mastitis according to Kitt. 
The Bacillus lactis aerogenes or Bacterium acidi lactict 
(Hueppe), described previously in connection with the 
acid-forming bacteria, may be regarded as a type of the 
aerogenes bacteria, of which there are a number of 
varieties. 
The optimum temperature of the coli-aerogenes bac- 
teria is 87° C. (98.6° F.), but they grow quite well at 
