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- 

 tici (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 uberis, which is one of the 

 causes of parenchymatous mastitis according to Kitt. 

 The Bacillus lactis aerogenes or Bacterium acidi lactici 

 (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 37 C. (98.6 F.), but they grow quite well at 



