374 PUBLIC HEALTH BACTERIOLOGY 



lactis aerogenes, and (2) Streptococcus lacticus (Kruse), 

 identical with the B . a cidi lactici (Leichmann) . Heinemann, 

 who has investigated the subject, states that the two 

 species are ordinarily present in naturally souring milk, 

 the former in abundance at the beginning, the latter 

 in the later stages when the acidity has reached a high 

 degree. The secret of the regularity of the presence of 

 these two species is their power of withstanding a much 

 higher degree of acidity than the other species present at 

 the first. In changing to lactic acid, the lactose is believed 

 to be first hydrolyzed into glucose and galactose. 



CifH.iOu + H 2 = C 6 H 12 6 + C 6 H 12 6 = 4C 3 H 6 3 



Coagulation of casein follows on acidification of the 

 milk, the amount of acid necessary to precipitate the 

 casein averaging 0-45 per cent ; the terminal amount may 

 reach 0-85 per cent. The casein precipitated by lactic 

 acid formation is never redissolved, because the high 

 acidity inhibits the proteolytic ferments. 



Casein precipitation, however, may also be due to a non- 

 acid coagulation caused by bacterial ferments. Casein 

 precipitated in this way may be redissolved by a bacterial 

 trypsin or casease, produced by the same or other bacteria, 

 and the milk hence may become entirely liquid, transparent, 

 and of a yellowish colour. 



B. bulgaricus in milk culture produces 2-5 per cent of 

 lactic acid, and 0-05 per cent of acetic and succinic acids, 

 is non-pathogenic, and exerts no putrefactive action upon 

 proteids. Metchnikoff suggested its use in milk cultures 

 as a food to inhibit, by its acid production, the growth in 

 the intestine of the class of bacteria which break up 

 proteids, the bacteria of putrefaction. This is Metchnikoff 's 

 bacteriotherapy, which has been extensively practised. 



B. bulgaricus is a large, non-motile, non-sporing, Gram- 

 positive bacillus, with square ends (like B. anthracis). It 

 forms short and long chains. It shows little or no growth 

 on ordinary media or below 37 C. Optimum temperature : 

 42 C. Grows in dextrose-peptone broth, to which calcium 

 carbonate has been added. 



Butyric Acid fermentation of milk occurs occasionally 

 in milk from the growth of anaerobic bacteria. It is a 



