472 THE LACTIC-ACID BACTERIA 



plants are found. It has already been stated that numerically the 

 most important source of bacteria in milk is cow's dung, in which 

 enormous numbers of bacilli of the colon-aerogenes group are found. 

 These are lactic-acid formers. Such bacteria, however, may also 

 enter the milk from other sources. It was pointed out in the chapter 

 on the Bacilli of the Colon Group that a number of investigators look 

 upon the Bacillus coli communis as a ubiquitous organism. If this 

 is the case its presence in milk cannot be taken as an absolute indi- 

 cation of fecal contamination. Rodgers and Ayers, in Circular 135 of 

 the Bureau of Animal Industry, state that in some middle Western 

 States organisms of the colon aerogenes group are commonly found 

 on grass, grain, and in slough holes, ,and that, therefore, these lactic- 

 acid and gas formers may not be derived from a fecal source when 

 cows are milked in open fields. The discovery that the lactic-acid 

 fermentation of milk is due to bacteria was first made by Pasteur and 

 later confirmed by Lister. Hiippe gave the first description of a 

 bacterium of this type and named it Bacillus acidi lactici, and orig- 

 inally believed that the latter was the only microbe which caused 

 lactic-acid fermentation of milk. It was, however, soon shown by a 

 number of investigators (Grotenfeld, Marpmann, Conn, Weigmann, 

 and others) that a great variety of bacteria occur in milk which possess 

 the power to ferment lactose. Some of them produce a dextrogyr, 

 others a sinistrogyr lactic acid. The former type was found first. 

 To the second class belongs Leichmann's Bacillus lactis acidi and 

 Micrococcus acidi levolactici and Kozai's Bacillus acidi levolactici. 



Classification. The lactic-acid bacteria are now, according to 

 Loehnis, as quoted by Weigmann, divided into four groups, namely: 



Group I. Plump, Gram-negative, gas-forming rods designated as 

 Bacterium pneumonia? of Friedlander or Bacterium acidi lactici of 

 Hiippe. 



Group II. Elongated, oval, or lancet-shaped, Gram-positive strep- 

 tococci, growing anaerobically and forming very little gas designated 

 as Streptococcus pyogenes or, better, as Streptococcus Guentheri. 

 This group contains the most important lactic-acid formers occurring 

 in milk. 



Group III. Long, slender, Gram-positive bacilli, growing better 

 anaerobically and forming little gas, designated as Bacterium 

 caucasicum or Bacterium casei. 



Group IV. Gram-positive staphylococci, aerobic, forming no gas, 

 generally liquefying gelatin and designated as Micrococcus pyogenes 

 or Micrococcus lactis acidi. 



Bacteria of Group I. The bacteria of the first group are generally 

 short rods, 1 to 1^ micra long and 0.75 to 1 micron thick. They 

 occur singly or in groups of two, and also in the form of longer chains 

 They are not motile and do not form spores, generally they are 

 Gram negative. They are aerobic and facultative anaerobic. As a 

 rule, they coagulate milk in from one to two days, sometimes later, 



