32 DAIRY BACTERIOLOGY 



produced 1,000 tons annually. Polarimetric observation reveals 

 three forms of lactic acid, dextro and Isevorotatory, and inactive. 

 The last- mentioned is a mixture of the two optically active acids 

 in equal proportions, and differs from these in the solubility of its 

 salts as well as in its optical properties 1 . In order to identify a 

 lactic acid bacterium, it is necessary to know what form of lactic 

 acid it produces under certain conditions. 



In 1857, Pasteur discovered that the lactic acid fermentation 

 was due to the action of certain bacteria 2 , and soon after the 

 discovery of the method of making gelatine plate cultures, Hueppe, 

 in 1884, succeeded in isolating one of these bacteria 3 . This 

 organism, which was named Bacillus acidi lactici, is not one of the 

 true lactic acid bacteria, but belongs to the aerogenes group, which 

 will be dealt with later. The organism which plays the principal 

 part in the self -souring of milk had already been correctly described 

 by Lister, in 1878, as an oval diplococcus, and given the name 

 Bacterium lactis 4 . A similar form was isolated by Grotenfeld in 

 1879, and named Streptococcus acidi lactici 5 . The importance of 

 this organism was, however, only recognised by ,eichmann in 

 1894 6 , and by Gunther and Thierfelder 7 in 1895, who demonstrated 

 that it produced dextro lactic acid in milk. Leichmann named it 

 very appropriately the bacterium of sour milk, Bacterium lactis 

 acidi. The large number of lactic acid bacteria now known really 

 only resemble one another in not forming spores, and therefore 

 being comparatively easily destroyed by heat. 



The True Lactic Acid Bacteria. These bacteria 

 ferment carbohydrates and higher alcohols to lactic acid ; they 

 only grow in presence of proteins or complexes of amino acids, and 

 not in presence of ammonium salts or single amino acids. They are 

 Gram-positive, non-motile, non-sporing rod or sphere forms. It 

 will only be possible here to mention the more important character- 

 istics by which nearly related forms are best distinguished. The 

 following is a brief summary of the results of the author's recent 

 researches on this subject 8 : 



1 The lactic acids are best identified by conversion into zinc salts. The 

 active zinc lactates rotate the plane of polarised light in senses opposite to 

 those due to the free acids, and they crystallise with two molecules of water, 

 corresponding to 12-9 per cent. H 2 0, which is not driven off below 140 C. 

 The inactive^zinc lactate is much less soluble, and crystallises with three 

 molecules of water (18-2 per cent. H Z 0), which is more readily driven off 

 on heating. 



' Comptes rendues," tome 45, p. 913. 



8 



rfc. 



Mitt. Kais. Ges.-Amt.," Bd. II., p. 309. 



Trans. Path. Soc. of Condon," vol. 29, p. 425. 



Fortschr. Medizin," Bd. VII., p. 121. 



Milchzeitung," Bd. XXIII., p. 523. 



Archiv. fur Hyg.," Bd. XXV., p. 164. 



The Lactic Acid Bacteria." Monograph published in English by the 



Danish Academy of Sciences, Copenhagen, 1919. 



