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4 



and we have also studied the most important of the lactic acid bacteria occurring in soured 

 beet slices, diffusion slices, potatoes and mash, as also in sour cabbage and sour dough. 

 Some experiments made by the State Committee for Plant Culture, with souring of beets 

 and potatoes, came in very conveniently for us in this respect. 



The strains of bacteria which we were able to isolate from these various sources were 

 further supplemented by related forms isolated by colleagues at home and abroad. For 

 a number of pathogenic strains, for instance, we are indebted to the colleagues mentioned 

 in the tables, and we hereby beg to express our thanks to the gentlemen in question for 

 their kindness in providing us with the cultures. 



Only 330 of the strains investigated will be mentioned in the work, as in the case 

 of strains of the same origin, and which in the course of the investigation proved entirely 

 identical, only one of them was included. On the other hand, we have included many 

 apparently identical strains of different origin, which have been constantly subjected to 

 parallel investigation in order to ascertain whether possible variations tended in the 

 same direction or not. 



The lactic acid bacteria of the dairy industry are, however, those which are most 

 numerously represented and most thoroughly dealt with, inter alia because we have had 

 them for observation from the very commencement of the work, whereas the lactic acid 

 bacteria of other origin only came in gradually, and have therefore in many cases only 

 been thoroughly tested a few times, which is not sufficient to determine the constancy 

 of their qualities. It is obvious that only the most constant qualities can be- used as species 

 character, and in investigations of this kind, therefore, all the qualities in each individual 

 strain must be tested again and again throughout a long period of years. The present 

 work has also, for this reason, taken up fully ten years of my time; partly also that of 

 my assistants, and I take this opportunity of thanking them — Miss Betzy Meyer 

 and my wife, Mrs. Orla -.Jensen — ■ for the numerous chemical analyses, microscopical 

 investigations and photographs to which they have devoted so much skill and care^). 



We shall commence by describing the methods employed in cultivation and investi- 

 gation, passing on then to description of the isolated species and their systematics, in con- 

 nection with which, a key will be furnished for determination of the lactic acid bacteria, 

 as also a survey showing where they are found. And finally, mention will be made of the 

 experiments we have carried out with a view to utilising the isolated species. 



For practical reasons, such of our microphotographs as it was found advisable to 

 reproduce have been collected in a special album, and I beg to offer my best thanks to 

 the Carlsberg Fund for having rendered it possible to publish the same. 1 also beg the 

 •Julius Skrike's Foundation to accept my best tlianks for having defrayed the 

 expenses of the English translation. 



September 1918. Orla-.Iensen. 



') I al-so beg to thank Mr. J. Lino, who has assisted me for a year, and Mr. Tholstbi p-Pederskn. 

 who in spite of liaving worked only a few months in the laboratory here has nevcrtlieless contributed 

 valuable assistance to the present work. The custodian of the laboratory, also, Mr. V. Jacohsen, I have 

 to thank for the help he afforded us. as an experienced dairjman, in the cheese-making experiments 

 mentioned at the close of the work. 



