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commercial production of yeast. Secondly the gradual introduction of 

 scientific principles into the dairy industry led to an increased interest 

 in the bacteriological processes which are the basis of butter and 

 cheese manufacture. 



Moreover, at the outset of BEIJERINCK'S bacteriological career, 

 various sour milk preparations, like kefir and yoghurt, were intro- 

 duced into Western-Europe, and it is only natural that a bacteriologist 

 with so wide an interest as BEIJERINCK had, should wish to take part 

 in the investigation of the agents which are active in the preparation 

 of such products. The first of BEIJERINCK'S papers to deal in detail 

 with a lactic acid bacterium referred to the microbiology of sour milk 

 preparations. In 1889 he published a study on kefir which can be 

 characterized even to-day as the most outstanding contribution to our 

 knowledge of this remarkable "milk-ferment" i). BEIJERINCK gave 

 convincing evidence that the kefir grains consist of zoogloea of a 

 lactic acid bacterium (Bacillus caucasicus) surrounded by layers of 

 cells of a special lactose-fermenting yeast, Saccharomyces kefyr. The 

 Russian investigator KERN who in 1882 had given a rather confusing 

 description of the micro-organisms present in kefir had proposed the 

 name of Dispora caucasica for the bacterial constituent, since he 

 thought he had seen the formation of two spores in each cell. It 

 remained for BEIJERINCK to prove that the organism in question was 

 a typical non-spore-forming lactic acid bacterium. Characteristic of 

 BEIJERINCK'S broad views is his emphasis on the fact that symbiosis 

 between yeasts and lactic acid bacteria is not at all restricted to kefir, 

 but is on the contrary, of quite general occurrence. He cited the ex- 

 amples of Edam cheese, ensilage, leaven, the faeces of breast-fed 

 infants, and the applications made in the fermentation industries. 



The detection of lactic acid bacteria was greatly facilitated by 

 BEIJERINCK'S suggestion either to cover suitable agar and gelatine 

 media with a thin layer of finely divided chalk, or to incorporate this 

 material into such media 2 ). 



Acid-producing colonies are then quickly surrounded by clear zones 

 which contrast markedly with the rest of the opaque plates. 



In a very short note published in 1893 BEIJERINCK reported the 

 rather startling observation that lactic acid bacteria were exceptions 

 to the rule universally accepted at that time, that every living cell has 

 the ability to decompose hydrogen peroxide into water and free 

 oxygen 3). In the Delft school of bacteriology this observation has 

 since been always applied for diagnostic purposes. For bacteria 

 growing under aerobic conditions the easily-established finding "cata- 

 lase-negative" practically always justifies the diagnosis of "lactic acid 

 bacterium". Only a very few exceptions to this rule have been found in 



\ivh. iuVi-1. d. sciem es exa< tes <{ naturclles 23, 428, 1889. 

 > rntralM. i. |;,ikt. H. Parasitenk. 9, 781, 1891. 

 1 Naturw. Rundschau 8, 671, 1893. 



