TRANSACTIONS OF SECTION K. 837 



cultures on ripening fruits still attached to the plant, but imprisoned in sterilised 

 glass vessels, that the yeasts and the moulds are separate forms, not genetically 

 connected, but merely associated in nature, as are so many other forms of yeasts, 

 bacteria and moulds. 



It is interesting to notice how here, as elsewhere, the lessons taught by pure 

 cultures are found to bear fruit, and how Hansen's work justifies the specialist's 

 laboratory. 



Among the most astonishing results that have come to us from such researches 

 are Hansen's discoveries that several of the yeasts furnish quite distinct races or 

 varieties in different breweries in various parts of the world, and it seems impos- 

 sible to avoid the conclusion that their race characteristics have been impressed on 

 the cells by the continued action of the conditions of culture to which they have 

 so long been exposed — they are, in fact, domestic races. 



Much work is now being done on the action of the environment on yeasts, and 

 several interesting results have been obtained. One of the most striking examples 

 is the fact observed by Sauer, who found that a given variety of yeast, whose 

 activity is normally inhibited when the alcohol attains a certain degree of concen- 

 tration in the liquid, can be induced to go on fermenting until a considerably higher 

 proportion of alcohol is formed if a certain lactic-acid bacterium is added to the 

 lermenting liquor. The bacterium, in fact, prepares the way for the yeast. Ex- 

 periments have shown that much damage may be done to beers and wines by 

 foreign or weed germs gaining access with the yeasts, and Hansen has proved that 

 several yeasts are inimical to the action of the required fermentation. But not all 

 pure fermentations give the desired results : partly because the race-varieties of 

 even the approved yeasts differ in their action, and partly, as it appears, on account 

 of causes as yet unknown. 



There are facts which lead to the suspicion that the search for the best possible 

 variety of yeast may not yield the desired results, if this particular form is used 

 as a pure culture. The researches of Hansen, Kothenbach, Belbriick, Van Laer, 

 and others, suggest that associated yeasts may ferment better than any single yeast 

 cultivated pure, and cases are cited where such a symbiotic union of two yeasts of 

 high fermenting power has given better results than either alone. 



If these statements are confirmed, they enhance the theoretical importance of 

 some investigations I had made several years previously. English ginger-beer 

 contains a curious symbiotic association of two organisms — a true yeast and a true 

 bacterium — so closely united that the yeast-cells imprisoned in the gelatinous 

 meshes of the bacterium remind one of the gonidia of a lichen entangled in the 

 hyphse of the fungus, except that there is no chlorophyll. Now it is a singular 

 fact that this symbiotic union of yeast and bacterium ferments the saccharine 

 liquid far more energetically than does either yeast or bacterium alone, and results 

 in a diflerent product, large quantities of lactic and carbonic acids being formed, 

 and little or no alcohol. 



In the kephir used in Europe for fermentmg milk, we find another symbiotic 

 association of a yeast and a bacterium ; indeed, Freudenreich declares that four 

 distinct organisms are here symbiotically active and necessary, a result not con- 

 firmed by my as yet incomplete investigation. I know of at least one other case 

 which may turn out to be different from either of the above. Moreover, examples 

 of these symbiotic fermentations are increasing in other directions. 



Kosai, Yabe, and others have Jately shown that in the fermentations of 

 rice to produce sake, the rice is first acted on by an Asper(/ilhs, which converts 

 the starch into sugars, and an associated yeast — hitherto regarded as a yeast-form 

 of the Aspergillus, but, as already said, now sliown to be a distinct fungus sym- 

 biotically associated with it — then ferments the sugar, and other similar cases are 

 on record. 



Starting from the demonstrated fact that the constitution of the medium pro- 

 foundly affects the physiological action of the fungus, there can be nothing sur- 

 prising in the discovery that the fungus is more active in a medium which has 

 been favourably altered by an associated organism, whether the latter aids the 

 fungus by directly altering the medium, or by ridding it of products of excretion 



