49 



127 



None of these stages in the investigation can be dispensed with, and none of them 

 can be taken as of decisive importance by itself, not even the reaction with a long series 

 of different sugars, though this does give such a valuable insight into the biology of the bac- 

 teria. In the first place, it is necessary of course to know whether the acid formed is lactic acid 

 at all, and in the second place, it is possible, as we shall see later on, for strains undoubtedly 

 belonging to the same species to react very differently towards certain sugars. We cannot 

 therefore lay too much stress upon the importance of avoiding the onesided classihcation 

 according to reaction with sugars, which has been largely practised in particular by 

 American writers. Only by taking equal note of all qualities can we arrive at a natural 

 bacteria system. 



Variability of Qualities. 



Among the particularly variable qualities of bacteria are colour formation, and to 

 some extent also the size and appearance of the surface growth; points which we shall 

 deal with more closely when discussing the tetracocci. In this connection it will suffice 

 to mention that comparatively anaerobic propionic acid bacteria transplanted from 

 agar have in the course of years become slimy, and developed a marked surface growth, 

 thus resembling in cultural respects the aerogenes bacteria. 



As mentioned, the bacteria can, under particular circumstances, accustom them- 

 selves to new sources of nitrogen, but the reverse process — the loss of power to utilise a 

 given source of nitrogen — is far more likely to occur, and on the whole, the varia- 

 tion which we have otherwise encountered in the lactic acid bacteria, 

 can be explained as a further development of already existent tenden- 

 cies, or more frequently as the results of weakness or degeneration. We 

 have never, for instance, found any of our strains acquiring the power to ferment a new 

 sugar, though we have occasionally found that its power of fermenting a certain sugar, 

 originally but slight, may be increased, and we have often noticed that it has lost the power 

 of fermenting certain sugars at all, those being chiefly those which it had always previously 

 found difficult to utilise. As an indication of weakness also, we should regard the point 

 already mentioned, that strains forming both dextro- and lævo-lactic acid may lose the 

 power of forming that of the two which it formed in the lesser quantity, and that strains 

 which besides lactic acid also formed abundant quantities of by-products lose altogether 

 or in part the power of so doing, by which they will of course be utilising their source of 

 energy to a less complete degree. 



A highly variable quality is the power of forming slime in milk, but this is, as men- 

 tioned, only due to further development of a tendency possessed by all lactic acid bac- 

 teria in a youthful state. 



It is interesting to note how closely-related strains are inclined to vary in the same 

 way; or may, indeed, when cultivated under like conditions, often be seen to vary — and 

 die off — at exactly the same time. All these apparent variations, therefore, are in reality 

 no hindrance at all to species determination; on the contrary, the manner in which a 

 bacterium is inclined to vary is often one of its most characteristic 

 qualities. 



I). K. D. Vitlensk. Selsk. Skr., naturvidensk. og niathem. Afil. 8. Række, V. 2. 17 



