TEXT-BOOK OF BACTERIOLOGY. i i 



could not dry up, there appeared on the surface a series of small, 

 white-colored points and drops, which increased quickly in extent 

 till they at length covered the whole slice. Microscopic examina- 

 tion showed that these spots were nothing but collections of micro- 

 organisms, and further, that such a point contained only bacteria 

 of one and the same species. This last fact was, at the period of 

 its discovery, something very striking, but its explanation was not 

 very difficult. The little aggregation had proceeded from germs 

 in the air, which, settling on the potato, found there a suitable 

 place for their development and growth; thej- were, however, com- 

 pelled by the existing conditions to develop and grow at that point 

 of the solid culture medium on which they had first fallen, so that 

 here only cells of the same species could arise. If, instead of fall- 

 ing on the potato, they had fallen into a glass of bouillon, they 

 would also have developed, but in a short time the different species 

 would have got mixed, and very soon all would have become a law- 

 less, inextricable chaos. 



On the potato it was impossible for one species to gain the as- 

 cendency, repressing the others and preventing their further de- 

 velopment; for here each germ found a quiet spot where it could 

 proceed to the reproduction of its kind. 



This collective grouping of individuals of one and the same 

 species only enabled us to observe these species more accuratelj' 

 and to determine distinguishing qualities with increased certainty. 

 With the herd before us, we succeeded in discovering many pecu- 

 liarities whch had altogether escaped observation when we had 

 only one specimen to examine; in a word, we had in this first col- 

 lection of bacteria on a potato (in a colony of bacteria, as it is called) 

 the point of departure for a pure culture with all its advantages, 

 and it proved easy to utilize this fact. 



In addition to this, the relations of the bacteria to their solid 

 food medium led to the discovery of a whole series of hitherto un- 

 known properties in these micro-organisms; properties which could 

 not be seen in the liquid food solutions. This gave rise to an abun- 

 dance of new ideas, and yielded extremely valuable criteria for the 

 comparison and discrimination of the separate species. 



Thus the potato all at once opened our ej^es to the advantages 

 of the solid culture media — the facilitj' which thej'^ offer to the ob- 

 taining and preserving of pure cultures, the certainty with whicla 

 they afford to each species an undisturbed and unfavored devel- 

 opment, and the revelation of a ncA'er-dreamt-of multitude of new 

 qualities in the bacteria which could not otherwise be recognized. 



The acuteness of Koch enabled him to fully appreciate these ad- 



