ROBERT KOCH 379 



ing on the pathogenic bacteria which I have described as distinct 

 species. The greatest stress, in investigations on bacteria, is justly laid 

 on the so-called pure cultivations, in which only one definite form of 

 bacterium is present. This evidently arises from the view that if, in 

 a series of cultivations, the same form of bacterium is always obtained, 

 a special significance must attach to this form : it must indeed be ac- 

 cepted as a constant form, or in a word as a species. Can, then, a 

 series of pure cultivations be carried out without admixture of other 

 bacteria? It can in truth be done, but only under very limited condi- 

 tions. Only such bacteria can be cultivated pure, with the aids at pres- 

 ent at command, which can always be known to be pure, either by their 

 size and easily recognizable form, as the bacillus anthracis, or by the 

 production of a characteristic coloring matter as the pigment bacteria. 

 When, during a series of cultivations, a strange species of bacteria has 

 by chance got in, as may occasionally happen under any circumstances, 

 it will in these cases be at once observed, and the unsuccessful experi- 

 ment will be thrown out of the series without the progress of investi- 

 gation being thereby necessarily interfered with. 



But the case is quite dififerent when attempts are made to carry out 

 cultivations of very small bacteria, which, perhaps, cannot be distin- 

 guished at all without staining ; how are we then to discover the occur- 

 rence of contamination? It is impossible to do so, and therefore all 

 attempts at pure cultivation in apparatus, however skilfully planned 

 and executed, must, as soon as small bacteria with but little charac- 

 teristic appearances are dealt with, be considered as subject to unavoid- 

 able sources of fallacy, and in themselves inconclusive. 



But nevertheless a pure cultivation is possible, even in the case of 

 the bacteria which are smallest and most difficult to recognise. This, 

 however, is not conducted in cultivation apparatus, but in the animal 

 body. My experiments demonstrate this. In all the cases of a dis- 

 tinct disease, e. g., of septicaemia of mice, only the small bacilli were 

 present, and no other form of bacterium was ever found with it, unless 

 in the case where that causing the tissue gangrene was intentionally 

 inoculated at the same time. In fact, there exists no better cultivation 

 apparatus for pathogenic bacteria than the animal body itself. Only a 

 very limited number of bacteria can grow in the body, and the pene- 

 tration of organisms into it is so difficult that the uninjured living body 

 may be regarded as completely isolated with respect to other forms of 



