208 



THE AMERICAN MONTHLY 



[Novembefj' 



^1 



duced accidentally grew only where 

 it fell, and thus a pure cultivation 

 from a pure part was always possible; 

 on the other hand, if these organisms 

 had been grown in a fluid, the 

 introduction of another form would 

 have rendered them impure forever. 

 Dr. Koch exhibited specimens oi Mi- 

 crococcus prodigiosus which produces 

 a red pigment, and also of the bacillus 

 which causes blue pus, and that which 

 causes blue milk. Other forms of 

 bacilli were shown which, microsco- 

 pically, were indistinguishable, but 

 which could be at once separated 

 from each other by differences in 

 their mode of growth on solid sub- 

 stances. The advantage of a solid, 

 rather than a liquid, cultivating ma- 

 terial being thus apparent. Dr. Koch 

 next turned his attention to the solidi- 

 fication of other cultivating materi- 

 als, such as would nourish pathogenic 

 bacteria, and he found that by the 

 addition of gelatin to the fluid used, 

 in the proportion of 3 or 4 per cent., 

 a solid cultivating material was ob- 

 tained, whose power of nourishing 

 organisms was not in any way inter- 

 fered with by the presence of the ge- 

 latin. 



Some of this material, being ren- 

 dered fluid by heating and spread out 

 on a slide, was allowed to solidify ; 

 then bacteria could be sown on it, 

 and their mode of growth watched 

 with a low power of the microscope. 

 Thus, a minute quantity of dry earth 

 was scattered over such a slide, and, 

 in a few hours development, could 

 be seen around almost every particle. 

 In this particular specimen seven dif- 

 ferent sorts of bacilli were present ; 

 many of these could not have been 

 distinguished from each other by the 

 microscope, but a difference was at 

 once observed between their modes 

 of growth on the solid substance — 

 some forming round balls, others 

 growing out in a star-shaped manner, 

 others growing in a fine net-work, etc. 



In the same way, the number and 

 nature of the organisms present in 

 any given quantity of air, could be 



estimated. A broad, shallow vessi 

 was filled with the gelatin mixtun 

 and exposed for a given time to th 

 air. At every point where an organ- 

 ism fell upon it, growth occurred, and 

 thus the number and nature of the 

 organisms present could be at once 

 ascertained. But, further, as each 

 organism was a pure cultivation, 

 pure flasks could be inoculated from 

 each variety, and thus its further life- 

 history and pathogenic characters 

 could be investigated. 



Similarly with water. The material 

 in a test-tube having been rendered 

 fluid, a given quantity of water was 

 shaken up with it until solidification 

 occurred. At every point where an 

 organism was present in the water, 

 development occurred, and thus the 

 number and nature of the organisms 

 present in a given specimen could be 

 at once ascertained. 



Dr. Koch also exhibited some of 

 his pathogenic bacteria. Animals 

 which had been killed with anthrax 

 were shown. The fatal nature of the 

 poison was demonstrated ; the con- 

 stant presence of the Bacillus anthra- 

 cis, its mode of growth in the gelatin 

 substance, and its virulent properties 

 after having been grown in it, were all 

 made apparent. The bacillus of 

 mouse-septicemia, which is described 

 in his work, was shown in a similar 

 manner. For several months this 

 organism was cultivated in gelatin- 

 blood-serum, forming a fine, cloudy 

 mass, and retaining its form and other 

 characteristics. A minute drop of 

 this was placed under the skin of a 

 mouse. This animal died in forty- 

 eight hours ; and in its blood were 

 numerous bacilli. Another mouse 

 inoculated from this blood also died. 

 In gelatin inoculated with this blood 

 these organisms developed ; and after 

 further cultivations with this, the mi- 

 nutest drop killed another mouse. 

 Septicemia was shown in pigeons, 

 rabbits, mice, etc., due to a minute 

 bacillus of peculiar form, resembling 

 in appearance the organism of the 

 choldra des poules of Pasteur. The 



