60 



SCIENCE 



[Vol. IT., No. 76. 



While all these investigations were going on, and 

 the contradictory and conflicting results derived from 

 them were being given to the world, other experi- 

 ments were being conducted, the results of which 

 were not announced as the work progressed, but were 

 kept from publication until they had been verified in 

 the most complete manner that modern methods 

 would permit. These experiments were those of 

 Kobert Koch, conducted by him at the laboratory of 

 the German board of health in Berlin, and pursued 

 with unremitting diligence and care for over two 

 years. The results were first made public under the 

 modest title of ' The etiology of tuberculosis,' at a 

 meeting of the Berlin physiological society in March, 

 1882, and were published in the Berliner klinische 

 wochenschrtft, 1882, No. 15. His method of work was 

 as follows: — 



Starting with the assumption that a micro-organ- 

 ism might be at the bottom of the disease, he care- 

 fully searched for some evidence to support this 

 theory by microscopic investigation of large numbers 

 of tuberculous tissues from various sources. As a 

 result, he found, that with favorable illumination of 

 the specimen, and good lenses, it was possible to make 

 out the almost constant presence, in tuberculous tis- 

 sues, of a rod-like organism much smaller and finer 

 than most of those that had hitherto been observed. 

 The occurrence of this organism was found to be so 

 frequent, and in such early stages of the disease, that 

 a suspicion of its causal relation to the pathological 

 process was forced upon him. 



Tbe discovery of the existence of this bacterium was 

 but the beginning of the investigation, however; and 

 the masterly series of experiments by which he went 

 on to prove its specific relationship must be read to 

 be appreciated. 



In tbe first place, it was necessary to isolate this 

 organism from its surroundings, and to propagate it 

 by itself; that is, to produce a 'pure culture.' The 

 best means to do this could only be ascertained by 

 experiment; and, after the conclusion of these ex- 

 periments, his results were these. The organism 

 was found to flourish best at a temperature of from 

 36° to 40° C, — a much higher range than is neces- 

 sary for most forms of bacteria. This being ascer- 

 tained, it was necessary to find some suitable culture- 

 soil upon which the organism could flourish ; for the 

 ordinary gelatine media would not remain solid at 

 this temperature. Here it was found that the serum 

 of the blood of sheep or cattle was the best medium 

 to be employed ; for, by exposure to a comparatively 

 low temperature (65° C), it would gradually solidify, 

 until, at tbe end of a few hours of such exposure, it 

 would become a transparent, amber-colored mass of 

 a jelly-like consistency, of course remaining solid at 

 a lower temperature for any length of time. After 

 this was done, there was still another peculiarity of 

 this organism to be appreciated, and that was its slow 

 growth. Up to this time, bacteria were supposed to 

 complete the cycle of their existence in a very short 

 time, — usually measured by minutes, occasionally 

 by hours. In this case, however, there was some- 

 thing entirely different ; and it was not until after a 



large number of experiments had failed, that it was 

 realized that they were dealing with an organism re- 

 quiring from a week to ten days for any appreciable 

 increase in its numbers to occur. 



Having found out all these peculiarities, it re- 

 mained to study what happened in the culture- 

 apparatus, as time went on. It was found that it 

 required from two to three weeks for such a growth 

 to take place upon the surface of the culture-medium ; 

 that a portion could be transferred to another nutri- 

 tive soil, and a new culture started : it was found 

 that the growth occurred in dry, whitish-gray scales, 

 that, under a low power of the microscope, were seen 

 to be rather of a sigmoid form ; and it was found that 

 when these scales were transferred to other soil and 

 broken up, each fragment would produce like scales, 

 and that this method of propagation could be kept up 

 for months at a time. Inoculation of animals of vari- 

 ous kinds with material from these cultures at any 

 and all stages of their growth, done under the most 

 rigid precautions for the exclusion of any impurities, 

 or possibly specific matter, was found invariably to 

 produce the disease tuberculosis. The tissues of 

 these animals, when killed, presented the nodular 

 appearances peculiar to the disease; and in these 

 nodules were always found organisms exactly similar 

 to those which had been injected. Cultures of these 

 organisms showed exactly the same peculiarities as 

 in the first instance; and inoculations with the result 

 of these cultures produced the same pathological ap- 

 pearances. 



In order to prove that these organisms alone would 

 produce the disease, Koch used other substances than 

 tuberculous matter for inoculation; and these sub- 

 stances being proven by microscopic examination to 

 contain no organisms, and being protected from ex- 

 ternal contamination by all known precautions, gave 

 entirely negative results, and in no case was tubercu- 

 losis produced. On the other hand, in no case was 

 there a failure to produce the disease, when materi- 

 als containing the organism, or the organism itself, 

 were employed as the inoculating material. 



Thus it will be seen that the conditions necessary 

 for the establishment of a specific causal relationship 

 between a micro-organism and a given disease were 

 fulfilled in this case, so far as it was possible for one 

 observer to bring them about. The constant occur- 

 rence of an organism in the varying forms of the dis- 

 ease in animals and in man; its isolation from the 

 tissues, and reproduction by artificial means; its in- 

 troduction into healthy animals, with the resulting 

 pathological processes exactly similar to the original 

 disease; its discovery in these inoculated animals; and 

 its re-isolation and observation, — all these require- 

 ments have been repeatedly fulfilled in the case 

 of the bacillus of tuberculosis, together with that 

 other trying one, that materials proven to contain 

 no bacilli invariably fail to act in any specific man- 

 ner. 



Not content with the work thus announced, Koch 

 went on for a year longer with his experiments, the 

 results of which were collected in July of last year, 

 but have only recently reached American readers 



