254 



SCIENCE 



[N. S. Vol. XLVI. No. 1185 



of the difficulty in applying Koch's last two 

 postulates to soil conditions. 



The second postulate of Koch's is that the 

 organism be isolated and cultivated in pure 

 culture. This can be applied without modi- 

 fication to soil conditions, and indeed is gen- 

 erally carried out by soil investigators. 



The thifd postulate is that the organism be 

 shown capable of producing the same disease 

 in healthy animals. The corresponding re- 

 quirement in regard to soil bacteria is ex- 

 tremely difficult to meet. It is possible to in- 

 oculate the organism in question into sterile 

 soil and study its activity under such condi- 

 tions — a test which is quite commonly made. 

 Such a test, however, does not furnish con- 

 clusive proof. Sterilized soil is always differ- 

 ent from natural soil; but worse still, activi- 

 ties in pure culture may be very different 

 from activities in mixed culture. To obtain 

 complete proof, the organism in question 

 should be inoculated into unsterilized soil, and 

 then if the activity under investigation occurs 

 the organism should be shown to be present 

 in large numbers. Such a procedure, how- 

 ever, is generally impossible, because of the 

 difficulty of getting an organism to grow vig- 

 orously in soil already stocked with a bacter- 

 ial flora of its own; and the interpretation of 

 results is difficult, because — in distinct con- 

 trast to the specific agency of microorganisms 

 in disease — the same chemical transformation 

 in soil may be caused by distinctly different 

 organisms. For this reason the best that can 

 ordinarily be done is to inoculate the organ- 

 ism in question into sterilized soil. To do so 

 furnishes better proof than to inoculate it 

 into any laboratory mediimi, but the unsatis- 

 factory nature of the test must be fully recog- 

 nized. Perhaps it is not overstating the case 

 to say that much of the past confusion in re- 

 gard to the activities of soil bacteria has 

 arisen from the fact that they have been 

 studied in piu-e culture while pure cultures 

 never occur naturally in soil. The inocula- 

 tion of sterilized soil is ordinarily the only 

 practical course, however, and has its value 

 as a means of confirming the tests carried 

 out in connection with the requirements al- 

 ready mentioned. 



Koch's last postulate is that the organism 

 be found in the tissues, blood or discharges 

 of the experimentally inoculated animals. 

 The corresponding requirement in regard to 

 soil activities is superfluous, provided sterile 

 soil is used for inoculation and contamination 

 is prevented during the experiment. If un- 

 sterilized soil is used, the presence of the or- 

 ganism in question should be demonstrated; 

 but the impractibility of using unsterilized 

 soil makes this last requirement of little value 

 as applied to soil conditions. 



Summing up, it may be said that to show 

 conclusively the agency of any microorganism 

 in any chemical transformation occurring in 

 soil, the following steps are necessary: (1) The 

 organism m\ist be shown to be present in ac- 

 tive form when the chemical transformation 

 under investigation is taking place; (2) it 

 must be shown to occur in larger numbers 

 under such conditions than in the same soil 

 in which the chemical change is not occur- 

 ring; (3) it must be isolated from the soil 

 and studied in pure culture; (4) the same 

 chemical change must be produced by the or- 

 ganism in experimentally inoculated soil, 

 making the test, if possible, in unsterilized 

 soil. The fourth reqiurement, however, can 

 ordinarily be carried out only by inoculating 

 sterilized soil, a procedure which does not 

 give rigid proof, but which is fairly conclusive 

 if carried out in connection with the other 

 three requirements. 



Sometimes these facts can be brought out 

 wholly by cultural methods, such as used in 

 the past. It must be remembered, however, 

 that cultural methods, at their best, are open 

 to serious error, as organisms that are nat- 

 urally inactive may become active under cul- 

 tural conditions, while under similar condi- 

 tions naturally active organisms may lose their 

 activity. This fact will make it necessary to 

 check up cultural methods with methods of 

 other sorts. Possibly the use of the micro- 

 scope^ will help solve some of the problems, 



1 See Conn, H. J., 1917, ' ' The Direct Microscopic 

 Examination of Bacteria in Soil." (Paper pre- 

 sented at New Haven meeting of the Society of 

 American Bacteriologists.) Abstract in "Ab- 

 stracts of Bact., " Vol. 1, p. 40. 



