PROBLEMS OF PATHOGENIC MICRO-ORGANISMS 223 



the plasticity of micro-organisms. It has long been known that the 

 pathogenic power of bacteria is reduced gradually in artificial cultures. 

 It is also well known that by a series of inoculations, or passages 

 through animals, the virulence may be restored, and even raised above 

 the natural level. Bacteria have been gradually accustomed to 

 originally destructive doses of poisons in culture-fluids. Very recently 

 it has been shown that they may be gradually trained to multiply in 

 strongly bactericidal serums and to refuse to be clumped in strongly 

 agglutinating serums. 



These adaptations persist for a certain time, and are transmitted 

 for a limited period, even in culture. In other words, the modifica- 

 tions are more or less gradually acquired and gradually lost. The same 

 is true of the anti-bodies of the host. The antitoxin circulates in 

 the blood of the horse long after the stimulation by toxins has ceased. 

 In the immunized animal the agglutinating properties do not disap- 

 pear at once. I am, therefore, inclined to believe that the bacterium 

 freshly removed from its usual environment will, at least for a time, 

 exercise all its functions, provided the special nutritive substances 

 which may be needed to carry on those functions are present. 



The theory of Professor Welch would then resolve itself into a ques- 

 tion of nutrition. In the body of the host there are certain substances 

 which give rise to special toxins when acted upon by special bac- 

 teria. If we could offer these special substances to freshly isolated 

 bacteria, there is no reason why the assumed toxin should not be 

 formed. We must, therefore, take into account two possibilities, the 

 adaptation of microbes to originally destructive agencies, and the pro- 

 duction of poisons from specific substances elaborated by the host. 



I have entered into this much of detail concerning the mutual 

 relation of micro-organisms and host, in order to make clear the 

 hypothesis, which, it seems to me, accounts very well for the general 

 phenomenon of infection. It is that the tendency of all invading 

 micro-organisms in their evolution toward a more highly parasitic 

 state is to act solely on the defensive while securing opportunity for 

 multiplication and escape to another host. By tendency 1 mean a 

 general slow movement through long periods of time. The following 

 data are in its favor : 



(1) The production of diffusible toxins survives parasitism inde- 

 finitely, and is readily brought about in cultures. 



(2) Where toxin-producing bacteria have become adapted to a de- 

 finite species, as in diphtheria, the toxin itself acts upon a number of 

 different species. In other words, the parasitic relation is far more 

 specialized than the chief pathogenic product. 



(3) No strictly invasive bacteria have yet been found producing 

 diffusible toxins which appear to be of any real significance in the 

 disease process. 



