THE INFLUENCE OF SYMBIOSIS. 79 



may exert upon the metabolism, the toxic and the resisting properties of 

 each, as well as the interaction of the two symbiotic elements which leads 

 to disease or death on the one hand or to what Smith calls a "condition of 

 balanced parasitism" on the other. 



For example, it is well known that certain cases of diphtheria compli- 

 cated by severe streptococcus infections are not benefited by antitoxin nor 

 are they cured by the administration of both antidiphtheritic and 

 specific antistreptococcic serums. It is probable that one reason for 

 failure in this instance is to be found in the fact that the toxic principle 

 is not a simple mixture of two toxins, but that it has the nature of a 

 new, definite, chemical compound, formed by the symbiosis of the two 

 organisms and therefore producing a type of poison differing not only 

 from that secreted by either, but also from any simple mixture of the two. 

 If this should be true, it would seem altogether probable that the proper 

 immunity unit could only be produced by the action upon animal tissues 

 of the compound toxin radical acting as a whole. 



BACTERIAL SYMBIOSIS. 



Before taking up a discussion of the influences which symbiosis may 

 exert upon the components produced by any bacterial coexistence, it is 

 necessary to remember that there are many other elements of environment 

 which may exert an influence on pure species of bacteria, similar perhaps 

 in results to the manifestations of the same influences on such organisms 

 in symbiosis. 



Variability in the virulence of bacteria is one of their most marked 

 features and the reason for this variability is but partially understood. 

 We know that it may be modified by many familiar methods, but no fixed 

 rules can be made or applied. The majority of pathogenic bacteria lose 

 more or less of their virulence by growth upon artificial media and 

 similarly their virulence is increased by passage through susceptible ani- 

 mals. However, this is not always true nor does it explain the varia- 

 bility. For example, there are bacteria which retain their virulence in 

 artificial cultures and others in which this property may be considerably 

 modified by the composition of the medium. Again, it may happen with 

 bacteria that passage through animals will cause a permanent attenuation 

 in some and in others increase of virulence, which for one species of 

 animals may be coexistent with a weakened virulence for some other 

 species. 



A satisfactory explanation of some of the alterations in virulence and 

 of certain of the peculiarities in reactions between bacteria and animal 

 tissues is found in what is now generally known as the hypothesis of 

 Welch which, succinctly put by Eicketts, is as follows: "If bacterial 

 toxins and the constituents of bacterial cells so act on the tissue cells 

 that the latter produce bodies (antibodies) which are inimical to the 

 bacteria, why may not the body fluids in turn so act on the bacteria 



