98 STUDIES IN IMMUNITY. 



again, the serum of a rabbit vaccinated against Massaouah (which, 

 however, was not very powerful) clumped the Massaouah organism, 

 and had only the slightest effect on the vibrio of Eastern Prussia as 

 we have already noted in conjunction with Mesnil. It is, therefore, 

 not certain that in using different sera we shall always obtain the 

 same result. 



Third: A micro-organism which ordinarily is clumped by a 

 given serum may undergo such changes as to be no longer distinctly 

 affected by this serum. In 1891 Metchnikoff found that the vibrio 

 Metchnikovi grown in the exudate of an immunized animal might 

 either form clumps or else grow as individual organisms, according 

 to the previous conditions under which the culture had lived. 

 Metchnikoff and I have recently convinced ourselves that a very 

 virulent vibrio from Eastern Prussia, after living for some time 

 within the protoplasm of phagocytes, was affected only very slightly 

 by serum from a horse vaccinated against cholera. This vibrio had 

 been injected subcutaneously into a horse and the exudate with- 

 drawn after phagocytosis was complete. Vibrios from the original 

 culture transplanted on agar were very strongly clumped by the 

 serum of this animal, whereas the organisms drawn from the exudate 

 of the animal remained scattered and caused a permanent cloudiness 

 in the fluid. This modification of the vibrio, moreover, lasts for 

 several generations. 



When dealing with an organism that has been recently isolated 

 we have very indefinite knowledge as to its previous conditions of 

 existence. It may well be that in nature certain influences change 

 organisms in such a way that they become insusceptible to the 

 clumping action of serum. Since the property of collecting bac- 

 teria into clumps does not belong exclusively to specific preventive 

 substances; and since the clumping effect of a preventive serum is 

 not absolutely specific; since bacteria are susceptible to changes in 

 respect to clumping, and their reaction to a given serum is incon- 

 stant and varying, it must be admitted that suppressing such a 

 diagnostic sign as the granular transformation does not add to the 

 accuracy of this type of investigation. 



