54 MASS. EXPERIMENT STATION BULLETIN 319 



structure. The correlation between colonial form and antigenic structure is by 

 no means constant. This also applies to other features. 



Since our knowledge concerning the underlying causes that bring about these 

 changes from the normal to the variant is meagre, it is apparent that a greater 

 insight with respect to the physiology and chemistry of the microorganism is 

 needed. 



Behavior of Variants 



According to the results obtained in the preceding experiments, variants may 

 differ greatly when placed under certain conditions. The phenomenon of clump- 

 ing, settling, and clearing of broth cultures was reported by Nicolle in 1898. 

 Other investigators have also made this observation, but Arkwright (1920) 

 associated this feature with other distinguishing characters, such as change in 

 colony form and serological properties. While this view may be correct for certain 

 variants, for others it does not seem to hold. The mechanism that is operating 

 when organisms settle out in broth is little understood. The fact that variants 

 may or may not exhibit this phenomenon indicates that it cannot be employed 

 as a general criterion for all variants. 



Salt-sensitiveness has also been designated as a means of detecting variants. 

 Certain variants studied in this investigation exhibited this property, but the 

 degree of instability in salt-solution did not appear to be constant. The degree 

 of roughness of the colony type could not be correlated with the degree of salt- 

 sensitivity. The reaction of auto-flocculating or settling was always completed 

 within 24 hours. As the salt concentration was reduced the stability of the 

 organism was increased. However, not all variants responded alike to diminished 

 salt concentration. 



The ability of the different variants to attack various fermentable substances 

 was practically comparable to that of normal types. 



In some cases the character of the growth was markedly influenced by the 

 acidity and possibly other environmental factors. The extreme variation in some 

 of the characteristics had little or no influence on the fermentative property. 

 That 5. pullorum may acquire or reveal the property to ferment substances in 

 addition to those that are attacked by the normal type was observed by the 

 author previously. A maltose-fermenting property was either acquired or revealed 

 which has remained stable for several years. This property may be looked upon 

 as a very unusual characteristic for S. pullorum. Investigational results pertaining 

 to this maltose-fermenting strain will appear in detail in another publication. 



The cultivation of pathogens in an environment which approaches the natural 

 was undertaken. Beyond a doubt, the most frequent mode of transmission of 

 5. pullorum from adult to chick is by means of the egg. Since the egg is frequently 

 a part of the natural environment of the organism, it appeared that it might 

 exert a definite influence on the variants. The egg medium employed in this 

 investigation caused little or no change in the character of the variants. It appears 

 to the author, therefore, that fresh eggs might prove more satisfactory in retaining 

 strains in their original state than the media ordinarily employed. 



In the experiment dealing with the passage of the parent type and the variants 

 through the natural host, it was demonstrated that the variants are less invasive 

 than the normal strains. Larger infecting doses than are ordinarily employed 

 with normal strains were necessary to obtain positive infection. Even then some 

 variants failed to establish themselves in the chick or adult bird. In one instance 

 (Variant B) it was demonstrated that partial reversion to the normal type had 



