VARIATION OF SALMONELLA PULLORUM 55 



occurred. This variant was similar in colony morphology to Variant VHI-a 

 which yielded smooth and apparently typical colonies only after a series of 

 injections. With both variants the chicks exhibited no clinical manifestations, 

 but the pathological picture was more extensive in the chicks injected with 

 Variant B than with Variant Vlll-a. The smooth type strain (VIII-c) derived 

 from Variant VI 1 1 -a through a series of inoculations into baby chicks apparently 

 exhibited the same degree of virulence for guinea pigs as the original parent 

 Strain VIII. That some variants may pass through the host unaltered in some 

 of their characteristics was demonstrated with Strains IX-b and XH-b. It is 

 extremely interesting to note that one may inoculate the natural host with a 

 variant as remote in colony form from the normal type as Variant IX-b and 

 recover the same apparently unchanged. 



The presence of the organism in the tissues was detected by tissue smears. 

 The observation of different pleomorphic types of cells outside of the host raised 

 the question of the morphology of the cells when present in the host. It was 

 demonstrated that tissue smears from organs which yielded pure growth on 

 primary culture might not reveal the organism. It was found that the organism 

 most frequently appeared in smears from the yolks of chicks. In this tissue some 

 atypical forms were observed, but for the most part the organisms appeared 

 slightly larger than the normal type isolated on meat extract agar. It does not 

 seem likely that extremely pleomorphic types would be found in body tissues 

 except possibly in tissues such as unabsorbed yolk, where there is a different 

 histological and physiological environment than is found in other parts of the host. 

 Whether the pathological ovary of an adult fowl is conducive to variation is 

 not known, except that the majority of positive isolations originating from this 

 organ seem typical. 



The experimental evidence concerning the pathogenicity of the variants 

 definitely suggests that the organisms have become void of some constituent or 

 physiological property that renders them avirulent. Some variants (Ill-a and 

 VI 1 1 -a) that appear markedly different from the normal type on plain agar, seem 

 to possess no invasive power and induce no local tissue response at the site of 

 inoculation. It appears that the defensive power, as well as the aggressive prop- 

 erty, has been lost. Such types are not expected to survive in the host for any 

 length of time, even when injected in large doses. Variants that have been dev- 

 eloped on liver infusion agar appear to be slightly more aggressive and offer a 

 greater resistance to the defense mechanism of the host than the type represented 

 by Ill-a and VHI-a. Such types may be encountered under natural conditions. 

 However, the type respresented by Variant E appeared the most virulent of all 

 types isolated. The fact that its parent strain XIV at times revealed tendencies 

 to vary may account for its high degree of virulence. The pleomorphic types 

 described by Plastridge and Rettger possessed a marked invasive and infective 

 power. None of the strains described in this investigation, with the possible 

 exception of strain XIV, approached these types in virulence. 



The serological investigations show that the variants underwent a profound 

 change in the antigenic structure, since they were unable to stimulate the pro- 

 duction ot agglutinins, and also were unable to absorb agglutinins from natural 

 and artificial immune sera. These observations appear very significant. One 

 must assume from them that such profound changes, may occur among stock 

 cultures that are employed for the preparation of antigens used in the macroscopic 

 agglutination test. The degree of sensitivity of an antigen and the type of agglu- 

 tination may be greatly influenced by the ratio of the variant to the normal type 

 in a mixed population of cells. 



