VARIATION OF SALMONELLA PULLORUM 45 



subject the variant to such a selective test or process as would eliminate the 

 weaker organism (the variant) and permit the stronger organism (the normal) 

 to survive. 



The results also suggest that all true variants possess a low degree of virulence, 

 or none at all. Strain E apparently seems to be as pathogenic as its parent Strain 

 XIV, which at times was in all probability a variant. Plastridge and Rettger 

 (1930) (1934) have clearly demonstrated that variants may be extremely virulent. 

 Such types, although apparently rare, are very baffling to the diagnostician and 

 difficult to identify, especially to one who has not before encountered such types. 



Pathogenicity for an Alien Host (Guinea Pig) 



In the study of the behavior of S. pullorum in the guinea pig, 10 strains (four 

 parent, five variants, and one apparently normal strain derived from a variant) 

 and 32 guinea pigs were employed. Each strain was inoculated intraperitoneally 

 into three pigs, with the exception of Strain I, which was given to five pigs. All 

 pigs received 0.5 cc. of a suspension of the organism having a turbidity equal to 

 that of tube 2 of the McFarland nephelometer, except animals 43, 44, and 45, 

 which received a concentration equal in turbidity to that of tube 1. The pigs 

 inoculated with the same strain were confined in one cage. 



Preliminary experiments revealed that typical normal strains of S. pullorum 

 would produce marked symptoms and even immediate death after one or more 

 inoculations, while the variants produced no ill effects. Therefore, antigens with 

 a greater concentration of organisms than previously employed were inoculated. 

 A series of six daily doses was administered to each pig, unless otherwise stated. 



The results of this experiment are in part briefly summarized in Table 15. 

 It may be seen that all of the normal strains and one variant produced a symptom- 

 atology which was quite similar for all groups. However, the time of response 

 to the inoculation varied greatly, as is revealed by pig 40, which appeared sick on 

 the second day after the first inoculation, and pig 39, which exhibited clinical 

 manifestations on the tenth day. The number of inoculations was guided by the 

 condition of the animal. Among the animals exhibiting symptoms only a few 

 did not succumb. Three pigs (numbers 16, 17, and 43) died following cardiac 

 bleeding, on the twelfth day. 5. pullorum was recovered from 12 of the 17 pigs 

 (one not cultured) inoculated with apparently typical strains, while no positive 

 isolations were obtained from the pigs inoculated with the variants. Sera were 

 tested for the presence of agglutinins in only 21 animals, since the remainder of 

 the group died as the result of the infection. Agglutinins were detected in two 

 sera, both obtained from pigs inoculated with a normal strain. It is of considerable 

 interest to note that the strain recovered from chicks in Lot XI and designated 

 as VIII-c in Table 15 behaved in a manner similar to the original normal type 

 Strain VIII when inoculated into guinea pigs. These observations point out that 

 virulence can be grossly masked or hidden in a culture that may be a variant with 

 respect to one or more of its characters, but through proper treatment of the 

 variant the virulent types may manifest themselves. 



Recognizing the fact that the number of pigs employed for each strain was small, 

 a second experiment was conducted by inoculating one parent strain (VIII) 

 and its variant (VHI-a) each into 14 guinea pigs. The procedure was quite 

 similar to that of the previous experiment. Table 16 shows that the results in the 

 main substantiated previous observations. It is clearly evident that the variants 

 used in the guinea pig inoculations did not possess the invasive power of their 

 parent strains. 



