404 HOST-PARASITE RELATIONSHIPS 



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mutates either toward virulence or toward avirulence quite readily. A virulent 

 culture placed on culture media favoring saprophytic growth for six months at refrigera- 

 tor temperature will frequently have no virulence to chickens. On the other hand, 

 avirulent gallinarium passed by blind passage through a series of as many as 20 chickens 

 normally will mutate to full virulence at some point along the way. 



Dr. Roderick: Do your selected and unselected lines show any differences in 

 symptomatic manifestations of the diseases? 



Dr. Gowen: In answer to Dr. Roderick's second question, our strains of mice do 

 show differences in the manifestiation of disease when inoculated with the same line 

 of Salmonella typhimurium 11C. There are distinct differences in morbidity. The S 

 mice show scarcely any effects of the disease even when exposed to 2,000,000 organisms. 

 The Ba mice are obviously sick when the exposure dose is 20 to 200 organisms. The 

 other strains act as though they were between these extremes. Clear-cut differences 

 in the survival curves are evident when they are analyzed for their different constants : 

 mean, mode, standard deviations, skewness, kurtosis, and type of curve. Incidentally, 

 there are no detectable immune bodies in the bloods of any of our strains before they 

 have contact with the pathogen. One strain has a characteristic alpha-<g\ob\\\m and 

 the other a characteristic fota-globulin. These behave as though controlled by single 

 Mendelian genes but seem to have little correlation with the pattern of disease. 



Dr. Weir has mentioned the differences in leucocytes. These differences in our 

 strains are correlated with differences in resistance to disease. However, in his 

 selections when differences in leucocytes were established in two separate strains 

 of mice, the differences were not accompanied by corresponding differences in 

 resistance to disease. Macrophages also differ in their abilities to retain visible 

 bacteria when examined microscopically during the course of infection. These differ- 

 ences may be correlated with digestive abilities of intracellular enzymes, since they are 

 correlated with the resistances of the strains. It may be said that one strain or the 

 other of our group has been found to differ significantly from the others in all of the 

 organs yet measured. Many of these differences are correlated with the visible 

 manifestations of the disease. 



