The pathogenicity of is a subject of considerable interest. It is believed that of colonial type 1 are pathogenic while those of type 4 are not. This is based on experimentation in human volunteers. The object of this study was to determine the reasons for the differences of susceptibility of chicken embryos to strains of colonial types 1, 4, 5 and 1R (a type-1 revertant from a non-pathogenic type 4 strain originally tested in human volunteers). Colonial types 1, 5, 1R and 4 caused mortality rates of 80, 70, 85 and 20% respectively. This variation in lethality appeared to depend upon the availability of free extra-cellular endotoxin and this was confirmed by chicken-embryo inoculation results and electronmicroscopy of normal and heated colonial types 1, 4 and 5. Similar results were obtained by inoculating purified endotoxins from these types into chicken embryos. The results of this study suggest that endotoxins play a major role in the pathogenicity of and that the variations in virulence of the colonial types depends on the stability of their cell walls.


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