Type D, bacillus of rabbit septicemia, exhibits marked fixity of the character of virulence. This is true for cultures that are regularly transplanted on serum agar or in broth. It is also to be observed when organisms of this variety are subjected to unfavorable conditions; e.g., remaining for 9½ days without transplant in plain broth. Under such conditions no decrease in virulence was observed. G forms which arose in the same culture during this time exhibited characteristic lack of invasibility.
Different individuals of a given culture of Type D do not vary to a noticeable extent in virulence. This was ascertained by test of virulence of cultures arising from six individuals fished from Type D cultures by Barber pipette.
A Type D culture subjected to passage in plain broth undergoes D → G mutation. Type D and G individuals can be demonstrated to be present in the same culture. The virulence of such a mixed culture is proportional to the relative member of Microbes D and G present. This throws light on certain mechanisms of attenuation.
The virulence of Type G can be raised to a considerable titer by animal passage. Such organisms do not, however, lose their characteristic of granular growth. This last, on the other hand, appears to intensify by animal passage. The acid agglutination zone of Type G strains which have been passed through animals shows a marked broadening.
Microbe D owes its superior invasive power at least in part to its antiphagocytic activity, a property apparently not possessed by Microbe G.