Abstract
The coexistence of two distinctly different types of microbe in cultures of the rabbit septicemia bacillus has been reported in a previous paper. 1 These varieties, once separated, appear to breed true to type for many passages. The organisms have been designated as types D and G. Type D is very virulent for rabbits, grows diffusely in liquid media, and yields highly fluorescent, rather opaque colonies on serum agar. Type G is of extremely low virulence, exhibits a granular sedimenting growth in fluid media, and grows in the form of translucent, non-fluorescing colonies on serum agar. The two types show no noticeable differences in morphology or in fermentation reactions. Immunization and agglutination reactions indicate their antigenic community.
It seemed necessary to determine whether the two varieties coexist in cultures isolated from infected rabbits or whether one variety arises from the other. Type D (virulent) is the microbe invariably obtained from the naturally infected rabbit. Type G has only been found after artificial cultivation has been carried on for some time. But since the primary isolations were made from colonies which conceivably might arise from two or more organisms, it would be unjustifiable to conclude that the original type D had changed into the microbe of the G variety. Consequently, 8 pure-line strains were isolated from a D culture by the Barber method. Single cells, removed from three-hour cultures, were planted in undiluted rabbit serum. The percentage of positive cultures obtained by performing the entire operation in serum was much higher than when broth was employed.
The resulting pure-line strains were planted daily in undiluted rabbit serum. Tests were carried out to determine the conditions under which the low-virulent type G makes its appearance.
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