Dissociation of microbic species. V. Further considerations in regard to the virulence of microbes D and G

The wide variations in virulence between microbes D and G, bacillus of rabbit septicemia, has been demonstrated in the first paper of this series. 1 Microbe D, the type found in natural infections, is possessed of powerful invasive properties, while its mutant form G is characterized by very low virulence.

The fixity of the character of high virulence for type D is demonstrated by the following experiment. Strain R-19, type D, was tested a few days after its isolation from a rabbit dead of broncho-pneumonia. The test was carried out by injecting high dilutions of a six-hour serum broth culture intrapleurally into young rabbits of 600 grams weight. The strain proved itself fatal in 10⁻⁸ c.c. of the serum broth culture. This culture was transplanted every seven days on serum agar. Tests made one and three months after the first experiment indicated its virulence to be still of the same titer.

The individuals of a given strain of type D appear to differ very little in the characteristic of virulence. Six pure-line strains, isolated by the Barber method from stock strain R-15, were tested for virulence by the method just described. All were fatal in dose of 10⁻⁶ c.c.

The virulence of type D not only remains constant during passage on serum agar, but persists under conditions that may be considered as distinctly unfavorable. For example, a pure-line strain of type D was planted in plain broth. It was allowed to remain at 37°C. for 9 days and 12 hours without further transplantation. At the end of this time a culture was streaked on a serum agar plate. Marked D ⇀ G mutation had occurred, counts showing D = 40, G = 60. A colony of each type was fished into serum broth tubes.