﻿STUDIES IN PLAGUE IMMUNITY. 167 



mained unchanged. It is perhaps needless to add that its morphology 

 and cultural characteristics are typical of other unquestionable strains 

 of Bacillus pestis and that it is agglutinated by a standard pest serum. 

 A small standard oese was employed throughout the work, of such a 

 caliber that one 48-hour second-generation culture of this strain yielded 

 about 10 oesen of growth. 



"Pest Avirulent."- — This strain was obtained through the kindness 

 of Prof. W. Kolle, at present Director of the Institut fur InfoMions- 

 hranVheiten, in Bern. It represents an attenuated strain of "Maassen 

 Alt," presently to be described. It forms colonies on agar typical of 

 other strains of Bacillus pestis and in other ordinary media its growth 

 resembles this organism. Its morphology on agar, while not perfectly 

 typical of Bacillus pestis, is suggestive of this organism, since a number 

 of bipolar staining bacilli may be distinguished in the cover slips made 

 from the fresh culture. 



A 19-hour agar slant culture of this organism was suspended in 

 bouillon and inoculated beneath the skin of the shaved abdomen of a 

 monkey. Six hours later an incision was made near the point of inocula- 

 tion and cover slip preparations and cultures secured from the drops of 

 blood which escaped from the incision. No bacilli were found in the 

 cover slips, but the cultures developed numerous colonies presenting the 

 typical appearance of those of Bacillus pestis. Microscopical prepara- 

 tions from these colonies revealed plump bacilli, a few involution forms, 

 and others with typical bipolar staining. Some of the organisms occurred 

 singly, others in pairs • or rarely in chains of three or four. Some of 

 them appeared encapsulated. In other instances also in which very large 

 amounts of this strain were inoculated intraperitoneally into guinea 

 pigs and in which the animal succumbed to pest intoxication, organisms 

 with the typical morphology of B. pestis have also been observed in 

 microscopical preparations made from the abdominal cavity. (See Series 

 32.) The strain "Pest Avirulent" is agglutinated by two standard pest 

 sera. Its virulence is so reduced that from 1 to 2 whole agar slant 

 cultures do not cause the death of guinea pigs when injected subcuta- 

 neously. In small animals under 175 grams in weight, one agar slant 

 injected intraperitoneally has occasionally caused death from plague 

 intoxication. (See Series 37, animal, number 2043, p. 207.) The un- 

 questionable proof that the organism really represents an attenuated strain 

 of the genuine pest bacillus is furnished by the fact that monkeys and 

 guinea pigs vaccinated with .this culture have later been shown to possess 

 high and undoubted pest immunity, by subsequently inoculating them 

 with multiple lethal doses of the strain "Pest Virulent" already described. 

 (See Series 4, 11, 12, 18, etc., pp. 199 to 212.) 



Pest "Maassen Alt." — This strain was also obtained by me from Pro- 

 fessor Kolle, who originally received it from Dr. Maassen. Both it and 



