6 QUATERCENTENARY STUDIES IN PATHOLOGY 



The toxic filtrates were, in their experience, fairly stable, but 20 

 minutes in water at a temperature of 100° C. completely destroyed the 

 toxicity for mice. 



By means of injections of the filtrates they were able to obtain 

 evidence of active immunity against toxin, " Giftfestigkeit," and the 

 animals which had acquired this showed also some increase of resistance 

 to bacterial infection. 



Markl (1898 and 1901) concluded, from a large number of experi- 

 ments, that the plague bacillus, when grown in bouillon, under aerobic 

 conditions, is capable of producing soluble toxins. The bacillus is 

 grown in bouillon, in flat flasks, to allow a free supply of oxygen, since, 

 under anaerobic conditions, no toxin formation takes place. Though 

 toxin may be demonstrated in a very short period, even within 24 hours, 

 two months at room temperature are required for the maximum toxicity 

 of the cultures to be attained. The optimum temperature for toxin 

 production is room temperature ; 37° C, while admitting of toxin 

 formation, is less favourable, a circumstance due partly to the injurious 

 influence on the bacillus of the higher temperature, partly to its 

 deleterious action on the already formed toxin. This susceptibility of 

 the toxin to the action of heat is a marked feature ; a temperature of 

 70° C, for 1 5 minutes completely destroys the toxicity of the filtrate for 

 mice, whereas it still remains toxic for rats, rabbits and guinea-pigs. 

 From this observation Markl argues that the filtrates contain two different, 

 but related, toxins. The relationship is best shown by the fact that 

 animals which have been injected with heated filtrates furnish an antitoxic 

 serum which is capable of protecting mice against the unheated toxin. 



Markl believes that, by injecting goats and horses, with toxin, an 

 antitoxin may be obtained, and he suggests the advisability of employing 

 an anti-plague serum, obtained by a combined method of immunisation 

 with bacilli and with toxins. 



He regards the toxin as a product of genuine bacillary secretion, and 

 not merely as the result of the degeneration and death of the bacilli, 

 with subsequent soaking-out of intracellular poisons. He bases his 

 opinion on a number of observations : — First, the toxin is demonstrable 

 at a very early period in the history of the culture, when the number of 

 dead and degenerated bacilli is too small to account for the toxin 

 present ; second, the toxin production bears a certain relationship to the 



(116) 



