iv MICROBES SIMULATING THE B. PESTIS 73 



microbe, would suggest that the disease is naturally 

 contracted by way of the respiratory organs. 



It might be inferred from the foregoing description 

 that this microbe is the diphtheria bacillus in an 

 attenuated state of virulence. Against such conclusion 

 there are, however, some very valid reasons : (a) the 

 microbe acts well on adult rats, animals which are 

 refractory towards even virulent diphtheria cultures ; 

 (b) diphtheria anti-toxin has no effect whatever in 

 neutralising the action of even a small dose of this 

 microbe. In this direction I have made a number of 

 experiments. A dose in each instance of a recent culture 

 of the microbe was subcutaneously injected into certain 

 control guinea-pigs, while a similar dose of the same cul- 

 ture mixed with 400, 500, and even 600 units of diph- 

 theria anti-toxin (obtained of Burroughs and Wellcome, 

 as also of Allen and Hanbury) was injected into each of 

 several other guinea-pigs. At the same time a dose of 

 diphtheria anti-toxin (100 and 200 units) mixed with a 

 lethal dose of culture of the true diphtheria bacilli was 

 injected into each of another series of guinea-pigs. These 

 latter remained without any disease, but the control 

 animals and those injected with mixture of culture of 

 the microbe which is in question and diphtheria anti- 

 toxin developed the characteristic tumour. It is quite 

 clear, then, that this microbe is in no way affected by 

 the diphtheria anti- toxin, and that it is, therefore, not 

 the true diphtheria bacillus, though it is a pathogenic 

 microbe belonging to the group of diphtheroid bacilli. 

 On account of its having been found in the rat I have 

 named it Bacterium muris. 



During the last twelve months I have had the oppor- 



