16 



our further study and search for the B. typhosus safely 

 exclude and disregard those red-haloed red colonies. But 

 the medium does more than this, for every red colony in 

 general, and every colony which is neutral in colour, i.e., 

 neither red nor blue, can likewise be excluded as being 

 certainly not a colony of B. typhosus, because the colonies 

 of B. typhosus after one, two, or, better, three days at 37 C. 

 appear bluish or more or less violet-blue. As will presently 

 be further stated, not every " blue " colony is necessarily one 

 of B. typhosus, but if we find in our plate colonies of blue 

 or violet-blue colour, we have a guide to which colonies we 

 have to direct our attention in our further study and tests. 

 How important a step in advance of all others this method 

 of Drigalski-Conradi is, can best be estimated by the following 

 illustration : if, for instance, we are working with a given 

 material (water, milk, shellfish), in which by previous 

 analysis by means of Drigalski plates we have ascertained 

 the number of B. coli and the absence of blue colonies in a 

 definite amount of that material, we can, after adding to the 

 material a trace of typhoid culture, without any difficulty 

 ascertain by Drigalski plates the number of B. typhosus in 

 any given amount of the material by merely counting the 

 blue or violet-blue colonies which have made their appear- 

 ance in the plates. Of course we would also be able, if 

 necessary, to make from these blue colonies the further tests 

 for B. typhosus. In the experiments to be presently described, 

 the enumeration by Drigalski plates of the number of B. 

 typhosus introduced into oysters or taken up by them while 

 in sea water, to which a small amount of a pure culture of 

 B. typhosus had been added, was easily carried out ; indeed, 

 such exact determination, as will presently be shown, was 

 made possible because we had this method at our disposal. 



The oysters, with the exception of those in Experiment IV, 

 selected for our experiments, as also the mussels and cockles, 

 were all clean, containing no microbes of the coli-typhoid 

 tyP e - The sea water used for the experiments was clean, and 

 free of any microbes of the coli-typhoid type in fact, the sea 

 water had been previously sterilised. The infecting material 



