67 



diagnosis is readily established. For B. Gaertner is shorter 

 than B. typhosus ; B. Gaertner gives positive neutral red broth 

 test, it turns the litmus milk at first slightly acid (red), but 

 after two or three days gradually alkaline (blue and slate 

 colour), and it produces acid and gas in MacConkey fluid. It 

 does not agglutinate with typhoid in anything like the high 

 dilution that B. typhosus does that is to say, it is by all the 

 above tests easily distinguished from the B. typhosus (see 

 a former page). There would be, therefore, none but a 

 preliminary difficulty in differentiating between the two 

 microbes. B. Gaertner in small doses is highly virulent to 

 rodents, both after subcutaneous injection, as also by feeding, 

 and in these respects differs from the B. typhosus. We have 

 already indicated that all colonies of B. coli communis can 

 by means of the Drigalski plates at 37 C. be recognised 

 already in 24 hours, they being several millimetres in dia- 

 meter, being red and surrounded by a distinct red halo ; other 

 acid-forming (red) coli-like microbes are slower in develop- 

 ing, and are much smaller. These are always numerous in 

 sewage, but being small and slow in coming, although red, 

 and even some with indication of red halo,, can at once be 

 neglected, as far as the search for specific microbes is 

 concerned. 



The B. dysenteric forms neither red nor blue colonies, 

 being neutral, like many other microbes not belonging to the 

 ooli-typhoid group. These, therefore, do not offer any basis 

 for further inquiry by means of the Drigalski medium.. 



But there are a number of species of microbes of sewage 

 and of faecal matters which, on account of their forming blue 

 or bluish colonies in the Drigalski medium, require special 

 considerations in reference to shellfish, in order to differentiate 

 them from B. typhosus and from B. Gaertner. 



I. MICEOBES OF SEWAGE FORMING "BLUE" COLONIES ON 

 DRIGALSKI MEDIUM. 



(a.) Amongst the many clean oysters which I have ex- 

 amined, I have not found any as yet which contain bacteria 



F 2 



