15 



culture medium), by which other bacteria not belonging to 

 the coli-typhoid 'group are kept back, while the bacteria of 

 the latter group grow undisturbed. The same, to a large extent, 

 is the case with Eisner potato gelatine. But it must be obvious 

 from what has been said above that such inhibition (for a 

 time, at any rate) of other bacteria not belonging to the coli- 

 typhoid group does not carry us very much further, because 

 the difficulty about the coli-bacteria amongst themselves is 

 still present. MacConkey's fluid is a further step in advance, 

 because in this medium the rapid production of acid red- 

 dening of the litmus fluid denotes already in 24 hours or 

 so, the presence of acid-forming bacteria, most probably 

 belonging to the coli-typhoid group, and possibly including 

 the B. typhosus. This medium keeps back non-acid pro- 

 ducers to a large extent, and, owing to the easily perceived 

 change in colour of the litmus to red, the presence of an 

 acid-producing microbe is at once made out. Unfortunately, 

 all coli-bacteria forming acid grow well in this medium, and, 

 therefore, it does not carry us much further in the isolation 

 of the B. typhosus than did Parietti's method. The same 

 remarks apply to all other media that have been hitherto 

 described, with the notable exception of the Drigalski- 

 Conradi medium, for by this medium used for surface plates, 

 as described by Drigalski-Conradi (" Zeitschrft f. Hygiene," 

 vol. 39, p. 283) (Nutrose, lactose, litmus, crystal violet, 

 agar), we are at once placed into this advantageous posi- 

 tion, that we can not only keep back or exclude (by the 

 use of the crystal violet) bacteria other than coli-typhoid, 

 but we can in positive fashion and in isolated aspect recog- 

 nise at once those colonies which are not B. typhosus. 

 Any colony which after 24-36 hours' incubation at 37 C. 

 appears on the medium red in colour surrounded by a red 

 halo (reddening of the litmus constituent of the medium due 

 to rapid acid production from, i.e., fermentation of the lactose 

 by the bacteria constituting the said colony), cannot be one 

 of B. typhosus, but must be one of B. coli, probably B. coli 

 communis ; if the medium did nothing else than this, it alone 

 would for obvious reasons be a great help, for we could from 



