ISOLATION OF SPECIFIC PATHOGENES 79 



turbid zone between the white opaque centre or nucleus 

 and the perfectly circular narrow white seam or edge. 



Stokes and Hachtel (1912) have suggested a modifica- 

 tion of the Hesse medium which consists in the increase 

 of the agar to 5.5 per cent and in the addition of 10 

 gm. of lactose and 50 gm. of glycerin to the formula 

 cited above. The agar is dried out at 105 degrees for 

 half an hour and dissolved in half a liter of water. The 

 meat extract is added to the other half liter and freed 

 from muscle sugar by inoculation with the colon bacillus 

 and incubation for 24 hours at 37 degrees. The sugar- 

 free broth thus prepared is filtered and to it is added 

 the peptone, lactose and salt. The two half liter por- 

 tions of the medium are then mixed and boiled for 

 30 minutes. The medium is filtered and adjusted to a 

 neutral reaction, the glycerin is added and the medium 

 is tinted with azolitmin solution before tubing and steril- 

 izing. Typhoid and para-typhoid organisms develop 

 medium-sized, pinkish colonies with concentric rings, 

 and may thus be distinguished from colonies of B. 

 alcaligenes, B. proteus and other motile forms. Organ- 

 isms of the B. subtilis group must be eliminated by 

 microscopic examination, using the Gram stain. 



Preliminary Enrichment. In most cases plate isola- 

 tion is preceded by some sort of enrichment process 

 designed to favor the typhoid bacilli at the expense 

 of the members of the B. coli group. The original 

 use of carbol broth has been already discussed. In 

 Europe caffein media have been used for this purpose 

 and in the United Staes bile media have been strongly 

 recommended. 



