7Sd ELEMENTS OF WATER BACTERIOLOGY 



these circumstances 4.5 grams may be used in the 

 formula instead of the 5 grams recommended by Hesse. 

 The medium must be stored in an ice chest with saturated 

 atmosphere. Plates must be made in sufficient dilu- 

 tion to give a few colonies on the plate; and where 

 this is done the typhoid colonies are sharply dis- 

 tinguished from those of B. coli by the fact that they 

 grow to a considerable size, often several centimeters 

 in diameter and show a broad translucent or scarcely 

 turbid zone between the white opaque centre or nucleus 

 and the perfectly circular narrow white seam or edge. 

 Stokes and Hachtel (191 2) 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- 



