662 Typhoid Fever 



In a careful study of the bile-salt media MacConkey* 

 points out an error, first discovered by Theobald Smith, that 

 depends upon the alkali production of the colon bacillus in 

 the absence of sugar. If too little sugar be added to the 

 medium, the alkali production masks the acid production 

 unless the oxygen be removed, and red colonies of the colon 

 bacillus grown upon the medium may in time turn dis- 

 tinctly blue. It becomes obvious, therefore, that the 

 medium should be as neutral as possible to the indicator 

 used. After trial he found neutral red preferable to litmus, 

 and makes the medium as follows: 



i. A stock solution is made: 



Sodium taurocholate (commercial from ox-bile 



and neutral to neutral red) 0.5 per cent. 



Peptone (White's) 2.0 



Water (distilled or tap) 100.0 c.c. 



(As calcium 0.03 per cent, is favorable to the growth of the 



organisms, it should be added if distilled water is used.) 



The ingredients should be mixed, steamed in a steam sterilizer for 

 one to two hours, filtered while hot, allowed to stand twenty-four to 

 forty-eight hours, then filtered cold through paper. A clear solution 

 should then result, which will keep indefinitely under proper conditions. 

 The various bile-salt media are prepared from this stock solution by 

 adding glucose, 0.5 per cent. ; lactose, i per cent. ; cane-sugar, i per cent. ; 

 dulcit, 0.5 per cent.; adonit, 0.5 per cent., or inulin, i per cent.; and 

 neutral red (i per cent, solution), 0.25 per cent., distributing into fer- 

 mentation-tubes and sterilizing in the steamer for fifteen minutes on 

 each of three successive days. 



Bile-salt agar-agar is made by dissolving 2 per cent, of agar-agar in 

 the stock fluid, either in the steamer or in the autoclave. The mixture 

 is cleared with an egg, filtered, neutral red added in the same proportion 

 as for the broth, and distributed into flasks in quantities of 80 c.c. 

 When required for use, the fermentable substance is added to the agar 

 in the flask, and the whole placed in a water-bath or steamer (care must 

 be taken not to heat either the fluid or solid medium beyond 100 C.). 

 When melted, the agar preparation is poured into Petri dishes, allowed 

 to solidify, and then dried in an incubator or warm room, the plate being 

 placed upside down with the bottom detached and propped up on the 

 edge of the cover. It is necessary that the surface of the agar-agar 

 should not be too wet, lest the colonies become confluent, nor too dry, 

 lest the growth be stunted. Inoculations are made by placing a loop- 

 ful of the material to be examined on the center of one plate, and 

 rubbed over the surface with a bent glass-rod; the same rod, without 

 recharging, being used to inoculate the surface of two other plates. 

 The plates are then incubated upside down. The colonies of the colon 

 bacillus appear yellow. 



* ''Journal of Hygiene," 1908, vm, p. 322. 



