BACTERIOLOGY OF WATER 237 



MacConTcey and Hill's Bile-Salt Broth. Sodium tauro- 

 cholate 0'5 gramme, glucose or lactose 0*5 gramme, 

 peptone 2'G grammes,* are dissolved in 100 c.c. of water 

 by heating; the mixture is filtered, and after filtration 

 sufficient neutral litmus solution is added to give a dis- 

 tinct colour. The medium is then distributed into 

 Durham's fermentation tubes, and sterilised for twenty 

 minutes on three successive days. This is of ' single 

 strength.' 



Neutral Red Bile-Salt Lactose Agar (MacConkey). 

 Sodium taurocholate 0*5 per cent., peptone 2 per cent.,* 

 lactose 1 per cent., agar 2 per cent. To be made with 

 tap-water, and 0-5 c.c. of a 1 per cent, solution of neutral 

 red added. 



Organisms, such as B. coli, which ferment lactose gener- 

 ally, but not invariably, give bright red colonies. B. ty- 

 phosus, which does not ferment this sugar, produces white 

 colonies, unless the peptone contains glucose. 



Conradi- Drigalslci Agar. Add peptone 10 grammes, 

 nutrose 10 grammes, sodium chloride 5 grammes, to 1 litre 

 of acid beef broth. Steam for one hour, and add 25 

 grammes of powdered agar. Steam for three hours, 

 bring to a reaction of + 10, and filter through papier 

 Chardin (A). 



Boil for a few minutes 100 c.c. of Kubel-Tiemann 

 litmus solution, add 15 grammes of pure powdered lactose, 

 and boil again for a few minutes (B). 



Add B to A, and to this mixture add 2 c.c. of a hot 

 10 per cent, solution of anhydrous sodium carbonate and 

 10 c.c. of a 0*1 per cent, solution of krystal violet. The 

 medium is then tubed, 10 c.c. being placed in each 

 test-tube, and sterilised. On this medium in forty-eight 

 hours B. coli forms large red colonies, typhoid small blue 

 ' dewdrop ' colonies, and streptococci small delicate red 

 colonies. 



This medium and the one preceding are employed as 

 surface plates. Tubes are melted in a water-bath, and 

 their contents poured out into sterile Petri dishes. When 

 set, the plates are placed in the blood-heat incubator for 

 two hours, with the lids slightly tilted at one edge, so that 

 the surface of the medium may dry somewhat. Other- 

 wise the moisture present would probably cause the 

 * The peptone used must be free from glucose. 



