SPECIAL EXAMINATIONS 361 



fact that the organism is easily outgrown by the other 

 excremental bacteria, and so is crowded out in most media. 

 When, to help matters, some agent, inhibitory to the 

 others, is added to the medium, the typhoid organism is 

 likewise affected, though to a lesser degree. 



From all these causes attempts at isolation of the B. 

 typhosus from water supplies usually fail. 



The first step is to concentrate the water. This can be 

 done by : 



(a.) Centrifugalization of large quantities, and 

 plating the sediment. 



(b.) Precipitation by entanglement in a chemical 

 precipitate, such as weak soda solution and ferrous 

 sulphate added to the water, or weak alum and 

 lime-water solutions. Centrifuge, dissolve precipi- 

 tate in neutral potassium tartrate, and plate on 

 solid media. 



(c.) Filtration through unglazed porcelain (Pas- 

 teur-Chamberland bougie) . Wash bougie by brush- 

 ing with 10 c.c. of sterile water, and use product to 

 smear plates. The candle should filter from without 

 inwards. Bacteria are apt to be lost in the pores of 

 the filter. Some filter from within outwards, add 

 broth, and thus get a primary culture. 



(d.) Evaporation of the bulk of the water at 



blood-heat under reduced atmospheric pressure. 



The enrichment method of Hoffman and Ficker is to 



add nutrose, caffein, and crystal violet to the sample, to 



make it a suitable growing medium for B. typhosus. For 



exact proportions see page 239. 



The first method that is, centrifugalization is mostly 

 favoured. 



One hundred c.c. of the sample are centrifuged. The 

 sediment is plated direct on to rebipelagar, and incubated 

 at 37 C. for twenty-four hours. All the colourless colonies 

 are subcultured on to agar slopes, and incubated, at 37 C. 

 for twenty-four hours. Thereafter subcultures are made 

 into gelatin stab, litmus milk, peptone water, and glucose, 

 lactose, saccharose, dulcite, mannite, adonite, and inulin 

 media. A motile organism, giving negative results to all 

 these reactions except litmus milk, in which it gives acid 



