234 AIDS TO BACTERIOLOGY 



Detection of the Typhoid Bacillus. The incubation 

 period of typhoid fever is generally ten to fourteen days, 

 a period often long enough for the disappearance of the 

 organism from the water before an examination is sug- 

 gested, leaving out of the question the period elapsing 

 before the water is suspected. A water containing typhoid 

 will almost certainly contain very much larger numbers 

 of the colon bacillus and other organisms which will 

 flourish on the media used for isolation of the typhoid 

 bacillus, crowd the plates, and perhaps outgrow the 

 organism sought. The last reason is less likely where 

 typhoid urine alone is responsible (see pp. 99 and 227), 

 and this class of cases happens to be that where typhoid 

 bacilli have been isolated and successfully identified. 

 Apart from the urine contamination cases, there has been 

 constant failure to find the typhoid bacillus in incrimi- 

 nated water, and considering all things this is no matter for 

 surprise. A negative result is worthless, and great danger 

 to the community may result from failure to realise this. 



The amount of actual typhoid pollution is almost 

 invariably very small, and direct plating of a suspected 

 water is waste of time. One of the following methods of 

 concentration is therefore adopted : 



(a) Filtration through a Porcelain Filter. A large 

 number of the organisms get entangled in the filter candle, 

 and the process is seldom used. 



(6) Willsori's Precipitation Method. A 10 per cent, 

 solution of alum in sterile distilled water is added to the 

 water till it contains 0-5 gramme of the salt to the litre. 

 The precipitate is allowed to settle, or, after the precipitate 

 of aluminium hydrate has formed, the vessel is well shaken 

 to distribute its contents evenly, and centrifuged for 

 fifteen minutes. The clear supernatant fluid is then 

 siphoned or poured carefully off from the precipitate, 

 and the mass of precipitate in the conical extremity of 

 the tube stirred up with the little fluid remaining. The 

 suspension is then plated out. If the water is very soft 

 it is advisable to add a little lime-water, or the precipitate 

 will not form well. The alum is said to have no bacteri- 

 cidal effect, but if too much alum be allowed on the surface 

 of a plate growth will be retarded. This method is the 

 most satisfactory. 



(c) Serum Agglutination (Schepilewsky). Ten to twenty 



