42 WATER 



rapidly concentrated without destruction of bacteria. The glass 

 reservoir containing the sample of water is connected with an 

 exhaust pump which rapidly lowers the pressure. The reservoir 

 is immersed in water which is accurately kept at 37 C. The 

 water boils at this reduced temperature and as a rule 24 hours 

 suffice to evaporate four litres of water almost to dryness. The 

 residue is spread over solid media plates as described below. 



This method can also be combined with "enrichment" by 

 the addition of broth or other nutrient medium to the water two 

 hours after the commencement of the evaporation, this delay 

 being necessary to avoid frothing. 



In a later paper Wilson and Dickson record that they were 

 able, using this method, to recover the typhoid bacillus from 

 water containing only 30 typhoid bacilli in four litres, equivalent 

 to one bacillus in 133 c.c. of water. 



2. Isolation. To isolate the typhoid bacilli from the concen- 

 trated material a large number of special media are now available. 

 These media do not so much show which colonies are typhoid 

 bacillus colonies as indicate the colonies which certainly are not 

 typhoid bacillus colonies. In this way a vast number of organisms 

 also present are either suppressed or, if they develop, are readily 

 distinguished by their colour, appearance, etc., from typhoid 

 bacillus colonies. All colonies possibly the latter must be 

 further investigated. 



Of these special media, the following may be mentioned : 

 Drigalski-Conradi agar, lactose bile salt neutral red agar (L.B.A.), 

 fuchsin agar, malachite green agar and brilliant green picric acid 

 agar as modified by Fawcus. 



The composition of these media is given in the Appendix. 



Houston in the investigations mentioned above used L.B.A. 

 modified by the addition of other sugars, etc. In addition to 

 lactose he added saccharose, adonite, raffinose and salicin, all 

 0*2 per cent, of each. By their addition the number of white 

 colonies was still further reduced and without risk of typhoid 

 bacilli being overlooked. 



With all these media the plates before or after inoculation 

 should be thoroughly dried uncovered, preferably in the incu- 

 bator. They are then covered, inverted and incubated. The 



