WATER 43 



colonies are sufficiently differentiated after 16 to 24 hours at 

 37 C. 



With Drigalski-Conradi agar the B. coli colonies are red, not 

 transparent, and have a diameter of 2 to 6 millimetres, but con- 

 siderable variation in size and degree of colour are met with. 

 The B. typhosus colonies are blue, with a violet tinge ; they are 

 transparent and resemble dewdrops, and have a diameter of I to 

 3 millimetres, seldom larger. 



With fuchsin agar the B. coli colonies are bright red, round 

 and have prominent margins ; the typhoid colonies are round, 

 colourless, very transparent, and have thin margins. 



With L.B.A. B. coli and other lactose fermenters grow as red 

 colonies while B. typhostis, Gaertner group bacilli and other non- 

 lactose fermenters form white colonies. 



Using Fawcus's brilliant green picric acid agar lactose fer- 

 menters form opaque colonies, while B. typhosus and other non- 

 lactose fermenters grow as clear colonies. 



Malachite green agar is more suitable for Gaertner group 

 work. Fuchsin agar does not keep well and Drigalski-Conradi 

 agar is troublesome to prepare and is not always satisfactory 

 to use. Fawcus's medium is praised by some workers but has 

 not been very satisfactory in the writer's hands. 



In general L.B.A., with or without the addition of further 

 sugars, etc., is recommended as the most generally useful, while 

 fuchsin agar is also decidedly valuable. 



3. Identification. All suspicious colonies found on the 

 different media in the plates are subcultivated into broth, and 

 incubated at 37 C. until next day. They are all then examined 

 in hanging drop, and those which show actively motile bacilli 

 are tested with anti-typhoid serum. A fairly powerful serum 

 should be available, and a dilution of not less than one per cent, 

 should be employed. 



A quicker and sometimes preferable method is to directly 

 test with the antityphoid serum each of the selected colonies by 

 rubbing up a little of the colony in a drop of one per cent, serum 

 on a coverglass. Only those which contain reacting bacilli are 

 subcultivated. 



All those which fail to show agglutination are rejected, while 



